信號量有一個很明顯的缺點，沒有區分臨界區的讀寫屬性，讀寫鎖允許多個線程進程並發的訪問臨界區，但是寫訪問只限於一個線程，在多處理器系統中允許多個讀者訪問共享資源，但是寫者有排他性，讀寫鎖的特性如下：允許多個讀者同時訪問臨界區，但是同一時間不能進入；同一時刻只允許一個寫者進入臨界區；讀者和寫者不能同時進入臨界區。讀寫鎖也有關閉中斷和下半部的版本。
RCU:read-copy-update 。。。。。。。。。。。。。。。。。。。。
問題：rcu相比讀寫鎖，解決了什么問題？ rcu的基本原理？
1、由於內核中spinlock mutex 等都使用了原子操作指令，即原子的訪問內存，但是當多cpu 競爭訪問臨界區時會讓cpu的cache命中率下降，性能下降。同時讀寫鎖有個缺陷，讀者和寫者不能同時存在。
rcu實現的目標就是要解決這個問題，為了使線程同步開銷小。不需要原子操作以及內存屏障而訪問數據，把同步的問題交給寫者線程，寫者線程等待所有的讀者線程完成后才會吧舊數據銷毀。當有多個寫者線程存在時，需要額外的保護機制。
原理
RCU原理：簡單理解為 記錄了所有指向共享數據的指針使用者，當要修改共享數據時，先創建一個副本，在副本中修改。所有讀者離開臨界區后，指針指向新的修改副本后的地方，並且刪除舊數據。
官方描述：RCU實際上是一種改進的rwlock，讀者幾乎沒有什么同步開銷，它不需要鎖，不使用原子指令，因此不會導致鎖競爭，內存延遲以及流水線停滯。不需要鎖也使得使用更容易，因為死鎖問題就不需要考慮了。
- 寫者的同步開銷比較大，它需要延遲數據結構的釋放，復制被修改的數據結構，它也必須使用某種鎖機制同步並行的其它寫者的修改操作。
- 讀者必須提供一個信號給寫者以便寫者能夠確定數據可以被安全地釋放或修改的時機。
- 有一個專門的垃圾收集器來探測讀者的信號，一旦所有的讀者都已經發送信號告知它們都不在使用被RCU保護的數據結構，垃圾收集器就調用回調函數完成最后的數據釋放或修改操作。
目前在內核中鏈表使用RCU較多。
在經典RCU中，RCU讀側臨界部分由rcu_read_lock() 和rcu_read_unlock()界定，它們可以嵌套。
對應的同步更新原語為synchronize_rcu()，還有同義的synchronize_net()，等待當前正執行的RCU讀側聞臨界部分運行完成。等待的時間稱為“寬限期”。
異步更新側原語call_rcu()在寬限期之后觸發指定的函數，如：call_rcu(p,f)調用觸發回調函數f(p)。有些情況，如：當卸載使用call_rcu()的模塊，必須等待所有RCU回調函數完成，原語rcu_barrier()起該作用。
在“RCU BH”列中，rcu_read_lock_bh() 和rcu_read_unlock_bh()界定讀側臨界部分，call_rcu_bh()在寬限期后觸發指定的函數。注意：RCU BH沒有同步接口synchronize_rcu_bh()，如果需要，用戶很容易添加同步接口函數。
直接操作指針的原語rcu_assign_pointer()和rcu_dereference()用於創建RCU保護的非鏈表數據結構，如：數組和樹
NOTE：讀者在訪問被RCU保護的共享數據期間不能被阻塞，這是RCU機制得以實現的一個基本前提，也就說當讀者在引用被RCU保護的共享數據期間，讀者所在的CPU不能發生上下文切換，spinlock和rwlock都需要這樣的前提。 寫者在訪問被RCU保護的共享數據時不需要和讀者競爭任何鎖，只有在有多於一個寫者的情況下需要獲得某種鎖以與其他寫者同步。寫者修改數據前首先拷貝一個被修改元素的副本，然后在副本上進行修改，修改完畢后它向垃圾回收器注冊一個回調函數以便在適當的時機執行真正的修改操作。等待適當時機的這一時期稱為寬限期（grace period），而CPU發生了上下文切換稱為經歷一個quiescent state，grace period就是所有CPU都經歷一次quiescent state所需要的等待的時間。垃圾收集器就是在grace period之后調用寫者注冊的回調函數來完成真正的數據修改或數據釋放操作的。
/* Please note that the "What is RCU?" LWN series is an excellent place to start learning about RCU: 1. What is RCU, Fundamentally? http://lwn.net/Articles/262464/ 2. What is RCU? Part 2: Usage http://lwn.net/Articles/263130/ 3. RCU part 3: the RCU API http://lwn.net/Articles/264090/ 4. The RCU API, 2010 Edition http://lwn.net/Articles/418853/ What is RCU? RCU is a synchronization mechanism that was added to the Linux kernel during the 2.5 development effort that is optimized for read-mostly situations. Although RCU is actually quite simple once you understand it, getting there can sometimes be a challenge. Part of the problem is that most of the past descriptions of RCU have been written with the mistaken assumption that there is "one true way" to describe RCU. Instead, the experience has been that different people must take different paths to arrive at an understanding of RCU. This document provides several different paths, as follows: 1. RCU OVERVIEW 2. WHAT IS RCU'S CORE API? 3. WHAT ARE SOME EXAMPLE USES OF CORE RCU API? 4. WHAT IF MY UPDATING THREAD CANNOT BLOCK? 5. WHAT ARE SOME SIMPLE IMPLEMENTATIONS OF RCU? 6. ANALOGY WITH READER-WRITER LOCKING 7. FULL LIST OF RCU APIs 8. ANSWERS TO QUICK QUIZZES People who prefer starting with a conceptual overview should focus on Section 1, though most readers will profit by reading this section at some point. People who prefer to start with an API that they can then experiment with should focus on Section 2. People who prefer to start with example uses should focus on Sections 3 and 4. People who need to understand the RCU implementation should focus on Section 5, then dive into the kernel source code. People who reason best by analogy should focus on Section 6. Section 7 serves as an index to the docbook API documentation, and Section 8 is the traditional answer key. So, start with the section that makes the most sense to you and your preferred method of learning. If you need to know everything about everything, feel free to read the whole thing -- but if you are really that type of person, you have perused the source code and will therefore never need this document anyway. ;-) 1. RCU OVERVIEW The basic idea behind RCU is to split updates into "removal" and "reclamation" phases. The removal phase removes references to data items within a data structure (possibly by replacing them with references to new versions of these data items), and can run concurrently with readers. The reason that it is safe to run the removal phase concurrently with readers is the semantics of modern CPUs guarantee that readers will see either the old or the new version of the data structure rather than a partially updated reference. The reclamation phase does the work of reclaiming (e.g., freeing) the data items removed from the data structure during the removal phase. Because reclaiming data items can disrupt any readers concurrently referencing those data items, the reclamation phase must not start until readers no longer hold references to those data items. Splitting the update into removal and reclamation phases permits the updater to perform the removal phase immediately, and to defer the reclamation phase until all readers active during the removal phase have completed, either by blocking until they finish or by registering a callback that is invoked after they finish. Only readers that are active during the removal phase need be considered, because any reader starting after the removal phase will be unable to gain a reference to the removed data items, and therefore cannot be disrupted by the reclamation phase. So the typical RCU update sequence goes something like the following: a. Remove pointers to a data structure, so that subsequent readers cannot gain a reference to it. b. Wait for all previous readers to complete their RCU read-side critical sections. c. At this point, there cannot be any readers who hold references to the data structure, so it now may safely be reclaimed (e.g., kfree()d). Step (b) above is the key idea underlying RCU's deferred destruction. The ability to wait until all readers are done allows RCU readers to use much lighter-weight synchronization, in some cases, absolutely no synchronization at all. In contrast, in more conventional lock-based schemes, readers must use heavy-weight synchronization in order to prevent an updater from deleting the data structure out from under them. This is because lock-based updaters typically update data items in place, and must therefore exclude readers. In contrast, RCU-based updaters typically take advantage of the fact that writes to single aligned pointers are atomic on modern CPUs, allowing atomic insertion, removal, and replacement of data items in a linked structure without disrupting readers. Concurrent RCU readers can then continue accessing the old versions, and can dispense with the atomic operations, memory barriers, and communications cache misses that are so expensive on present-day SMP computer systems, even in absence of lock contention. In the three-step procedure shown above, the updater is performing both the removal and the reclamation step, but it is often helpful