Re: [PATCH rcu 11/18] rcu: Mark lockless ->qsmask read in rcu_check_boost_fail()

[Boqun Feng <boqun.feng@gmail.com>]

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@kernel.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())? ;-)

Regards,
Boqun

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