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diff --git a/Documentation/RCU/whatisRCU.rst b/Documentation/RCU/whatisRCU.rst new file mode 100644 index 0000000000..e488c8e557 --- /dev/null +++ b/Documentation/RCU/whatisRCU.rst @@ -0,0 +1,1294 @@ +.. _whatisrcu_doc: + +What is RCU? -- "Read, Copy, Update" +====================================== + +Please note that the "What is RCU?" LWN series is an excellent place +to start learning about RCU: + +| 1. What is RCU, Fundamentally? https://lwn.net/Articles/262464/ +| 2. What is RCU? Part 2: Usage https://lwn.net/Articles/263130/ +| 3. RCU part 3: the RCU API https://lwn.net/Articles/264090/ +| 4. The RCU API, 2010 Edition https://lwn.net/Articles/418853/ +| 2010 Big API Table https://lwn.net/Articles/419086/ +| 5. The RCU API, 2014 Edition https://lwn.net/Articles/609904/ +| 2014 Big API Table https://lwn.net/Articles/609973/ +| 6. The RCU API, 2019 Edition https://lwn.net/Articles/777036/ +| 2019 Big API Table https://lwn.net/Articles/777165/ + +For those preferring video: + +| 1. Unraveling RCU Mysteries: Fundamentals https://www.linuxfoundation.org/webinars/unraveling-rcu-usage-mysteries +| 2. Unraveling RCU Mysteries: Additional Use Cases https://www.linuxfoundation.org/webinars/unraveling-rcu-usage-mysteries-additional-use-cases + + +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, making effective use +of it requires you to think differently about your code. Another part +of the problem is the mistaken assumption that there is "one true way" to +describe and to use RCU. Instead, the experience has been that different +people must take different paths to arrive at an understanding of RCU, +depending on their experiences and use cases. This document provides +several different paths, as follows: + +:ref:`1. RCU OVERVIEW <1_whatisRCU>` + +:ref:`2. WHAT IS RCU'S CORE API? <2_whatisRCU>` + +:ref:`3. WHAT ARE SOME EXAMPLE USES OF CORE RCU API? <3_whatisRCU>` + +:ref:`4. WHAT IF MY UPDATING THREAD CANNOT BLOCK? <4_whatisRCU>` + +:ref:`5. WHAT ARE SOME SIMPLE IMPLEMENTATIONS OF RCU? <5_whatisRCU>` + +:ref:`6. ANALOGY WITH READER-WRITER LOCKING <6_whatisRCU>` + +:ref:`7. ANALOGY WITH REFERENCE COUNTING <7_whatisRCU>` + +:ref:`8. FULL LIST OF RCU APIs <8_whatisRCU>` + +:ref:`9. ANSWERS TO QUICK QUIZZES <9_whatisRCU>` + +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_whatisRCU: + +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_whatisRCU: + +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); + + This temporal primitive is 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); + + This temporal primitives is 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); + + This temporal primitive 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 an asynchronous 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.rst for some approaches to limiting the update rate. + +rcu_assign_pointer() +^^^^^^^^^^^^^^^^^^^^ + void 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 spatial macro 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 is a spatial (as + opposed to temporal) macro. It does not evaluate to an rvalue, + but it does execute any memory-barrier instructions required + for a given CPU architecture. Its ordering properties are that + of a store-release operation. + + 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 the spatial rcu_dereference() macro to fetch + an RCU-protected pointer, which returns a value that may + then be safely dereferenced. Note that rcu_dereference() + 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 a + volatile load. + + 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 [1]_. + 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() [2]_. + +.. [1] The variant rcu_dereference_protected() can be used outside + of an RCU read-side critical section as long as the usage is + protected by locks acquired by the update-side code. This variant + avoids the lockdep warning that would happen when using (for + example) rcu_dereference() without rcu_read_lock() protection. + Using rcu_dereference_protected() also has the advantage + of permitting compiler optimizations that rcu_dereference() + must prohibit. The rcu_dereference_protected() variant takes + a lockdep expression to indicate which locks must be acquired + by the caller. If the indicated protection is not provided, + a lockdep splat is emitted. See Design/Requirements/Requirements.rst + and the API's code comments for more details and example usage. + +.. [2] If the list_for_each_entry_rcu() instance might be used by + update-side code as well as by RCU readers, then an additional + lockdep expression can be added to its list of arguments. + For example, given an additional "lock_is_held(&mylock)" argument, + the RCU lockdep