for an entirely different thread to do the reclamation, as is in fact the case in the Linux kernel's directory-entry cache (dcache). Even if the same thread performs both the update step (step (a) above) and the reclamation step (step (c) above), it is often helpful to think of them separately. For example, RCU readers and updaters need not communicate at all, but RCU provides implicit low-overhead communication between readers and reclaimers, namely, in step (b) above. So how the heck can a reclaimer tell when a reader is done, given that readers are not doing any sort of synchronization operations??? Read on to learn about how RCU's API makes this easy. 2. WHAT IS RCU'S CORE API? The core RCU API is quite small: a. rcu_read_lock() b. rcu_read_unlock() c. synchronize_rcu() / call_rcu() d. rcu_assign_pointer() e. rcu_dereference() There are many other members of the RCU API, but the rest can be expressed in terms of these five, though most implementations instead express synchronize_rcu() in terms of the call_rcu() callback API. The five core RCU APIs are described below, the other 18 will be enumerated later. See the kernel docbook documentation for more info, or look directly at the function header comments. rcu_read_lock() void rcu_read_lock(void); Used by a reader to inform the reclaimer that the reader is entering an RCU read-side critical section. It is illegal to block while in an RCU read-side critical section, though kernels built with CONFIG_PREEMPT_RCU can preempt RCU read-side critical sections. Any RCU-protected data structure accessed during an RCU read-side critical section is guaranteed to remain unreclaimed for the full duration of that critical section. Reference counts may be used in conjunction with RCU to maintain longer-term references to data structures. rcu_read_unlock() void rcu_read_unlock(void); Used by a reader to inform the reclaimer that the reader is exiting an RCU read-side critical section. Note that RCU read-side critical sections may be nested and/or overlapping. synchronize_rcu() void synchronize_rcu(void); Marks the end of updater code and the beginning of reclaimer code. It does this by blocking until all pre-existing RCU read-side critical sections on all CPUs have completed. Note that synchronize_rcu() will -not- necessarily wait for any subsequent RCU read-side critical sections to complete. For example, consider the following sequence of events: CPU 0 CPU 1 CPU 2 ----------------- ------------------------- --------------- 1. rcu_read_lock() 2. enters synchronize_rcu() 3. rcu_read_lock() 4. rcu_read_unlock() 5. exits synchronize_rcu() 6. rcu_read_unlock() To reiterate, synchronize_rcu() waits only for ongoing RCU read-side critical sections to complete, not necessarily for any that begin after synchronize_rcu() is invoked. Of course, synchronize_rcu() does not necessarily return -immediately- after the last pre-existing RCU read-side critical section completes. For one thing, there might well be scheduling delays. For another thing, many RCU implementations process requests in batches in order to improve efficiencies, which can further delay synchronize_rcu(). Since synchronize_rcu() is the API that must figure out when readers are done, its implementation is key to RCU. For RCU to be useful in all but the most read-intensive situations, synchronize_rcu()'s overhead must also be quite small. The call_rcu() API is a callback form of synchronize_rcu(), and is described in more detail in a later section. Instead of blocking, it registers a function and argument which are invoked after all ongoing RCU read-side critical sections have completed. This callback variant is particularly useful in situations where it is illegal to block or where update-side performance is critically important. However, the call_rcu() API should not be used lightly, as use of the synchronize_rcu() API generally results in simpler code. In addition, the synchronize_rcu() API has the nice property of automatically limiting update rate should grace periods be delayed. This property results in system resilience in face of denial-of-service attacks. Code using call_rcu() should limit update rate in order to gain this same sort of resilience. See checklist.txt for some approaches to limiting the update rate. rcu_assign_pointer() typeof(p) rcu_assign_pointer(p, typeof(p) v); Yes, rcu_assign_pointer() -is- implemented as a macro, though it would be cool to be able to declare a function in this manner. (Compiler experts will no doubt disagree.) The updater uses this function to assign a new value to an RCU-protected pointer, in order to safely communicate the change in value from the updater to the reader. This function returns the new value, and also executes any memory-barrier instructions required for a given CPU architecture. Perhaps just as important, it serves to document (1) which pointers are protected by RCU and (2) the point at which a given structure becomes accessible to other CPUs. That said, rcu_assign_pointer() is most frequently used indirectly, via the _rcu list-manipulation primitives such as list_add_rcu(). rcu_dereference() typeof(p) rcu_dereference(p); Like rcu_assign_pointer(), rcu_dereference() must be implemented as a macro. The reader uses rcu_dereference() to fetch an RCU-protected pointer, which returns a value that may then be safely dereferenced. Note that rcu_deference() does not actually dereference the pointer, instead, it protects the pointer for later dereferencing. It also executes any needed memory-barrier instructions for a given CPU architecture. Currently, only Alpha needs memory barriers within rcu_dereference() -- on other CPUs, it compiles to nothing, not even a compiler directive. Common coding practice uses rcu_dereference() to copy an RCU-protected pointer to a local variable, then dereferences this local variable, for example as follows: p = rcu_dereference(head.next); return p->data; However, in this case, one could just as easily combine these into one statement: return rcu_dereference(head.next)->data; If you are going to be fetching multiple fields from the RCU-protected structure, using the local variable is of course preferred. Repeated rcu_dereference() calls look ugly, do not guarantee that the same pointer will be returned if an update happened while in the critical section, and incur unnecessary overhead on Alpha CPUs. Note that the value returned by rcu_dereference() is valid only within the enclosing RCU read-side critical section. For example, the following is -not- legal: rcu_read_lock(); p = rcu_dereference(head.next); rcu_read_unlock(); x = p->address; /* BUG!!! */ rcu_read_lock(); y = p->data; /* BUG!!! */ rcu_read_unlock(); Holding a reference from one RCU read-side critical section to another is just as illegal as holding a reference from one lock-based critical section to another! Similarly, using a reference outside of the