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; }
