Re: [PATCH v2] rcutorture: Reuse value read using READ_ONCE instead of re-reading it

[Paul E. McKenney]

On Wed, Mar 06, 2024 at 10:05:20AM +0800, linke li wrote:
> rp->rtort_pipe_count is modified concurrently by rcu_torture_writer(). To
> prevent a data race, reuse i which is read using READ_ONCE before instead
> of re-reading it.
> 
> Signed-off-by: linke li <lilinke99@xxxxxx>
> Reviewed-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
> ---

Thank you both!

This topic got quite a bit of discussion [1], with the result that I
took your patch, but edited your commit log.  Could you please take a
look below and let me know if I messed anything up?

							Thanx, Paul

[1] https://lore.kernel.org/all/20240306103719.1d241b93@xxxxxxxxxxxxxxxxxx/

------------------------------------------------------------------------

commit e3038bbf5d746fd4c72975b792abbb63fa3f3421
Author: linke li <lilinke99@xxxxxx>
Date:   Wed Mar 6 19:51:10 2024 -0800

    rcutorture: Re-use value stored to ->rtort_pipe_count instead of re-reading
    
    Currently, the rcu_torture_pipe_update_one() writes the value (i + 1)
    to rp->rtort_pipe_count, then immediately re-reads it in order to compare
    it to RCU_TORTURE_PIPE_LEN.  This re-read is pointless because no other
    update to rp->rtort_pipe_count can occur at this point.  This commit
    therefore instead re-uses the (i + 1) value stored in the comparison
    instead of re-reading rp->rtort_pipe_count.
    
    Signed-off-by: linke li <lilinke99@xxxxxx>
    Reviewed-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
    Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
    Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>

diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
index 0cb5452ecd945..dd7d5ba457409 100644
--- a/kernel/rcu/rcutorture.c
+++ b/kernel/rcu/rcutorture.c
@@ -467,7 +467,7 @@ rcu_torture_pipe_update_one(struct rcu_torture *rp)
 	atomic_inc(&rcu_torture_wcount[i]);
 	WRITE_ONCE(rp->rtort_pipe_count, i + 1);
 	ASSERT_EXCLUSIVE_WRITER(rp->rtort_pipe_count);
-	if (rp->rtort_pipe_count >= RCU_TORTURE_PIPE_LEN) {
+	if (i + 1 >= RCU_TORTURE_PIPE_LEN) {
 		rp->rtort_mbtest = 0;
 		return true;
 	}