From: Paul E. McKenney Date: Thu, 5 Dec 2013 22:56:54 +0000 (-0800) Subject: documentation: Document call_rcu() safety mechanisms and limitations X-Git-Url: https://git.karo-electronics.de/?a=commitdiff_plain;h=6e67669678d2d51b2bcf0411aeb629b4353a9880;p=linux-beck.git documentation: Document call_rcu() safety mechanisms and limitations The call_rcu() family of primitives will take action to accelerate grace periods when the number of callbacks pending on a given CPU becomes excessive. Although this safety mechanism can be useful, it is no substitute for users of call_rcu() having rate-limit controls in place. This commit adds this nuance to the documentation. Reported-by: "Michael S. Tsirkin" Reported-by: Gleb Natapov Signed-off-by: Paul E. McKenney Reviewed-by: Josh Triplett --- diff --git a/Documentation/RCU/checklist.txt b/Documentation/RCU/checklist.txt index 91266193b8f4..9d10d1db16a5 100644 --- a/Documentation/RCU/checklist.txt +++ b/Documentation/RCU/checklist.txt @@ -256,10 +256,10 @@ 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 @@ -268,7 +268,8 @@ 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. @@ -276,6 +277,13 @@ over a rather long period of time, but improvements are always welcome! 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(), and list_for_each_safe_rcu(), must be either within an RCU read-side