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rcu: Do a single rhp->func read in rcu_head_after_call_rcu() 

The rcu_head_after_call_rcu() function reads the rhp->func pointer twice,
which can result in a false-positive WARN_ON_ONCE() if the callback
were passed to call_rcu() between the two reads.  Although racing
rcu_head_after_call_rcu() with call_rcu() is to be a dubious use case
(the return value is not reliable in that case), intermittent and
irreproducible warnings are also quite dubious.  This commit therefore
uses a single READ_ONCE() to pick up the value of rhp->func once, then
tests that value twice, thus guaranteeing consistent processing within
rcu_head_after_call_rcu()().

Neverthless, racing rcu_head_after_call_rcu() with call_rcu() is still
a dubious use case.

Signed-off-by: Neeraj Upadhyay <neeraju@codeaurora.org>
[ paulmck: Add blank line after declaration per checkpatch.pl. ]
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
This commit is contained in:
Neeraj Upadhyay 2019-03-11 17:28:03 +05:30 committed by Paul E. McKenney
parent 6973032a60
commit b699cce160
1 changed files with 4 additions and 2 deletions
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6
include/linux/rcupdate.h
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@ -878,9 +878,11 @@ static inline void rcu_head_init(struct rcu_head *rhp)
static inline bool
rcu_head_after_call_rcu(struct rcu_head *rhp, rcu_callback_t f)
{
if (READ_ONCE(rhp->func) == f)
rcu_callback_t func = READ_ONCE(rhp->func);
if (func == f)
return true;
WARN_ON_ONCE(READ_ONCE(rhp->func) != (rcu_callback_t)~0L);
WARN_ON_ONCE(func != (rcu_callback_t)~0L);
return false;
}