critical section in which it was acquired is just as illegal as doing so with normal locking. As with rcu_assign_pointer(), an important function of rcu_dereference() is to document which pointers are protected by RCU, in particular, flagging a pointer that is subject to changing at any time, including immediately after the rcu_dereference(). And, again like rcu_assign_pointer(), rcu_dereference() is typically used indirectly, via the _rcu list-manipulation primitives, such as list_for_each_entry_rcu(). The following diagram shows how each API communicates among the reader, updater, and reclaimer. rcu_assign_pointer() +--------+ +---------------------->| reader |---------+ | +--------+ | | | | | | | Protect: | | | rcu_read_lock() | | | rcu_read_unlock() | rcu_dereference() | | +---------+ | | | updater |<---------------------+ | +---------+ V | +-----------+ +----------------------------------->| reclaimer | +-----------+ Defer: synchronize_rcu() & call_rcu() The RCU infrastructure observes the time sequence of rcu_read_lock(), rcu_read_unlock(), synchronize_rcu(), and call_rcu() invocations in order to determine when (1) synchronize_rcu() invocations may return to their callers and (2) call_rcu() callbacks may be invoked. Efficient implementations of the RCU infrastructure make heavy use of batching in order to amortize their overhead over many uses of the corresponding APIs. There are no fewer than three RCU mechanisms in the Linux kernel; the diagram above shows the first one, which is by far the most commonly used. The rcu_dereference() and rcu_assign_pointer() primitives are used for all three mechanisms, but different defer and protect primitives are used as follows: Defer Protect a. synchronize_rcu() rcu_read_lock() / rcu_read_unlock() call_rcu() rcu_dereference() b. synchronize_rcu_bh() rcu_read_lock_bh() / rcu_read_unlock_bh() call_rcu_bh() rcu_dereference_bh() c. synchronize_sched() rcu_read_lock_sched() / rcu_read_unlock_sched() call_rcu_sched() preempt_disable() / preempt_enable() local_irq_save() / local_irq_restore() hardirq enter / hardirq exit NMI enter / NMI exit rcu_dereference_sched() These three mechanisms are used as follows: a. RCU applied to normal data structures. b. RCU applied to networking data structures that may be subjected to remote denial-of-service attacks. c. RCU applied to scheduler and interrupt/NMI-handler tasks. Again, most uses will be of (a). The (b) and (c) cases are important for specialized uses, but are relatively uncommon. 3. WHAT ARE SOME EXAMPLE USES OF CORE RCU API? This section shows a simple use of the core RCU API to protect a global pointer to a dynamically allocated structure. More-typical uses of RCU may be found in listRCU.txt, arrayRCU.txt, and NMI-RCU.txt. struct foo { int a; char b; long c; }; DEFINE_SPINLOCK(foo_mutex); struct foo __rcu *gbl_foo; /* * Create a new struct foo that is the same as the one currently * pointed to by gbl_foo, except that field "a" is replaced * with "new_a". Points gbl_foo to the new structure, and * frees up the old structure after a grace period. * * Uses rcu_assign_pointer() to ensure that concurrent readers * see the initialized version of the new structure. * * Uses synchronize_rcu() to ensure that any readers that might * have references to the old structure complete before freeing * the old structure. */ void foo_update_a(int new_a) { struct foo *new_fp; struct foo *old_fp; new_fp = kmalloc(sizeof(*new_fp), GFP_KERNEL); spin_lock(&foo_mutex); old_fp = rcu_dereference_protected(gbl_foo, lockdep_is_held(&foo_mutex)); *new_fp = *old_fp; new_fp->a = new_a; rcu_assign_pointer(gbl_foo, new_fp); spin_unlock(&foo_mutex); synchronize_rcu(); kfree(old_fp); } /* * Return the value of field "a" of the current gbl_foo * structure. Use rcu_read_lock() and rcu_read_unlock() * to ensure that the structure does not get deleted out * from under us, and use rcu_dereference() to ensure that * we see the initialized version of the structure (important * for DEC Alpha and for people reading the code). */ int foo_get_a(void) { int retval; rcu_read_lock(); retval = rcu_dereference(gbl_foo)->a; rcu_read_unlock(); return retval; } So, to sum up: o Use rcu_read_lock() and rcu_read_unlock() to guard RCU read-side critical sections. o Within an RCU read-side critical section, use rcu_dereference() to dereference RCU-protected pointers. o Use some solid scheme (such as locks or semaphores) to keep concurrent updates from interfering with each other. o Use rcu_assign_pointer() to update an RCU-protected pointer. This primitive protects concurrent readers from the updater, -not- concurrent updates from each other! You therefore still need to use locking (or something similar) to keep concurrent rcu_assign_pointer() primitives from interfering with each other. o Use synchronize_rcu() -after- removing a data element from an RCU-protected data structure, but -before- reclaiming/freeing the data element, in order to wait for the completion of all RCU read-side critical sections that might be referencing that data item. See checklist.txt for additional rules to follow when using RCU. And again, more-typical uses of RCU may be found in listRCU.txt, arrayRCU.txt, and NMI-RCU.txt. 4. WHAT IF MY UPDATING THREAD CANNOT BLOCK? In the example above, foo_update_a() blocks until a grace period elapses. This is quite simple, but in some cases one cannot afford to wait so long -- there might be other high-priority work to be done. In such cases, one uses call_rcu() rather than synchronize_rcu(). The call_rcu() API is as follows: void call_rcu(struct rcu_head * head, void (*func)(struct rcu_head *head)); This function invokes func(head) after a grace period has elapsed. This invocation might happen from either softirq or process context, so the function is not permitted to block. The foo struct needs to have an rcu_head structure added, perhaps as follows: struct foo { int a; char b; long c; struct rcu_head rcu; }; The foo_update_a() function might then be written as follows: /* * Create a new struct foo that is the same as the one currently * pointed to by gbl_foo, except that field "a" is replaced * with "new_a". Points gbl_foo to the new structure, and * frees up the old structure after a grace period. * * Uses rcu_assign_pointer() to ensure that concurrent readers * see the initialized version of the new structure. * * Uses call_rcu() to ensure that any readers that might have * references to the old structure complete before freeing the * old structure. */ void foo_update_a(int new_a) { struct foo *new_fp; struct foo *old_fp; new_fp = kmalloc(sizeof(*new_fp), GFP_KERNEL); spin_lock(&foo_mutex); old_fp = rcu_dereference_protected(gbl_foo, lockdep_is_held(&foo_mutex)); *new_fp = *old_fp; new_fp->a = new_a; rcu_assign_pointer(gbl_foo, new_fp); spin_unlock(&foo_mutex); call_rcu(&old_fp->rcu, foo_reclaim); } The foo_reclaim() function might appear as