Re: [PATCH] rcuupdate: Do a single rhp->func read in rcu_head_after_call_rcu

["Paul E. McKenney" <paulmck@linux.ibm.com>]

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@codeaurora.org>
> >
> >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@codeaurora.org>
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@codeaurora.org>
    [ paulmck: Add blank line after declaration per checkpatch.pl. ]
    Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>

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;
 }