Review Checklist for RCU Patches — The Linux Kernel 7.0.10-gnu-parabola1-1 documentation

# Review Checklist for RCU Patches

This document contains a checklist for producing and reviewing patches that make use of RCU. Violating any of the rules listed below will result in the same sorts of problems that leaving out a locking primitive would cause. This list is based on experiences reviewing such patches over a rather long period of time, but improvements are always welcome!

0. Is RCU being applied to a read-mostly situation? If the data structure is updated more than about 10% of the time, then you should strongly consider some other approach, unless detailed performance measurements show that RCU is nonetheless the right tool for the job. Yes, RCU does reduce read-side overhead by increasing write-side overhead, which is exactly why normal uses of RCU will do much more reading than updating.

   Another exception is where performance is not an issue, and RCU provides a simpler implementation. An example of this situation is the dynamic NMI code in the Linux 2.6 kernel, at least on architectures where NMIs are rare.

   Yet another exception is where the low real-time latency of RCU’s read-side primitives is critically important.

   One final exception is where RCU readers are used to prevent the ABA problem (https://en.wikipedia.org/wiki/ABA_problem) for lockless updates. This does result in the mildly counter-intuitive situation where `rcu_read_lock()` and `rcu_read_unlock()` are used to protect updates, however, this approach can provide the same simplifications to certain types of lockless algorithms that garbage collectors do.

1. Does the update code have proper mutual exclusion?

   RCU does allow readers to run (almost) naked, but writers must still use some sort of mutual exclusion, such as:

   1. locking,

   2. atomic operations, or

   3. restricting updates to a single task.

   If you choose #b, be prepared to describe how you have handled memory barriers on weakly ordered machines (pretty much all of them -- even x86 allows later loads to be reordered to precede earlier stores), and be prepared to explain why this added complexity is worthwhile. If you choose #c, be prepared to explain how this single task does not become a major bottleneck on large systems (for example, if the task is updating information relating to itself that other tasks can read, there by definition can be no bottleneck). Note that the definition of “large” has changed significantly: Eight CPUs was “large” in the year 2000, but a hundred CPUs was unremarkable in 2017.

2. Do the RCU read-side critical sections make proper use of `rcu_read_lock()` and friends? These primitives are needed to prevent grace periods from ending prematurely, which could result in data being unceremoniously freed out from under your read-side code, which can greatly increase the actuarial risk of your kernel.

   As a rough rule of thumb, any dereference of an RCU-protected pointer must be covered by `rcu_read_lock()`, `rcu_read_lock_bh()`, `rcu_read_lock_sched()`, or by the appropriate update-side lock. Explicit disabling of preemption (`preempt_disable()`, for example) can serve as `rcu_read_lock_sched()`, but is less readable and prevents lockdep from detecting locking issues. Acquiring a raw spinlock also enters an RCU read-side critical section.

   The guard(rcu)() and scoped_guard(rcu) primitives designate the remainder of the current scope or the next statement, respectively, as the RCU read-side critical section. Use of these guards can be less error-prone than `rcu_read_lock()`, `rcu_read_unlock()`, and friends.

   Please note that you cannot rely on code known to be built only in non-preemptible kernels. Such code can and will break, especially in kernels built with CONFIG_PREEMPT_COUNT=y.

   Letting RCU-protected pointers “leak” out of an RCU read-side critical section is every bit as bad as letting them leak out from under a lock. Unless, of course, you have arranged some other means of protection, such as a lock or a reference count before letting them out of the RCU read-side critical section.

3. Does the update code tolerate concurrent accesses?

   The whole point of RCU is to permit readers to run without any locks or atomic operations. This means that readers will be running while updates are in progress. There are a number of ways to handle this concurrency, depending on the situation:

   1. Use the RCU variants of the list and hlist update primitives to add, remove, and replace elements on an RCU-protected list. Alternatively, use the other RCU-protected data structures that have been added to the Linux kernel.

      This is almost always the best approach.