code would complain only if this instance was + invoked outside of an RCU read-side critical section and without + the protection of mylock. + +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 temporal 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. +The rcu_assign_pointer() and rcu_dereference() invocations communicate +spatial changes via stores to and loads from the RCU-protected pointer in +question. + +There are at least three flavors of RCU usage in the Linux kernel. The diagram +above shows the most common one. On the updater side, the rcu_assign_pointer(), +synchronize_rcu() and call_rcu() primitives used are the same for all three +flavors. However for protection (on the reader side), the primitives used vary +depending on the flavor: + +a. rcu_read_lock() / rcu_read_unlock() + rcu_dereference() + +b. rcu_read_lock_bh() / rcu_read_unlock_bh() + local_bh_disable() / local_bh_enable() + rcu_dereference_bh() + +c. rcu_read_lock_sched() / rcu_read_unlock_sched() + preempt_disable() / preempt_enable() + local_irq_save() / local_irq_restore() + hardirq enter / hardirq exit + NMI enter / NMI exit + rcu_dereference_sched() + +These three flavors 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. The SRCU, RCU-Tasks, +RCU-Tasks-Rude, and RCU-Tasks-Trace have similar relationships among +their assorted primitives. + +.. _3_whatisRCU: + +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.rst, arrayRCU.rst, and NMI-RCU.rst. +:: + + 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: + +- Use rcu_read_lock() and rcu_read_unlock() to guard RCU + read-side critical sections. + +- Within an RCU read-side critical section, use rcu_dereference() + to dereference RCU-protected pointers. + +- Use some solid design (such as locks or semaphores) to + keep concurrent updates from interfering with each other. + +- 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. + +- 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.rst for additional rules to follow when using RCU. +And again, more-typical uses of RCU may be found in listRCU.rst, +arrayRCU.rst, and NMI-RCU.rst. + +.. _4_whatisRCU: + +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, rcu_callback_t func); + +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(): + +- 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); + +If the occasional sleep is permitted, the single-argument form may +be used, omitting the rcu_head structure from struct foo. + + kfree_rcu_mightsleep(old_fp); + +This variant almost never blocks, but might do so by invoking +synchronize_rcu() in response to memory-allocation failure. + +Again, see checklist.rst for additional rules governing the use of RCU. + +.. _5_whatisRCU: + +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/rcu/update.c for a +production-quality implementation, and see: + + https://docs.google.com/document/d/1X0lThx8OK0ZgLMqVoXiR4ZrGURHrXK6NyLRbeXe3Xac/edit + +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. It also assumes recursive +reader-writer locks: If you try this with non-recursive locks, and +you allow nested rcu_read_lock() calls, you can deadlock. + +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); + smp_mb__after_spinlock(); + write_unlock(&rcu_gp_mutex); + } + +[You can ignore rcu_assign_pointer() and rcu_dereference() without missing +much. But here are simplified versions anyway. And whatever you do, +don't forget about them when submitting patches making use of RCU!]:: + + #define rcu_assign_pointer(p, v) \ + ({ \ + smp_store_release(&(p), (v)); \ + }) + + #define rcu_dereference(p) \ + ({ \ + typeof(p) _________p1 = READ_ONCE(p); \ + (_________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 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. The smp_mb__after_spinlock() +promotes synchronize_rcu() to a full memory barrier in compliance with +the "Memory-Barrier Guarantees" listed in: + + Design/Requirements/Requirements.rst + +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. + +.. _quiz_1: + +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? + +:ref:`Answers to Quick Quiz <9_whatisRCU>` + +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_PREEMPTION +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. + +.. _quiz_2: + +Quick Quiz #2: + Give an example where Classic RCU's read-side + overhead is **negative**. + +:ref:`Answers to Quick Quiz <9_whatisRCU>` + +.. _quiz_3: + +Quick Quiz #3: + If it is illegal to block in an RCU read-side + critical section, what the heck do you do in + CONFIG_PREEMPT_RT, where normal spinlocks can block??? + +:ref:`Answers to Quick Quiz <9_whatisRCU>` + +.. _6_whatisRCU: + +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. +:: + + @@ -5,5 +5,5 @@ struct el { + int data; + /* Other