From: "Paul E. McKenney" <paul...@linux.vnet.ibm.com>
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" <m...@redhat.com>
Reported-by: Gleb Natapov <g...@redhat.com>
Signed-off-by: Paul E. McKenney <paul...@linux.vnet.ibm.com>
---
Documentation/RCU/checklist.txt | 19 ++++++++++++++-----
1 file changed, 14 insertions(+), 5 deletions(-)
diff --git a/Documentation/RCU/checklist.txt b/Documentation/RCU/checklist.txt
index 91266193b8f4..5733e31836b5 100644
--- a/Documentation/RCU/checklist.txt
+++ b/Documentation/RCU/checklist.txt
@@ -256,10 +256,11 @@ 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 takes this
+ approach -- updates were guarded by 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 +269,8 @@ 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 +278,13 @@ 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
--
1.8.1.5
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