   2. Proceed as in (a) above, but also maintain per-element locks (that are acquired by both readers and writers) that guard per-element state. Fields that the readers refrain from accessing can be guarded by some other lock acquired only by updaters, if desired.

      This also works quite well.

   3. Make updates appear atomic to readers. For example, pointer updates to properly aligned fields will appear atomic, as will individual atomic primitives. Sequences of operations performed under a lock will not appear to be atomic to RCU readers, nor will sequences of multiple atomic primitives. One alternative is to move multiple individual fields to a separate structure, thus solving the multiple-field problem by imposing an additional level of indirection.

      This can work, but is starting to get a bit tricky.

   4. Carefully order the updates and the reads so that readers see valid data at all phases of the update. This is often more difficult than it sounds, especially given modern CPUs’ tendency to reorder memory references. One must usually liberally sprinkle memory-ordering operations through the code, making it difficult to understand and to test. Where it works, it is better to use things like `smp_store_release()` and `smp_load_acquire()`, but in some cases the `smp_mb()` full memory barrier is required.

      As noted earlier, it is usually better to group the changing data into a separate structure, so that the change may be made to appear atomic by updating a pointer to reference a new structure containing updated values.

4. Weakly ordered CPUs pose special challenges. Almost all CPUs are weakly ordered -- even x86 CPUs allow later loads to be reordered to precede earlier stores. RCU code must take all of the following measures to prevent memory-corruption problems:

   1. Readers must maintain proper ordering of their memory accesses. The `rcu_dereference()` primitive ensures that the CPU picks up the pointer before it picks up the data that the pointer points to. This really is necessary on Alpha CPUs.

      The `rcu_dereference()` primitive is also an excellent documentation aid, letting the person reading the code know exactly which pointers are protected by RCU. Please note that compilers can also reorder code, and they are becoming increasingly aggressive about doing just that. The `rcu_dereference()` primitive therefore also prevents destructive compiler optimizations. However, with a bit of devious creativity, it is possible to mishandle the return value from `rcu_dereference()`. Please see PROPER CARE AND FEEDING OF RETURN VALUES FROM rcu_dereference() for more information.

      The `rcu_dereference()` primitive is used by the various “`_rcu()`” list-traversal primitives, such as the `list_for_each_entry_rcu()`. Note that it is perfectly legal (if redundant) for update-side code to use `rcu_dereference()` and the “`_rcu()`” list-traversal primitives. This is particularly useful in code that is common to readers and updaters. However, lockdep will complain if you access `rcu_dereference()` outside of an RCU read-side critical section. See RCU and lockdep checking to learn what to do about this.

      Of course, neither `rcu_dereference()` nor the “`_rcu()`” list-traversal primitives can substitute for a good concurrency design coordinating among multiple updaters.

   2. If the list macros are being used, the `list_add_tail_rcu()` and `list_add_rcu()` primitives must be used in order to prevent weakly ordered machines from misordering structure initialization and pointer planting. Similarly, if the hlist macros are being used, the `hlist_add_head_rcu()` primitive is required.

   3. If the list macros are being used, the `list_del_rcu()` primitive must be used to keep `list_del()`’s pointer poisoning from inflicting toxic effects on concurrent readers. Similarly, if the hlist macros are being used, the `hlist_del_rcu()` primitive is required.

      The `list_replace_rcu()` and `hlist_replace_rcu()` primitives may be used to replace an old structure with a new one in their respective types of RCU-protected lists.

   4. Rules similar to (4b) and (4c) apply to the “hlist_nulls” type of RCU-protected linked lists.

   5. Updates must ensure that initialization of a given structure happens before pointers to that structure are publicized. Use the `rcu_assign_pointer()` primitive when publicizing a pointer to a structure that can be traversed by an RCU read-side critical section.

5. If any of `call_rcu()`, `call_srcu()`, `call_rcu_tasks()`, or `call_rcu_tasks_trace()` is used, the callback function may be invoked from softirq context, and in any case with bottom halves disabled. In particular, this callback function cannot block. If you need the callback to block, run that code in a workqueue handler scheduled from the callback. The `queue_rcu_work()` function does this for you in the case of `call_rcu()`.

