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

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Message-ID: <YQJsxtypw7M/3XBD@boqun-archlinux>
Date:   Thu, 29 Jul 2021 16:54:30 +0800
From:   Boqun Feng <boqun.feng@...il.com>
To:     "Paul E. McKenney" <paulmck@...nel.org>
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 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?

Regards,
Boqun

>  				return false;
>  			} else {
>  				if (READ_ONCE(rnp->gp_tasks))
> -- 
> 2.31.1.189.g2e36527f23
>