On Sat, Feb 15, 2020 at 02:58:03AM -0800, Paul E. McKenney wrote: > On Fri, Feb 14, 2020 at 10:47:43PM -0500, Steven Rostedt wrote: > > On Fri, 14 Feb 2020 15:55:43 -0800 > > paulmck@xxxxxxxxxx wrote: > > > > > From: "Paul E. McKenney" <paulmck@xxxxxxxxxx> > > > > > > The rcu_node structure's ->exp_seq_rq field is read locklessly, so > > > this commit adds the WRITE_ONCE() to a load in order to provide proper > > > documentation and READ_ONCE()/WRITE_ONCE() pairing. > > > > > > This data race was reported by KCSAN. Not appropriate for backporting > > > due to failure being unlikely. > > > > > > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx> > > > --- > > > kernel/rcu/tree_exp.h | 2 +- > > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > > > diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h > > > index d7e0484..85b009e 100644 > > > --- a/kernel/rcu/tree_exp.h > > > +++ b/kernel/rcu/tree_exp.h > > > @@ -314,7 +314,7 @@ static bool exp_funnel_lock(unsigned long s) > > > sync_exp_work_done(s)); > > > return true; > > > } > > > - rnp->exp_seq_rq = s; /* Followers can wait on us. */ > > > + WRITE_ONCE(rnp->exp_seq_rq, s); /* Followers can wait on us. */ > > > > Didn't Linus say this is basically bogus? > > > > Perhaps just using it as documenting that it's read locklessly, but is > > it really needed? > > Yes, Linus explicitly stated that WRITE_ONCE() is not required in > this case, but he also said that he was OK with it being there for > documentation purposes. Just to add, PeterZ does approve of WRITE_ONCE() to prevent store-tearing where applicable. And I have reproduced Will's example [1] with the arm64 Clang compiler shipping with the latest Android NDK just now. It does break up stores when writing zeroes to 64-bit valyes, this is a real problem which WRITE_ONCE() resolves. And I've verified GCC on arm64 does break up 64-bit immediate value writes without WRITE_ONCE(). thanks, - Joel [1] https://lore.kernel.org/lkml/20190821103200.kpufwtviqhpbuv2n@willie-the-truck/ > And within RCU, I -do- need it because I absolutely need to see if a > given patch introduced new KCSAN reports. So I need it for the same > reason that I need the build to proceed without warnings. > > Others who are working with less concurrency-intensive code might quite > reasonably make other choices, of course. And my setting certain KCSAN > config options in my own builds doesn't inconvenience them, so we should > all be happy, right? :-) > > Thanx, Paul > > > -- Steve > > > > > > > > > spin_unlock(&rnp->exp_lock); > > > trace_rcu_exp_funnel_lock(rcu_state.name, rnp->level, > > > rnp->grplo, rnp->grphi, TPS("nxtlvl")); > >
