On Mon, Mar 11, 2019 at 09:22:30PM +0530, Neeraj Upadhyay wrote: > > > On 3/11/19 9:18 PM, Paul E. McKenney wrote: > >On Mon, Mar 11, 2019 at 05:28:03PM +0530, Neeraj Upadhyay wrote: > >>Read rhp->func pointer in rcu_head_after_call_rcu() only once, > >>to avoid warning in the case, where call_rcu() happens between > >>two reads of rhp->func. > >> > >>Signed-off-by: Neeraj Upadhyay <neeraju@xxxxxxxxxxxxxx> > > > >This would more gracefully handle racing rcu_head_after_call_rcu() > >with call_rcu(). > > > >But this thing is not yet used, so let's see what Neil Brown says. > >If he isn't going to use it, my thought is to instead just remove > >this. > > Agree, makes sense. And Neil said that he intends to use it, so I applied your patch, updated as shown below. Ah, and please use scripts/checkpatch.pl -- it sometimes gets overly enthusiastic, but the blank line following the declarations is good practice. Thanx, Paul ------------------------------------------------------------------------ commit fcf4326ee3fb7e0925fe0f299c385c31f5d62fbf Author: Neeraj Upadhyay <neeraju@xxxxxxxxxxxxxx> Date: Mon Mar 11 17:28:03 2019 +0530 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@xxxxxxxxxxxxxx> [ paulmck: Add blank line after declaration per checkpatch.pl. ] Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxx> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h index 6cdb1db776cf..922bb6848813 100644 --- a/include/linux/rcupdate.h +++ b/include/linux/rcupdate.h @@ -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; }