follows: void foo_reclaim(struct rcu_head *rp) { struct foo *fp = container_of(rp, struct foo, rcu); foo_cleanup(fp->a); kfree(fp); } The container_of() primitive is a macro that, given a pointer into a struct, the type of the struct, and the pointed-to field within the struct, returns a pointer to the beginning of the struct. The use of call_rcu() permits the caller of foo_update_a() to immediately regain control, without needing to worry further about the old version of the newly updated element. It also clearly shows the RCU distinction between updater, namely foo_update_a(), and reclaimer, namely foo_reclaim(). The summary of advice is the same as for the previous section, except that we are now using call_rcu() rather than synchronize_rcu(): o Use call_rcu() -after- removing a data element from an RCU-protected data structure in order to register a callback function that will be invoked after the completion of all RCU read-side critical sections that might be referencing that data item. If the callback for call_rcu() is not doing anything more than calling kfree() on the structure, you can use kfree_rcu() instead of call_rcu() to avoid having to write your own callback: kfree_rcu(old_fp, rcu); Again, see checklist.txt for additional rules governing the use of RCU. 5. WHAT ARE SOME SIMPLE IMPLEMENTATIONS OF RCU? One of the nice things about RCU is that it has extremely simple "toy" implementations that are a good first step towards understanding the production-quality implementations in the Linux kernel. This section presents two such "toy" implementations of RCU, one that is implemented in terms of familiar locking primitives, and another that more closely resembles "classic" RCU. Both are way too simple for real-world use, lacking both functionality and performance. However, they are useful in getting a feel for how RCU works. See kernel/rcupdate.c for a production-quality implementation, and see: http://www.rdrop.com/users/paulmck/RCU for papers describing the Linux kernel RCU implementation. The OLS'01 and OLS'02 papers are a good introduction, and the dissertation provides more details on the current implementation as of early 2004. 5A. "TOY" IMPLEMENTATION #1: LOCKING This section presents a "toy" RCU implementation that is based on familiar locking primitives. Its overhead makes it a non-starter for real-life use, as does its lack of scalability. It is also unsuitable for realtime use, since it allows scheduling latency to "bleed" from one read-side critical section to another. However, it is probably the easiest implementation to relate to, so is a good starting point. It is extremely simple: static DEFINE_RWLOCK(rcu_gp_mutex); void rcu_read_lock(void) { read_lock(&rcu_gp_mutex); } void rcu_read_unlock(void) { read_unlock(&rcu_gp_mutex); } void synchronize_rcu(void) { write_lock(&rcu_gp_mutex); write_unlock(&rcu_gp_mutex); } [You can ignore rcu_assign_pointer() and rcu_dereference() without missing much. But here they are anyway. And whatever you do, don't forget about them when submitting patches making use of RCU!] #define rcu_assign_pointer(p, v) ({ \ smp_wmb(); \ (p) = (v); \ }) #define rcu_dereference(p) ({ \ typeof(p) _________p1 = p; \ smp_read_barrier_depends(); \ (_________p1); \ }) The rcu_read_lock() and rcu_read_unlock() primitive read-acquire and release a global reader-writer lock. The synchronize_rcu() primitive write-acquires this same lock, then immediately releases it. This means that once synchronize_rcu() exits, all RCU read-side critical sections that were in progress before synchronize_rcu() was called are guaranteed to have completed -- there is no way that synchronize_rcu() would have been able to write-acquire the lock otherwise. It is possible to nest rcu_read_lock(), since reader-writer locks may be recursively acquired. Note also that rcu_read_lock() is immune from deadlock (an important property of RCU). The reason for this is that the only thing that can block rcu_read_lock() is a synchronize_rcu(). But synchronize_rcu() does not acquire any locks while holding rcu_gp_mutex, so there can be no deadlock cycle. Quick Quiz #1: Why is this argument naive? How could a deadlock occur when using this algorithm in a real-world Linux kernel? How could this deadlock be avoided? 5B. "TOY" EXAMPLE #2: CLASSIC RCU This section presents a "toy" RCU implementation that is based on "classic RCU". It is also short on performance (but only for updates) and on features such as hotplug CPU and the ability to run in CONFIG_PREEMPT kernels. The definitions of rcu_dereference() and rcu_assign_pointer() are the same as those shown in the preceding section, so they are omitted. void rcu_read_lock(void) { } void rcu_read_unlock(void) { } void synchronize_rcu(void) { int cpu; for_each_possible_cpu(cpu) run_on(cpu); } Note that rcu_read_lock() and rcu_read_unlock() do absolutely nothing. This is the great strength of classic RCU in a non-preemptive kernel: read-side overhead is precisely zero, at least on non-Alpha CPUs. And there is absolutely no way that rcu_read_lock() can possibly participate in a deadlock cycle! The implementation of synchronize_rcu() simply schedules itself on each CPU in turn. The run_on() primitive can be implemented straightforwardly in terms of the sched_setaffinity() primitive. Of course, a somewhat less "toy" implementation would restore the affinity upon completion rather than just leaving all tasks running on the last CPU, but when I said "toy", I meant -toy-! So how the heck is this supposed to work??? Remember that it is illegal to block while in an RCU read-side critical section. Therefore, if a given CPU executes a context switch, we know that it must have completed all preceding RCU read-side critical sections. Once -all- CPUs have executed a context switch, then -all- preceding RCU read-side critical sections will have completed. So, suppose that we remove a data item from its structure and then invoke synchronize_rcu(). Once synchronize_rcu() returns, we are guaranteed that there are no RCU read-side critical sections holding a reference to that data item, so we can safely reclaim it. Quick Quiz #2: Give an example where Classic RCU's read-side overhead is -negative-. Quick Quiz #3: If it is illegal to block in an RCU read-side critical section, what the heck do you do in PREEMPT_RT, where normal spinlocks can block??? 