6. Since `synchronize_rcu()` can block, it cannot be called from any sort of irq context. The same rule applies for `synchronize_srcu()`, `synchronize_rcu_expedited()`, `synchronize_srcu_expedited()`, `synchronize_rcu_tasks()`, `synchronize_rcu_tasks_rude()`, and `synchronize_rcu_tasks_trace()`.

   The expedited forms of these primitives have the same semantics as the non-expedited forms, but expediting is more CPU intensive. Use of the expedited primitives should be restricted to rare configuration-change operations that would not normally be undertaken while a real-time workload is running. Note that IPI-sensitive real-time workloads can use the rcupdate.rcu_normal kernel boot parameter to completely disable expedited grace periods, though this might have performance implications.

   In particular, if you find yourself invoking one of the expedited primitives repeatedly in a loop, please do everyone a favor: Restructure your code so that it batches the updates, allowing a single non-expedited primitive to cover the entire batch. This will very likely be faster than the loop containing the expedited primitive, and will be much much easier on the rest of the system, especially to real-time workloads running on the rest of the system. Alternatively, instead use asynchronous primitives such as `call_rcu()`.

7. As of v4.20, a given kernel implements only one RCU flavor, which is RCU-sched for PREEMPTION=n and RCU-preempt for PREEMPTION=y. If the updater uses `call_rcu()` or `synchronize_rcu()`, then the corresponding readers may use: (1) `rcu_read_lock()` and `rcu_read_unlock()`, (2) any pair of primitives that disables and re-enables softirq, for example, `rcu_read_lock_bh()` and `rcu_read_unlock_bh()`, or (3) any pair of primitives that disables and re-enables preemption, for example, `rcu_read_lock_sched()` and `rcu_read_unlock_sched()`. If the updater uses `synchronize_srcu()` or `call_srcu()`, then the corresponding readers must use `srcu_read_lock()` and `srcu_read_unlock()`, and with the same srcu_struct. The rules for the expedited RCU grace-period-wait primitives are the same as for their non-expedited counterparts.

   Similarly, it is necessary to correctly use the RCU Tasks flavors:

   1. If the updater uses `synchronize_rcu_tasks()` or `call_rcu_tasks()`, then the readers must refrain from executing voluntary context switches, that is, from blocking.

   2. If the updater uses `call_rcu_tasks_trace()` or `synchronize_rcu_tasks_trace()`, then the corresponding readers must use `rcu_read_lock_trace()` and `rcu_read_unlock_trace()`.

   3. If an updater uses `synchronize_rcu_tasks_rude()`, then the corresponding readers must use anything that disables preemption, for example, `preempt_disable()` and `preempt_enable()`.

   Mixing things up will result in confusion and broken kernels, and has even resulted in an exploitable security issue. Therefore, when using non-obvious pairs of primitives, commenting is of course a must. One example of non-obvious pairing is the XDP feature in networking, which calls BPF programs from network-driver NAPI (softirq) context. BPF relies heavily on RCU protection for its data structures, but because the BPF program invocation happens entirely within a single `local_bh_disable()` section in a NAPI poll cycle, this usage is safe. The reason that this usage is safe is that readers can use anything that disables BH when updaters use `call_rcu()` or `synchronize_rcu()`.

8. Although `synchronize_rcu()` is slower than is `call_rcu()`, it usually results in simpler code. So, unless update performance is critically important, the updaters cannot block, or the latency of `synchronize_rcu()` is visible from userspace, `synchronize_rcu()` should be used in preference to `call_rcu()`. Furthermore, `kfree_rcu()` and `kvfree_rcu()` usually result in even simpler code than does `synchronize_rcu()` without `synchronize_rcu()`’s multi-millisecond latency. So please take advantage of `kfree_rcu()`’s and `kvfree_rcu()`’s “fire and forget” memory-freeing capabilities where it applies.