data fields */ + }; + -rwlock_t listmutex; + +spinlock_t listmutex; + struct el head; + + @@ -13,15 +14,15 @@ + struct list_head *lp; + struct el *p; + + - read_lock(&listmutex); + - 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(&listmutex); + + rcu_read_unlock(); + return 1; + } + } + - read_unlock(&listmutex); + + 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 rwlock_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(&listmutex); 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(&listmutex); 10 rcu_read_unlock(); + 11 return 1; 11 return 1; + 12 } 12 } + 13 } 13 } + 14 read_unlock(&listmutex); 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_whatisRCU: + +7. ANALOGY WITH REFERENCE COUNTING +----------------------------------- + +The reader-writer analogy (illustrated by the previous section) is not +always the best way to think about using RCU. Another helpful analogy +considers RCU an effective reference count on everything which is +protected by RCU. + +A reference count typically does not prevent the referenced object's +values from changing, but does prevent changes to type -- particularly the +gross change of type that happens when that object's memory is freed and +re-allocated for some other purpose. Once a type-safe reference to the +object is obtained, some other mechanism is needed to ensure consistent +access to the data in the object. This could involve taking a spinlock, +but with RCU the typical approach is to perform reads with SMP-aware +operations such as smp_load_acquire(), to perform updates with atomic +read-modify-write operations, and to provide the necessary ordering. +RCU provides a number of support functions that embed the required +operations and ordering, such as the list_for_each_entry_rcu() macro +used in the previous section. + +A more focused view of the reference counting behavior is that, +between rcu_read_lock() and rcu_read_unlock(), any reference taken with +rcu_dereference() on a pointer marked as ``__rcu`` can be treated as +though a reference-count on that object has been temporarily increased. +This prevents the object from changing type. Exactly what this means +will depend on normal expectations of objects of that type, but it +typically includes that spinlocks can still be safely locked, normal +reference counters can be safely manipulated, and ``__rcu`` pointers +can be safely dereferenced. + +Some operations that one might expect to see on an object for +which an RCU reference is held include: + + - Copying out data that is guaranteed to be stable by the object's type. + - Using kref_get_unless_zero() or similar to get a longer-term + reference. This may fail of course. + - Acquiring a spinlock in the object, and checking if the object still + is the expected object and if so, manipulating it freely. + +The understanding that RCU provides a reference that only prevents a +change of type is particularly visible with objects allocated from a +slab cache marked ``SLAB_TYPESAFE_BY_RCU``. RCU operations may yield a +reference to an object from such a cache that has been concurrently freed +and the memory reallocated to a completely different object, though of +the same type. In this case RCU doesn't even protect the identity of the +object from changing, only its type. So the object found may not be the +one expected, but it will be one where it is safe to take a reference +(and then potentially acquiring a spinlock), allowing subsequent code +to check whether the identity matches expectations. It is tempting +to simply acquire the spinlock without first taking the reference, but +unfortunately any spinlock in a ``SLAB_TYPESAFE_BY_RCU`` object must be +initialized after each and every call to kmem_cache_alloc(), which renders +reference-free spinlock acquisition completely unsafe. Therefore, when +using ``SLAB_TYPESAFE_BY_RCU``, make proper use of a reference counter. +(Those willing to use a kmem_cache constructor may also use locking, +including cache-friendly sequence locking.) + +With traditional reference counting -- such as that implemented by the +kref library in Linux -- there is typically code that runs when the last +reference to an object is dropped. With kref, this is the function +passed to kref_put(). When RCU is being used, such finalization code +must not be run until all ``__rcu`` pointers referencing the object have +been updated, and then a grace period has passed. Every remaining +globally visible pointer to the object must be considered to be a +potential counted reference, and the finalization code is typically run +using call_rcu() only after all those pointers have been changed. + +To see how to choose between these two analogies -- of RCU as a +reader-writer lock and RCU as a reference counting system -- it is useful +to reflect on the scale of the thing being protected. The reader-writer +lock analogy looks at larger multi-part objects such as a linked list +and shows how RCU can facilitate concurrency while elements are added +to, and removed from, the list. The reference-count analogy looks at +the individual objects and looks at how they can be accessed safely +within whatever whole they are a part of. + +.. _8_whatisRCU: + +8. 