linux-kernel - Re: [PATCH tip/core/rcu 1/4] srcu: Fix __call_srcu()/process

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Message-ID: <20200217181802.GQ2935@paulmck-ThinkPad-P72>
Date:   Mon, 17 Feb 2020 10:18:02 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...com, mingo@...nel.org, jiangshanlai@...il.com,
        dipankar@...ibm.com, akpm@...ux-foundation.org,
        mathieu.desnoyers@...icios.com, josh@...htriplett.org,
        tglx@...utronix.de, rostedt@...dmis.org, dhowells@...hat.com,
        edumazet@...gle.com, fweisbec@...il.com, oleg@...hat.com,
        joel@...lfernandes.org
Subject: Re: [PATCH tip/core/rcu 1/4] srcu: Fix __call_srcu()/process_srcu()
 datarace

On Mon, Feb 17, 2020 at 01:42:31PM +0100, Peter Zijlstra wrote:
> On Fri, Feb 14, 2020 at 04:29:29PM -0800, paulmck@...nel.org wrote:
> > From: "Paul E. McKenney" <paulmck@...nel.org>
> > 
> > The srcu_node structure's ->srcu_gp_seq_needed_exp field is accessed
> > locklessly, so updates must use WRITE_ONCE().  This commit therefore
> > adds the needed WRITE_ONCE() invocations.
> > 
> > This data race was reported by KCSAN.  Not appropriate for backporting
> > due to failure being unlikely.
> 
> This does indeed look like there can be a failure; but this Changelog
> fails to describe an actual problem.

As before, within RCU, the mere fact of a KCSAN report motivates a change.
I am not going to waste time brainstorming overly creative compiler
optimizations, present or future.

							Thanx, Paul

> > Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
> > ---
> >  kernel/rcu/srcutree.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
> > index 657e6a7..b1edac9 100644
> > --- a/kernel/rcu/srcutree.c
> > +++ b/kernel/rcu/srcutree.c
> > @@ -550,7 +550,7 @@ static void srcu_gp_end(struct srcu_struct *ssp)
> >  		snp->srcu_have_cbs[idx] = gpseq;
> >  		rcu_seq_set_state(&snp->srcu_have_cbs[idx], 1);
> >  		if (ULONG_CMP_LT(snp->srcu_gp_seq_needed_exp, gpseq))
> > -			snp->srcu_gp_seq_needed_exp = gpseq;
> > +			WRITE_ONCE(snp->srcu_gp_seq_needed_exp, gpseq);
> >  		mask = snp->srcu_data_have_cbs[idx];
> >  		snp->srcu_data_have_cbs[idx] = 0;
> >  		spin_unlock_irq_rcu_node(snp);
> > @@ -660,7 +660,7 @@ static void srcu_funnel_gp_start(struct srcu_struct *ssp, struct srcu_data *sdp,
> >  		if (snp == sdp->mynode)
> >  			snp->srcu_data_have_cbs[idx] |= sdp->grpmask;
> >  		if (!do_norm && ULONG_CMP_LT(snp->srcu_gp_seq_needed_exp, s))
> > -			snp->srcu_gp_seq_needed_exp = s;
> > +			WRITE_ONCE(snp->srcu_gp_seq_needed_exp, s);
> >  		spin_unlock_irqrestore_rcu_node(snp, flags);
> >  	}
> >  
> > -- 
> > 2.9.5
> >