   An especially important property of the `synchronize_rcu()` primitive is that it automatically self-limits: if grace periods are delayed for whatever reason, then the `synchronize_rcu()` primitive will correspondingly delay updates. In contrast, code using `call_rcu()` should explicitly limit update rate in cases where grace periods are delayed, as failing to do so can result in excessive realtime latencies or even OOM conditions.

   Ways of gaining this self-limiting property when using `call_rcu()`, `kfree_rcu()`, or `kvfree_rcu()` include:

   1. Keeping a count of the number of data-structure elements used by the RCU-protected data structure, including those waiting for a grace period to elapse. Enforce a limit on this number, stalling updates as needed to allow previously deferred frees to complete. Alternatively, limit only the number awaiting deferred free rather than the total number of elements.

      One way to stall the updates is to acquire the update-side mutex. (Don’t try this with a spinlock -- other CPUs spinning on the lock could prevent the grace period from ever ending.) Another way to stall the updates is for the updates to use a wrapper function around the memory allocator, so that this wrapper function simulates OOM when there is too much memory awaiting an RCU grace period. There are of course many other variations on this theme.

   2. Limiting update rate. For example, if updates occur only once per hour, then no explicit rate limiting is required, unless your system is already badly broken. Older versions of the dcache subsystem take this approach, guarding updates with a global lock, limiting their rate.

   3. Trusted update -- if updates can only be done manually by superuser or some other trusted user, then it might not be necessary to automatically limit them. The theory here is that superuser already has lots of ways to crash the machine.

   4. Periodically invoke `rcu_barrier()`, permitting a limited number of updates per grace period.

   The same cautions apply to `call_srcu()`, `call_rcu_tasks()`, and `call_rcu_tasks_trace()`. This is why there is an `srcu_barrier()`, `rcu_barrier_tasks()`, and `rcu_barrier_tasks_trace()`, respectively.

   Note that although these primitives do take action to avoid memory exhaustion when any given CPU has too many callbacks, a determined user or administrator can still exhaust memory. This is especially the case if a system with a large number of CPUs has been configured to offload all of its RCU callbacks onto a single CPU, or if the system has relatively little free memory.

9. All RCU list-traversal primitives, which include `rcu_dereference()`, `list_for_each_entry_rcu()`, and `list_for_each_safe_rcu()`, must be either within an RCU read-side critical section or must be protected by appropriate update-side locks. RCU read-side critical sections are delimited by `rcu_read_lock()` and `rcu_read_unlock()`, or by similar primitives such as `rcu_read_lock_bh()` and `rcu_read_unlock_bh()`, in which case the matching `rcu_dereference()` primitive must be used in order to keep lockdep happy, in this case, `rcu_dereference_bh()`.

   The reason that it is permissible to use RCU list-traversal primitives when the update-side lock is held is that doing so can be quite helpful in reducing code bloat when common code is shared between readers and updaters. Additional primitives are provided for this case, as discussed in RCU and lockdep checking.

   One exception to this rule is when data is only ever added to the linked data structure, and is never removed during any time that readers might be accessing that structure. In such cases, `READ_ONCE()` may be used in place of `rcu_dereference()` and the read-side markers (`rcu_read_lock()` and `rcu_read_unlock()`, for example) may be omitted.

10. Conversely, if you are in an RCU read-side critical section, and you don’t hold the appropriate update-side lock, you must use the “`_rcu()`” variants of the list macros. Failing to do so will break Alpha, cause aggressive compilers to generate bad code, and confuse people trying to understand your code.

11. Any lock acquired by an RCU callback must be acquired elsewhere with softirq disabled, e.g., via `spin_lock_bh()`. Failing to disable softirq on a given acquisition of that lock will result in deadlock as soon as the RCU softirq handler happens to run your RCU callback while interrupting that acquisition’s critical section.