6. ANALOGY WITH READER-WRITER LOCKING Although RCU can be used in many different ways, a very common use of RCU is analogous to reader-writer locking. The following unified diff shows how closely related RCU and reader-writer locking can be. @@ -13,15 +14,15 @@ struct list_head *lp; struct el *p; - read_lock(); - list_for_each_entry(p, head, lp) { + rcu_read_lock(); + list_for_each_entry_rcu(p, head, lp) { if (p->key == key) { *result = p->data; - read_unlock(); + rcu_read_unlock(); return 1; } } - read_unlock(); + rcu_read_unlock(); return 0; } @@ -29,15 +30,16 @@ { struct el *p; - write_lock(&listmutex); + spin_lock(&listmutex); list_for_each_entry(p, head, lp) { if (p->key == key) { - list_del(&p->list); - write_unlock(&listmutex); + list_del_rcu(&p->list); + spin_unlock(&listmutex); + synchronize_rcu(); kfree(p); return 1; } } - write_unlock(&listmutex); + spin_unlock(&listmutex); return 0; } Or, for those who prefer a side-by-side listing: 1 struct el { 1 struct el { 2 struct list_head list; 2 struct list_head list; 3 long key; 3 long key; 4 spinlock_t mutex; 4 spinlock_t mutex; 5 int data; 5 int data; 6 /* Other data fields */ 6 /* Other data fields */ 7 }; 7 }; 8 spinlock_t listmutex; 8 spinlock_t listmutex; 9 struct el head; 9 struct el head; 1 int search(long key, int *result) 1 int search(long key, int *result) 2 { 2 { 3 struct list_head *lp; 3 struct list_head *lp; 4 struct el *p; 4 struct el *p; 5 5 6 read_lock(); 6 rcu_read_lock(); 7 list_for_each_entry(p, head, lp) { 7 list_for_each_entry_rcu(p, head, lp) { 8 if (p->key == key) { 8 if (p->key == key) { 9 *result = p->data; 9 *result = p->data; 10 read_unlock(); 10 rcu_read_unlock(); 11 return 1; 11 return 1; 12 } 12 } 13 } 13 } 14 read_unlock(); 14 rcu_read_unlock(); 15 return 0; 15 return 0; 16 } 16 } 1 int delete(long key) 1 int delete(long key) 2 { 2 { 3 struct el *p; 3 struct el *p; 4 4 5 write_lock(&listmutex); 5 spin_lock(&listmutex); 6 list_for_each_entry(p, head, lp) { 6 list_for_each_entry(p, head, lp) { 7 if (p->key == key) { 7 if (p->key == key) { 8 list_del(&p->list); 8 list_del_rcu(&p->list); 9 write_unlock(&listmutex); 9 spin_unlock(&listmutex); 10 synchronize_rcu(); 10 kfree(p); 11 kfree(p); 11 return 1; 12 return 1; 12 } 13 } 13 } 14 } 14 write_unlock(&listmutex); 15 spin_unlock(&listmutex); 15 return 0; 16 return 0; 16 } 17 } Either way, the differences are quite small. Read-side locking moves to rcu_read_lock() and rcu_read_unlock, update-side locking moves from a reader-writer lock to a simple spinlock, and a synchronize_rcu() precedes the kfree(). However, there is one potential catch: the read-side and update-side critical sections can now run concurrently. In many cases, this will not be a problem, but it is necessary to check carefully regardless. For example, if multiple independent list updates must be seen as a single atomic update, converting to RCU will require special care. Also, the presence of synchronize_rcu() means that the RCU version of delete() can now block. If this is a problem, there is a callback-based mechanism that never blocks, namely call_rcu() or kfree_rcu(), that can be used in place of synchronize_rcu(). 7. FULL LIST OF RCU APIs The RCU APIs are documented in docbook-format header comments in the Linux-kernel source code, but it helps to have a full list of the APIs, since there does not appear to be a way to categorize them in docbook. Here is the list, by category. RCU list traversal: list_entry_rcu list_first_entry_rcu list_next_rcu list_for_each_entry_rcu list_for_each_entry_continue_rcu hlist_first_rcu hlist_next_rcu hlist_pprev_rcu hlist_for_each_entry_rcu hlist_for_each_entry_rcu_bh hlist_for_each_entry_continue_rcu hlist_for_each_entry_continue_rcu_bh hlist_nulls_first_rcu hlist_nulls_for_each_entry_rcu hlist_bl_first_rcu hlist_bl_for_each_entry_rcu RCU pointer/list update: rcu_assign_pointer list_add_rcu list_add_tail_rcu list_del_rcu list_replace_rcu hlist_add_behind_rcu hlist_add_before_rcu hlist_add_head_rcu hlist_del_rcu hlist_del_init_rcu hlist_replace_rcu list_splice_init_rcu() hlist_nulls_del_init_rcu hlist_nulls_del_rcu hlist_nulls_add_head_rcu hlist_bl_add_head_rcu hlist_bl_del_init_rcu hlist_bl_del_rcu hlist_bl_set_first_rcu RCU: Critical sections Grace period Barrier rcu_read_lock synchronize_net rcu_barrier rcu_read_unlock synchronize_rcu rcu_dereference synchronize_rcu_expedited rcu_read_lock_held call_rcu rcu_dereference_check kfree_rcu rcu_dereference_protected bh: Critical sections Grace period Barrier rcu_read_lock_bh call_rcu_bh rcu_barrier_bh rcu_read_unlock_bh synchronize_rcu_bh rcu_dereference_bh synchronize_rcu_bh_expedited rcu_dereference_bh_check rcu_dereference_bh_protected rcu_read_lock_bh_held sched: Critical sections Grace period Barrier rcu_read_lock_sched synchronize_sched rcu_barrier_sched rcu_read_unlock_sched call_rcu_sched [preempt_disable] synchronize_sched_expedited [and friends] rcu_read_lock_sched_notrace rcu_read_unlock_sched_notrace rcu_dereference_sched rcu_dereference_sched_check rcu_dereference_sched_protected rcu_read_lock_sched_held SRCU: Critical sections Grace period Barrier srcu_read_lock synchronize_srcu srcu_barrier srcu_read_unlock call_srcu srcu_dereference synchronize_srcu_expedited srcu_dereference_check srcu_read_lock_held SRCU: Initialization/cleanup init_srcu_struct cleanup_srcu_struct All: lockdep-checked RCU-protected pointer access rcu_access_pointer rcu_dereference_raw RCU_LOCKDEP_WARN rcu_sleep_check RCU_NONIDLE See the comment headers in the source code (or the docbook generated from them) for more information. However, given that there are no fewer than four families of RCU APIs in the Linux kernel, how do you choose which one to use? The following list can be helpful: a. Will readers need to block? If so, you need SRCU. b. What about the -rt patchset? If readers would need to block in an non-rt kernel, you need SRCU. If readers would block in a -rt kernel, but not in a non-rt kernel, SRCU is not necessary. c. Do you need to treat NMI handlers, hardirq handlers, and code segments with preemption disabled (whether via preempt_disable(), local_irq_save(), local_bh_disable(), or some other mechanism) as if they were explicit RCU readers? If so, RCU-sched is the only choice that will work for you. d. Do you need RCU grace periods to complete even in the face of softirq monopolization of one or more of the CPUs? For example, is your code subject to network-based denial-of-service attacks? If so, you need RCU-bh. e. Is your workload too update-intensive for normal use of RCU, but inappropriate for other synchronization mechanisms? If so, consider SLAB_DESTROY_BY_RCU. But please be careful! f. Do you need read-side critical sections that are respected even though they are in the middle of the idle loop, during user-mode execution, or on an offlined CPU? If so, SRCU is the only choice that will work for you. g. Otherwise, use RCU. Of course, this all assumes that you have determined that RCU is in fact the right tool for your job. 8. ANSWERS TO QUICK QUIZZES Quick Quiz #1: Why is this argument naive? How could a deadlock occur when using this algorithm in a real-world Linux kernel? [Referring to the lock-based "toy" RCU algorithm.] Answer: Consider the following sequence of events: 1. CPU 0 acquires some unrelated lock, call it "problematic_lock", disabling irq via spin_lock_irqsave(). 2. CPU 1 enters synchronize_rcu(), write-acquiring rcu_gp_mutex. 