[ Upstream commit b699cce1604e828f19c39845252626eb78cdf38a ]
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 Signed-off-by: Sasha Levin sashal@kernel.org --- include/linux/rcupdate.h | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-)
diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h index 4db8bcacc51ae..991d97cf395a7 100644 --- a/include/linux/rcupdate.h +++ b/include/linux/rcupdate.h @@ -890,9 +890,11 @@ 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; }