12. RCU callbacks can be and are executed in parallel. In many cases, the callback code simply wrappers around `kfree()`, so that this is not an issue (or, more accurately, to the extent that it is an issue, the memory-allocator locking handles it). However, if the callbacks do manipulate a shared data structure, they must use whatever locking or other synchronization is required to safely access and/or modify that data structure.

    Do not assume that RCU callbacks will be executed on the same CPU that executed the corresponding `call_rcu()`, `call_srcu()`, `call_rcu_tasks()`, or `call_rcu_tasks_trace()`. For example, if a given CPU goes offline while having an RCU callback pending, then that RCU callback will execute on some surviving CPU. (If this was not the case, a self-spawning RCU callback would prevent the victim CPU from ever going offline.) Furthermore, CPUs designated by rcu_nocbs= might well always have their RCU callbacks executed on some other CPUs, in fact, for some real-time workloads, this is the whole point of using the rcu_nocbs= kernel boot parameter.

    In addition, do not assume that callbacks queued in a given order will be invoked in that order, even if they all are queued on the same CPU. Furthermore, do not assume that same-CPU callbacks will be invoked serially. For example, in recent kernels, CPUs can be switched between offloaded and de-offloaded callback invocation, and while a given CPU is undergoing such a switch, its callbacks might be concurrently invoked by that CPU’s softirq handler and that CPU’s rcuo kthread. At such times, that CPU’s callbacks might be executed both concurrently and out of order.

13. Unlike most flavors of RCU, it is permissible to block in an SRCU read-side critical section (demarked by `srcu_read_lock()` and `srcu_read_unlock()`), hence the “SRCU”: “sleepable RCU”. As with RCU, guard(srcu)() and scoped_guard(srcu) forms are available, and often provide greater ease of use. Please note that if you don’t need to sleep in read-side critical sections, you should be using RCU rather than SRCU, because RCU is almost always faster and easier to use than is SRCU.

    Also unlike other forms of RCU, explicit initialization and cleanup is required either at build time via `DEFINE_SRCU()`, `DEFINE_STATIC_SRCU()`, `DEFINE_SRCU_FAST()`, or `DEFINE_STATIC_SRCU_FAST()` or at runtime via either `init_srcu_struct()` or `init_srcu_struct_fast()` and `cleanup_srcu_struct()`. These last three are passed a `struct srcu_struct` that defines the scope of a given SRCU domain. Once initialized, the srcu_struct is passed to `srcu_read_lock()`, `srcu_read_unlock()` `synchronize_srcu()`, `synchronize_srcu_expedited()`, and `call_srcu()`. A given `synchronize_srcu()` waits only for SRCU read-side critical sections governed by `srcu_read_lock()` and `srcu_read_unlock()` calls that have been passed the same srcu_struct. This property is what makes sleeping read-side critical sections tolerable -- a given subsystem delays only its own updates, not those of other subsystems using SRCU. Therefore, SRCU is less prone to OOM the system than RCU would be if RCU’s read-side critical sections were permitted to sleep.

    The ability to sleep in read-side critical sections does not come for free. First, corresponding `srcu_read_lock()` and `srcu_read_unlock()` calls must be passed the same srcu_struct. Second, grace-period-detection overhead is amortized only over those updates sharing a given srcu_struct, rather than being globally amortized as they are for other forms of RCU. Therefore, SRCU should be used in preference to rw_semaphore only in extremely read-intensive situations, or in situations requiring SRCU’s read-side deadlock immunity or low read-side realtime latency. You should also consider percpu_rw_semaphore when you need lightweight readers.

    SRCU’s expedited primitive (`synchronize_srcu_expedited()`) never sends IPIs to other CPUs, so it is easier on real-time workloads than is `synchronize_rcu_expedited()`.

    It is also permissible to sleep in RCU Tasks Trace read-side critical section, which are delimited by `rcu_read_lock_trace()` and `rcu_read_unlock_trace()`. However, this is a specialized flavor of RCU, and you should not use it without first checking with its current users. In most cases, you should instead use SRCU. As with RCU and SRCU, guard(rcu_tasks_trace)() and scoped_guard(rcu_tasks_trace) are available, and often provide greater ease of use.