3. CPU 0 enters rcu_read_lock(), but must wait because CPU 1 holds rcu_gp_mutex. 4. CPU 1 is interrupted, and the irq handler attempts to acquire problematic_lock. The system is now deadlocked. One way to avoid this deadlock is to use an approach like that of CONFIG_PREEMPT_RT, where all normal spinlocks become blocking locks, and all irq handlers execute in the context of special tasks. In this case, in step 4 above, the irq handler would block, allowing CPU 1 to release rcu_gp_mutex, avoiding the deadlock. Even in the absence of deadlock, this RCU implementation allows latency to "bleed" from readers to other readers through synchronize_rcu(). To see this, consider task A in an RCU read-side critical section (thus read-holding rcu_gp_mutex), task B blocked attempting to write-acquire rcu_gp_mutex, and task C blocked in rcu_read_lock() attempting to read_acquire rcu_gp_mutex. Task A's RCU read-side latency is holding up task C, albeit indirectly via task B. Realtime RCU implementations therefore use a counter-based approach where tasks in RCU read-side critical sections cannot be blocked by tasks executing synchronize_rcu(). Quick Quiz #2: Give an example where Classic RCU's read-side overhead is -negative-. Answer: Imagine a single-CPU system with a non-CONFIG_PREEMPT kernel where a routing table is used by process-context code, but can be updated by irq-context code (for example, by an "ICMP REDIRECT" packet). The usual way of handling this would be to have the process-context code disable interrupts while searching the routing table. Use of RCU allows such interrupt-disabling to be dispensed with. Thus, without RCU, you pay the cost of disabling interrupts, and with RCU you don't. One can argue that the overhead of RCU in this case is negative with respect to the single-CPU interrupt-disabling approach. Others might argue that the overhead of RCU is merely zero, and that replacing the positive overhead of the interrupt-disabling scheme with the zero-overhead RCU scheme does not constitute negative overhead. In real life, of course, things are more complex. But even the theoretical possibility of negative overhead for a synchronization primitive is a bit unexpected. ;-) Quick Quiz #3: If it is illegal to block in an RCU read-side critical section, what the heck do you do in PREEMPT_RT, where normal spinlocks can block??? Answer: Just as PREEMPT_RT permits preemption of spinlock critical sections, it permits preemption of RCU read-side critical sections. It also permits spinlocks blocking while in RCU read-side critical sections. Why the apparent inconsistency? Because it is it possible to use priority boosting to keep the RCU grace periods short if need be (for example, if running short of memory). In contrast, if blocking waiting for (say) network reception, there is no way to know what should be boosted. Especially given that the process we need to boost might well be a human being who just went out for a pizza or something. And although a computer-operated cattle prod might arouse serious interest, it might also provoke serious objections. Besides, how does the computer know what pizza parlor the human being went to??? ACKNOWLEDGEMENTS My thanks to the people who helped make this human-readable, including Jon Walpole, Josh Triplett, Serge Hallyn, Suzanne Wood, and Alan Stern. For more information, see http://www.rdrop.com/users/paulmck/RCU. */ Using RCU to Protect Read-Mostly Linked Lists One of the best applications of RCU is to protect read-mostly linked lists ("struct list_head" in list.h). One big advantage of this approach is that all of the required memory barriers are included for you in the list macros. This document describes several applications of RCU, with the best fits first. Example 1: Read-Side Action Taken Outside of Lock, No In-Place Updates The best applications are cases where, if reader-writer locking were used, the read-side lock would be dropped before taking any action based on the results of the search. The most celebrated example is the routing table. Because the routing table is tracking the state of equipment outside of the computer, it will at times contain stale data. Therefore, once the route has been computed, there is no need to hold the routing table static during transmission of the packet. After all, you can hold the routing table static all you want, but that won't keep the external Internet from changing, and it is the state of the external Internet that really matters. In addition, routing entries are typically added or deleted, rather than being modified in place. A straightforward example of this use of RCU may be found in the system-call auditing support. For example, a reader-writer locked implementation of audit_filter_task() might be as follows: static enum audit_state audit_filter_task(struct task_struct *tsk) { struct audit_entry *e; enum audit_state state; read_lock(&auditsc_lock); /* Note: audit_netlink_sem held by caller. */ list_for_each_entry(e, &audit_tsklist, list) { if (audit_filter_rules(tsk, &e->rule, NULL, &state)) { read_unlock(&auditsc_lock); return state; } } read_unlock(&auditsc_lock); return AUDIT_BUILD_CONTEXT; } Here the list is searched under the lock, but the lock is dropped before the corresponding value is returned. By the time that this value is acted on, the list may well have been modified. This makes sense, since if you are turning auditing off, it is OK to audit a few extra system calls. This means that RCU can be easily applied to the read side, as follows: static enum audit_state audit_filter_task(struct task_struct *tsk) { struct audit_entry *e; enum audit_state state; rcu_read_lock(); /* Note: audit_netlink_sem held by caller. */ list_for_each_entry_rcu(e, &audit_tsklist, list) { if (audit_filter_rules(tsk, &e->rule, NULL, &state)) { rcu_read_unlock(); return state; } } rcu_read_unlock(); return AUDIT_BUILD_CONTEXT; } The read_lock() and read_unlock() calls have become rcu_read_lock() and rcu_read_unlock(), respectively, and the list_for_each_entry() has become list_for_each_entry_rcu(). The _rcu() list-traversal primitives insert the read-side memory barriers that are required on DEC Alpha CPUs. The changes to the update side are also straightforward. A reader-writer lock might be used as follows for deletion and insertion: static inline int audit_del_rule(struct audit_rule *rule, struct list_head *list) { struct audit_entry *e; write_lock(&auditsc_lock); list_for_each_entry(e, list, list) { if (!audit_compare_rule(rule, &e->rule)) { list_del(&e->list); write_unlock(&auditsc_lock); return 0; } } write_unlock(&auditsc_lock); return -EFAULT; /* No matching rule */ } static inline int audit_add_rule(struct audit_entry *entry, struct list_head *list) { write_lock(&auditsc_lock); if (entry->rule.flags & AUDIT_PREPEND) { entry->rule.flags &= ~AUDIT_PREPEND; list_add(&entry->list, list); } else { list_add_tail(&entry->list, list); } write_unlock(&auditsc_lock); return 0; } Following are the RCU equivalents for these two functions: static inline int audit_del_rule(struct audit_rule *rule, struct list_head *list) { struct audit_entry *e; /* Do not use the _rcu iterator here, since this is the only * deletion routine. */ list_for_each_entry(e, list, list) { if (!audit_compare_rule(rule, &e->rule)) { list_del_rcu(&e->list); call_rcu(&e->rcu, audit_free_rule); return 0; } } return -EFAULT; /* No matching rule */ } static inline int audit_add_rule(struct audit_entry *entry, struct list_head *list) { if (entry->rule.flags & AUDIT_PREPEND) { entry->rule.flags &= ~AUDIT_PREPEND; list_add_rcu(&entry->list, list); } else { list_add_tail_rcu(&entry->list, list); } return 0; } Normally, the write_lock() and write_unlock() would be replaced by a spin_lock() and a spin_unlock(), but in this case, all callers hold audit_netlink_sem, so no additional locking is required. The auditsc_lock can therefore be eliminated, since use of RCU eliminates the need for writers to exclude readers. Normally, the write_lock() calls would be converted into spin_lock() calls. The list_del(), list_add(), and list_add_tail() primitives have been replaced by list_del_rcu(), list_add_rcu(), and list_add_tail_rcu(). The _rcu() list-manipulation primitives add memory barriers that are needed on weakly ordered CPUs (most of them!). The list_del_rcu() primitive omits the pointer poisoning debug-assist code that would otherwise cause concurrent readers to fail spectacularly. So, when readers can tolerate stale data and when entries are either added or deleted, without in-place modification, it is very easy to use RCU! Example 2: Handling In-Place Updates The system-call auditing code does not update auditing rules in place. However, if it did, reader-writer-locked code to do so might look as follows (presumably, the field_count is only permitted to decrease, otherwise, the added fields would need to be filled in): static inline int audit_upd_rule(struct audit_rule *rule, struct list_head *list, __u32 newaction, __u32 newfield_count) { struct audit_entry *e; struct audit_newentry *ne; write_lock(&auditsc_lock); /* Note: audit_netlink_sem held by caller. */ list_for_each_entry(e, list, list) { if (!audit_compare_rule(rule, &e->rule)) { e->rule.action = newaction; e->rule.file_count = newfield_count; write_unlock(&auditsc_lock); return 0; } } write_unlock(&auditsc_lock); return -EFAULT; /* No matching rule */ } The RCU version creates a copy, updates the copy, then replaces the old entry with the newly updated entry. This sequence of actions, allowing concurrent reads while doing a copy to perform an update, is what gives RCU ("read-copy update") its name. The RCU code is as follows: static inline int audit_upd_rule(struct audit_rule *rule, struct list_head *list, __u32 newaction, __u32 newfield_count) { struct audit_entry *e; struct audit_newentry *ne; list_for_each_entry(e, list, list) { if (!audit_compare_rule(rule, &e->rule)) { ne = kmalloc(sizeof(*entry), GFP_ATOMIC); if (ne == NULL) return -ENOMEM; audit_copy_rule(&ne->rule, &e->rule); ne->rule.action = newaction; ne->rule.file_count = newfield_count; list_replace_rcu(&e->list, &ne->list); call_rcu(&e->rcu, audit_free_rule); return 0; } } return -EFAULT; /* No matching rule */ } Again, this assumes that the caller holds audit_netlink_sem. Normally, the reader-writer lock would become a spinlock in this sort of code. Example 3: Eliminating Stale Data The auditing examples above tolerate stale data, as do most algorithms that are tracking external state. Because there is a delay from the time the external state changes before Linux becomes aware of the change, additional RCU-induced staleness is normally not a problem. However, there are many examples where stale data cannot be tolerated. One example in the Linux kernel is the System V IPC (see the ipc_lock() function in ipc/util.c). This code checks a "deleted" flag under a per-entry spinlock, and, if the "deleted" flag is set, pretends that the entry does not exist. For this to be helpful, the search function must return holding the per-entry spinlock, as ipc_lock() does in fact do. Quick Quiz: Why does the search function need to return holding the per-entry lock for this deleted-flag technique to be helpful? If the system-call audit module were to ever need to reject stale data, one way to accomplish this would be to add a "deleted" flag and a "lock" spinlock to the audit_entry structure, and modify audit_filter_task() as follows: static enum audit_state audit_filter_task(struct task_struct *tsk) { struct audit_entry *e; enum audit_state state; rcu_read_lock(); list_for_each_entry_rcu(e, &audit_tsklist, list) { if (audit_filter_rules(tsk, &e->rule, NULL, &state)) { spin_lock(&e->lock); if (e->deleted) { spin_unlock(&e->lock); rcu_read_unlock(); return AUDIT_BUILD_CONTEXT; } rcu_read_unlock(); return state; } } rcu_read_unlock(); return AUDIT_BUILD_CONTEXT; } Note that this example assumes that entries are only added and deleted. Additional mechanism is required to deal correctly with the update-in-place performed by audit_upd_rule(). For one thing, audit_upd_rule() would need additional memory barriers to ensure that the list_add_rcu() was really executed before the list_del_rcu(). The audit_del_rule() function would need to set the "deleted" flag under the spinlock as follows: static inline int audit_del_rule(struct audit_rule *rule, struct list_head *list) { struct audit_entry *e; /* Do not need to use the _rcu iterator here, since this * is the only deletion routine. */ list_for_each_entry(e, list, list) { if (!audit_compare_rule(rule, &e->rule)) { spin_lock(&e->lock); list_del_rcu(&e->list); e->deleted = 1; spin_unlock(&e->lock); call_rcu(&e->rcu, audit_free_rule); return 0; } } return -EFAULT; /* No matching rule */ } Summary Read-mostly list-based data structures that can tolerate stale data are the most amenable to use of RCU. The simplest case is where entries are either added or deleted from the data structure (or atomically modified in place), but non-atomic in-place modifications can be handled by making a copy, updating the copy, then replacing the original with the copy. If stale data cannot be tolerated, then a "deleted" flag may be used in conjunction with a per-entry spinlock in order to allow the search function to reject newly deleted data. Answer to Quick Quiz Why does the search function need to return holding the per-entry lock for this deleted-flag technique to be helpful? If the search function drops the per-entry lock before returning, then the caller will be processing stale data in any case. If it is really OK to be processing stale data, then you don't need a "deleted" flag. If processing stale data really is a problem, then you need to hold the per-entry lock across all of the code that uses the value that was returned.
