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| author | Paul E. McKenney <paulmck@linux.vnet.ibm.com> | 2013-12-05 23:56:54 +0100 |
|---|---|---|
| committer | Paul E. McKenney <paulmck@linux.vnet.ibm.com> | 2014-02-17 23:55:58 +0100 |
| commit | 6e67669678d2d51b2bcf0411aeb629b4353a9880 (patch) | |
| tree | 954ea91326c4ca225a698ccb3d6e3c3f8ee4b3d0 | |
| parent | Linux 3.14-rc3 (diff) | |
| download | linux-6e67669678d2d51b2bcf0411aeb629b4353a9880.tar.xz linux-6e67669678d2d51b2bcf0411aeb629b4353a9880.zip | |
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" <mst@redhat.com>
Reported-by: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
| -rw-r--r-- | Documentation/RCU/checklist.txt | 18 |
1 files changed, 13 insertions, 5 deletions
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 |