linux-kernel - Re: [PATCH rcu 11/18] rcu: Mark lockless ->qsmask read in rcu_check_boost

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Message-ID: <20210730032656.GF4397@paulmck-ThinkPad-P17-Gen-1>
Date:   Thu, 29 Jul 2021 20:26:56 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Boqun Feng <boqun.feng@...il.com>
Cc:     rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...com, mingo@...nel.org, jiangshanlai@...il.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
        rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
        fweisbec@...il.com, oleg@...hat.com, joel@...lfernandes.org
Subject: Re: [PATCH rcu 11/18] rcu: Mark lockless ->qsmask read in
 rcu_check_boost_fail()

On Fri, Jul 30, 2021 at 10:28:58AM +0800, Boqun Feng wrote:
> On Thu, Jul 29, 2021 at 07:03:17AM -0700, Paul E. McKenney wrote:
> > On Thu, Jul 29, 2021 at 04:54:30PM +0800, Boqun Feng wrote:
> > > On Wed, Jul 21, 2021 at 01:21:19PM -0700, Paul E. McKenney wrote:
> > > > Accesses to ->qsmask are normally protected by ->lock, but there is an
> > > > exception in the diagnostic code in rcu_check_boost_fail().  This commit
> > > > therefore applies data_race() to this access to avoid KCSAN complaining
> > > > about the C-language writes protected by ->lock.
> > > > 
> > > > Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
> > > > ---
> > > >  kernel/rcu/tree_stall.h | 2 +-
> > > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > > 
> > > > diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h
> > > > index 42847caa3909b..6dd6c9aa3f757 100644
> > > > --- a/kernel/rcu/tree_stall.h
> > > > +++ b/kernel/rcu/tree_stall.h
> > > > @@ -766,7 +766,7 @@ bool rcu_check_boost_fail(unsigned long gp_state, int *cpup)
> > > >  
> > > >  	rcu_for_each_leaf_node(rnp) {
> > > >  		if (!cpup) {
> > > > -			if (READ_ONCE(rnp->qsmask)) {
> > > > +			if (data_race(READ_ONCE(rnp->qsmask))) {
> > > 
> > > If the write sides allow normal writes, i.e. allowing store tearing, the
> > > READ_ONCE() here could read incomplete writes, which could be anything
> > > basically? And we get the same result if we remove the READ_ONCE(),
> > > don't we? Yes, I know, without the READ_ONCE(), compilers can do
> > > anything to optimize on rnp->qsmask, but the end result is same or
> > > similar to reading incomplete writes (which is also a result by compiler
> > > optimization). So if we mark something as data_race(), **in theory**, it
> > > makes no difference with or without the READ_ONCE(), so I think maybe we
> > > can remove the READ_ONCE() here?
> > 
> > In this particular case, perhaps.  But there is also the possibility
> > of ASSERT_EXCLUSIVE_WRITER() in conjunction with WRITE_ONCE(), and
> > data_race(READ_ONCE(()) handles all such cases properly.
> > 
> > Again, the point here is to prevent the compiler from messing with
> > the load in strange and unpredictable ways while also telling KCSAN
> > that this particular read should not be considered to be part of the
> > concurrent algorithm.
> 
> Thanks for explaining this ;-)
> 
> I guess I'm just a little concerned that things may end up with using
> data_race(READ_ONCE()) everywhere instead of data_race(), because who
> doesn't want his/her racy code to be 1) not reported by KCSan (using
> data_race()), and 2) less racy (using READ_ONCE())? ;-)

There always is the risk of a too-attractive "easy way out",
isn't there?  Just make sure to understand exactly where
the "out" leads to longer term.  ;-)

							Thanx, Paul

> Regards,
> Boqun
> 
> > 							Thanx, Paul
> > 
> > > Regards,
> > > Boqun
> > > 
> > > >  				return false;
> > > >  			} else {
> > > >  				if (READ_ONCE(rnp->gp_tasks))
> > > > -- 
> > > > 2.31.1.189.g2e36527f23
> > > >