diff options
Diffstat (limited to 'Documentation/RCU/checklist.txt')
| -rw-r--r-- | Documentation/RCU/checklist.txt | 89 |
1 files changed, 53 insertions, 36 deletions
diff --git a/Documentation/RCU/checklist.txt b/Documentation/RCU/checklist.txt index fc103d7a047..877947130eb 100644 --- a/Documentation/RCU/checklist.txt +++ b/Documentation/RCU/checklist.txt @@ -114,12 +114,16 @@ over a rather long period of time, but improvements are always welcome! http://www.openvms.compaq.com/wizard/wiz_2637.html The rcu_dereference() primitive is also an excellent - documentation aid, letting the person reading the code - know exactly which pointers are protected by RCU. + 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. + 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 rcu_dereference.txt in this directory for + more information. The rcu_dereference() primitive is used by the various "_rcu()" list-traversal primitives, such @@ -202,8 +206,8 @@ over a rather long period of time, but improvements are always welcome! updater uses call_rcu_sched() or synchronize_sched(), then the corresponding readers must disable preemption, possibly by calling rcu_read_lock_sched() and rcu_read_unlock_sched(). - If the updater uses synchronize_srcu() or call_srcu(), - the the corresponding readers must use srcu_read_lock() and + 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 primitives are the same as for their non-expedited counterparts. Mixing things up will result in confusion and @@ -217,9 +221,14 @@ over a rather long period of time, but improvements are always welcome! whether the increased speed is worth it. 8. Although synchronize_rcu() is slower than is call_rcu(), it - usually results in simpler code. So, unless update performance - is critically important or the updaters cannot block, - synchronize_rcu() should be used in preference to call_rcu(). + 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() usually results in even simpler code than does + synchronize_rcu() without synchronize_rcu()'s multi-millisecond + latency. So please take advantage of kfree_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 @@ -251,10 +260,10 @@ over a rather long period of time, but improvements are always welcome! variations on this theme. b. 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. The dcache - subsystem takes this approach -- updates are guarded - by a global lock, limiting their rate. + 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. c. Trusted update -- if updates can only be done manually by superuser or some other trusted user, then it might not @@ -263,23 +272,31 @@ over a rather long period of time, but improvements are always welcome! the machine. d. Use call_rcu_bh() rather than call_rcu(), in order to take - advantage of call_rcu_bh()'s faster grace periods. + advantage of call_rcu_bh()'s faster grace periods. (This + is only a partial solution, though.) e. Periodically invoke synchronize_rcu(), permitting a limited number of updates per grace period. - The same cautions apply to call_rcu_bh() and call_rcu_sched(). + The same cautions apply to call_rcu_bh(), call_rcu_sched(), + call_srcu(), and kfree_rcu(). + + Note that although these primitives do take action to avoid memory + exhaustion when any given CPU has too many callbacks, a determined + user could 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(), - list_for_each_continue_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(). + 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 @@ -297,9 +314,9 @@ over a rather long period of time, but improvements are always welcome! all currently executing rcu_read_lock()-protected RCU read-side critical sections complete. It does -not- necessarily guarantee that all currently running interrupts, NMIs, preempt_disable() - code, or idle loops will complete. Therefore, if you do not have - rcu_read_lock()-protected read-side critical sections, do -not- - use synchronize_rcu(). + code, or idle loops will complete. Therefore, if your + read-side critical sections are protected by something other + than rcu_read_lock(), do -not- use synchronize_rcu(). Similarly, disabling preemption is not an acceptable substitute for rcu_read_lock(). Code that attempts to use preemption @@ -310,6 +327,12 @@ over a rather long period of time, but improvements are always welcome! code under the influence of preempt_disable(), you instead need to use synchronize_irq() or synchronize_sched(). + This same limitation also applies to synchronize_rcu_bh() + and synchronize_srcu(), as well as to the asynchronous and + expedited forms of the three primitives, namely call_rcu(), + call_rcu_bh(), call_srcu(), synchronize_rcu_expedited(), + synchronize_rcu_bh_expedited(), and synchronize_srcu_expedited(). + 12. Any lock acquired by an RCU callback must be acquired elsewhere with softirq disabled, e.g., via spin_lock_irqsave(), spin_lock_bh(), etc. Failing to disable irq on a given @@ -343,12 +366,6 @@ over a rather long period of time, but improvements are always welcome! using RCU rather than SRCU, because RCU is almost always faster and easier to use than is SRCU. - If you need to enter your read-side critical section in a - hardirq or exception handler, and then exit that same read-side - critical section in the task that was interrupted, then you need - to srcu_read_lock_raw() and srcu_read_unlock_raw(), which avoid - the lockdep checking that would otherwise this practice illegal. - Also unlike other forms of RCU, explicit initialization and cleanup is required via init_srcu_struct() and cleanup_srcu_struct(). These are passed a "struct srcu_struct" @@ -396,9 +413,9 @@ over a rather long period of time, but improvements are always welcome! read-side critical sections. It is the responsibility of the RCU update-side primitives to deal with this. -17. Use CONFIG_PROVE_RCU, CONFIG_DEBUG_OBJECTS_RCU_HEAD, and - the __rcu sparse checks to validate your RCU code. These - can help find problems as follows: +17. Use CONFIG_PROVE_RCU, CONFIG_DEBUG_OBJECTS_RCU_HEAD, and the + __rcu sparse checks (enabled by CONFIG_SPARSE_RCU_POINTER) to + validate your RCU code. These can help find problems as follows: CONFIG_PROVE_RCU: check that accesses to RCU-protected data structures are carried out under the proper RCU |