在使用RCU時，對共享資源的訪問在大部分時間應該是只讀的，寫訪問應該相對較少，因為寫訪問多了必然相對於其他鎖機制而已更占系統資源，影響效率。其次是讀者在持有rcu_read_lock（RCU讀鎖定函數）的時候，不能發生進程上下文切換，否則，因為寫者需要等待讀者完成方可進行，則此時寫者進程也會一直被阻塞，影響系統的正常運行。再次寫者執行完畢后需要調用回調函數，此時發生上下文切換，當前進程進入睡眠，則系統將一直不能調用回調函數，更槽糕的是，此時其它進程若再去執行共享的臨界區，必然造成一定的錯誤。最后一點是受RCU機制保護的資源必須是通過指針訪問。因為從RCU機制上看，幾乎所有操作都是針對指針數據的;
同步函數最為重要，即synchronize_rcu()。讀者函數的實質其實很簡單：禁止搶占，也就是說在RCU期間不允許發生進程上下文切換，原因上述已提及，即是寫者需要等待讀者完成方可進行，則此時寫者進程也會一直被阻塞，影響系統的正常運行等，故而不允許在RCU期間發生進程上下文切換
關於寫者函數，主要就是call_rcu和call_rcu_bh兩個函數。其中call_rcu能實現的功能是它不會使寫者阻塞，因而它可在中斷上下文及軟中斷使用，該函數將函數func掛接到RCU的回調函數鏈表上，然后立即返回，讀者函數中提及的synchronize_rcu()函數在實現時也調用了該函數。而call_rcu_bh函數實現的功能幾乎與call_rcu完全相同，唯一的差別是它將軟中斷的完成當作經歷一個quiescent state（靜默狀態，本節一開始有提及這個概念）， 因此若寫者使用了該函數，那么讀者需對應的使用rcu_read_lock_bh() 和rcu_read_unlock_bh()。
· 使用rcu_read_lock_bh() 和rcu_read_unlock_bh()函數的原因是由於call_rcu_bh函數不會使寫者阻塞，可在中斷上下文及軟中斷使用。這表明此時系統中的中斷和軟中斷並沒有被關閉。那么寫者在調用call_rcu_bh函數訪問臨界區時，RCU機制下的讀者也能訪問臨界區。此時對於讀者而言，它若是需要讀取臨界區的內容，它必須把軟中斷關閉，以免讀者在當前的進程上下文過程中被軟中斷打斷（上述內容提過軟中斷可以打斷當前的進程上下文）。而rcu_read_lock_bh() 和rcu_read_unlock_bh()函數的實質是調用local_bh_disable()和local_bh_enable()函數，顯然這是實現了禁止軟中斷和使能軟中斷的功能。
另外在Linux源碼中關於call_rcu_bh函數的注釋中還明確說明了如果當前的進程是在中斷上下文中，則需要執行rcu_read_lock()和rcu_read_unlock()，結合這兩個函數的實現實質表明它實際上禁止或使能內核的搶占調度，原因不言而喻，避免當前進程在執行讀寫過程中被其它進程搶占。同時內核注釋還表明call_rcu_bh這個接口函數的使用條件是在大部分的讀臨界區操作發生在軟中斷上下文中，原因還是需從它實現的功能出發，相信很容易理解，主要是要從執行效率方面考慮。
static inline void rcu_read_lock_bh(void); static inline void rcu_read_unlock_bh(void);
這個變種只在修改是通過 call_rcu_bh進行的情況下使用，因為 call_rcu_bh將把 softirq 的執行完畢也認為是一個 quiescent state，因此如果修改是通過 call_rcu_bh 進行的，在進程上下文的讀端臨界區必須使用這一變種
每一個 CPU 維護兩個數據結構 rcu_sched_data，rcu_bh_data，它們用於保存回調函數。函數call_rcu和函數call_rcu_bh用於注冊回調函數，前者把回調函數注冊到rcu_sched_data，而后者則把回調函數注冊到rcu_bh_data，在每一個數據結構上，回調函數被組成一個鏈表，先注冊的排在前頭，后注冊的排在末尾；時鍾中斷處理函數（update_process_times）調用函數rcu_check_callbacks
函數rcu_check_callbacks首先檢查該CPU是否經歷了一個quiescent state，如果(或)：
- 當前進程運行在用戶態；
- 當前進程為idle且當前不處在運行softirq狀態，也不處在運行IRQ處理函數的狀態；
該CPU已經經歷了一個quiescent state，因此通過調用函數rcu_sched_qs和rcu_bh_qs標記該CPU的數據結構rcu_sched_data和rcu_bh_data的標記字段passed_quiesc，以記錄該CPU已經經歷一個quiescent state。
否則，如果當前不處在運行softirq狀態，那么，只標記該CPU的數據結構rcu_bh_data的標記字段passed_quiesc，以記錄該CPU已經經歷一個quiescent state。注意，該標記只對rcu_bh_data有效。
然后，函數rcu_check_callbacks將調用開啟RCU_SOFTIRQ。
synchronize_rcu()在RCU中是一個最核心的函數,它用來等待之前的讀者全部退出。
在完整的寬限期結束后，即在所有當前正在執行的RCU讀取端臨界區完成之后，控制權會在一段時間后返回給調用者。