    Note that `rcu_assign_pointer()` relates to SRCU just as it does to other forms of RCU, but instead of `rcu_dereference()` you should use `srcu_dereference()` in order to avoid lockdep splats.

14. The whole point of `call_rcu()`, `synchronize_rcu()`, and friends is to wait until all pre-existing readers have finished before carrying out some otherwise-destructive operation. It is therefore critically important to first remove any path that readers can follow that could be affected by the destructive operation, and only then invoke `call_rcu()`, `synchronize_rcu()`, or friends.

    Because these primitives only wait for pre-existing readers, it is the caller’s responsibility to guarantee that any subsequent readers will execute safely.

15. The various RCU read-side primitives do not necessarily contain memory barriers. You should therefore plan for the CPU and the compiler to freely reorder code into and out of RCU read-side critical sections. It is the responsibility of the RCU update-side primitives to deal with this.

    For SRCU readers, you can use `smp_mb__after_srcu_read_unlock()` immediately after an `srcu_read_unlock()` to get a full barrier.

16. Use CONFIG_PROVE_LOCKING, CONFIG_DEBUG_OBJECTS_RCU_HEAD, and the __rcu sparse checks to validate your RCU code. These can help find problems as follows:

    CONFIG_PROVE_LOCKING:

      check that accesses to RCU-protected data structures are carried out under the proper RCU read-side critical section, while holding the right combination of locks, or whatever other conditions are appropriate.

    CONFIG_DEBUG_OBJECTS_RCU_HEAD:

      check that you don’t pass the same object to `call_rcu()` (or friends) before an RCU grace period has elapsed since the last time that you passed that same object to `call_rcu()` (or friends).

    CONFIG_RCU_STRICT_GRACE_PERIOD:

      combine with KASAN to check for pointers leaked out of RCU read-side critical sections. This Kconfig option is tough on both performance and scalability, and so is limited to four-CPU systems.

    __rcu sparse checks:

      tag the pointer to the RCU-protected data structure with __rcu, and sparse will warn you if you access that pointer without the services of one of the variants of `rcu_dereference()`.

    These debugging aids can help you find problems that are otherwise extremely difficult to spot.

17. If you pass a callback function defined within a module to one of `call_rcu()`, `call_srcu()`, `call_rcu_tasks()`, or `call_rcu_tasks_trace()`, then it is necessary to wait for all pending callbacks to be invoked before unloading that module. Note that it is absolutely not sufficient to wait for a grace period! For example, `synchronize_rcu()` implementation is not guaranteed to wait for callbacks registered on other CPUs via `call_rcu()`. Or even on the current CPU if that CPU recently went offline and came back online.

    You instead need to use one of the barrier functions:

    - `call_rcu()` -> `rcu_barrier()`

    - `call_srcu()` -> `srcu_barrier()`

    - `call_rcu_tasks()` -> `rcu_barrier_tasks()`

    - `call_rcu_tasks_trace()` -> `rcu_barrier_tasks_trace()`

    However, these barrier functions are absolutely not guaranteed to wait for a grace period. For example, if there are no `call_rcu()` callbacks queued anywhere in the system, `rcu_barrier()` can and will return immediately.

    So if you need to wait for both a grace period and for all pre-existing callbacks, you will need to invoke both functions, with the pair depending on the flavor of RCU:

    - Either `synchronize_rcu()` or `synchronize_rcu_expedited()`, together with `rcu_barrier()`

    - Either `synchronize_srcu()` or `synchronize_srcu_expedited()`, together with and `srcu_barrier()`

    - `synchronize_rcu_tasks()` and `rcu_barrier_tasks()`

    - `synchronize_tasks_trace()` and `rcu_barrier_tasks_trace()`

    If necessary, you can use something like workqueues to execute the requisite pair of functions concurrently.

    See RCU and Unloadable Modules for more information.