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_entry_lockless + list_first_entry_rcu + list_next_rcu + list_for_each_entry_rcu + list_for_each_entry_continue_rcu + list_for_each_entry_from_rcu + list_first_or_null_rcu + list_next_or_null_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_from_rcu + 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_add_tail_rcu + hlist_del_rcu + hlist_del_init_rcu + hlist_replace_rcu + list_splice_init_rcu + list_splice_tail_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 rcu_barrier + rcu_read_unlock_bh synchronize_rcu + [local_bh_disable] synchronize_rcu_expedited + [and friends] + rcu_dereference_bh + rcu_dereference_bh_check + rcu_dereference_bh_protected + rcu_read_lock_bh_held + +sched:: + + Critical sections Grace period Barrier + + rcu_read_lock_sched call_rcu rcu_barrier + rcu_read_unlock_sched synchronize_rcu + [preempt_disable] synchronize_rcu_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 + + +RCU-Tasks:: + + Critical sections Grace period Barrier + + N/A call_rcu_tasks rcu_barrier_tasks + synchronize_rcu_tasks + + +RCU-Tasks-Rude:: + + Critical sections Grace period Barrier + + N/A call_rcu_tasks_rude rcu_barrier_tasks_rude + synchronize_rcu_tasks_rude + + +RCU-Tasks-Trace:: + + Critical sections Grace period Barrier + + rcu_read_lock_trace call_rcu_tasks_trace rcu_barrier_tasks_trace + rcu_read_unlock_trace synchronize_rcu_tasks_trace + + +SRCU:: + + Critical sections Grace period Barrier + + srcu_read_lock call_srcu srcu_barrier + srcu_read_unlock synchronize_srcu + srcu_dereference synchronize_srcu_expedited + srcu_dereference_check + srcu_read_lock_held + +SRCU: Initialization/cleanup:: + + DEFINE_SRCU + DEFINE_STATIC_SRCU + init_srcu_struct + cleanup_srcu_struct + +All: lockdep-checked RCU utility APIs:: + + RCU_LOCKDEP_WARN + rcu_sleep_check + +All: Unchecked RCU-protected pointer access:: + + rcu_dereference_raw + +All: Unchecked RCU-protected pointer access with dereferencing prohibited:: + + rcu_access_pointer + +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. Will readers need to block and are you doing tracing, for + example, ftrace or BPF? If so, you need RCU-tasks, + RCU-tasks-rude, and/or RCU-tasks-trace. + +c. What about the -rt patchset? If readers would need to block in + an non-rt kernel, you need SRCU. If readers would block when + acquiring spinlocks in a -rt kernel, but not in a non-rt kernel, + SRCU is not necessary. (The -rt patchset turns spinlocks into + sleeplocks, hence this distinction.) + +d. 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 readers are the only choice that will work + for you, but since about v4.20 you use can use the vanilla RCU + update primitives. + +e. 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 should disable softirq across your readers, for + example, by using rcu_read_lock_bh(). Since about v4.20 you + use can use the vanilla RCU update primitives. + +f. Is your workload too update-intensive for normal use of + RCU, but inappropriate for other synchronization mechanisms? + If so, consider SLAB_TYPESAFE_BY_RCU (which was originally + named SLAB_DESTROY_BY_RCU). But please be careful! + +g. Do you need read-side critical sections that are respected even + on CPUs that are deep in the idle loop, during entry to or exit + from user-mode execution, or on an offlined CPU? If so, SRCU + and RCU Tasks Trace are the only choices that will work for you, + with SRCU being strongly preferred in almost all cases. + +h. Otherwise, use RCU. + +Of course, this all assumes that you have determined that RCU is in fact +the right tool for your job. + +.. _9_whatisRCU: + +9. 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(). + +:ref:`Back to Quick Quiz #1 <quiz_1>` + +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_PREEMPTION + 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. ;-) + +:ref:`Back to Quick Quiz #2 <quiz_2>` + +Quick Quiz #3: + If it is illegal to block in an RCU read-side + critical section, what the heck do you do in + CONFIG_PREEMPT_RT, where normal spinlocks can block??? + +Answer: + Just as CONFIG_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 + 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??? + +:ref:`Back to Quick Quiz #3 <quiz_3>` + +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. |