Re: [PATCH tip/core/rcu 1/4] srcu: Fix __call_srcu()/process_srcu() datarace

[Joel Fernandes <joel@joelfernandes.org>]

On Mon, Feb 17, 2020 at 01:42:31PM +0100, Peter Zijlstra wrote:
> On Fri, Feb 14, 2020 at 04:29:29PM -0800, paulmck@kernel.org wrote:
> > From: "Paul E. McKenney" <paulmck@kernel.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.

Peter it sounds like you have a failure scenario in mind. Could you describe
more if so?

I am curious if you were thinking of invented-stores issue here.

For educational purposes, I was trying to come up with an example where my
compiler does something bad to code without WRITE_ONCE(). So far I only can
reproduce a write-tearing example when write with an immediate value is split
into 2 writes, like Will mentioned:
http://lore.kernel.org/r/20190821103200.kpufwtviqhpbuv2n@willie-the-truck
But that does not seem to apply to this code.

thanks,

 - Joel


> > Signed-off-by: Paul E. McKenney <paulmck@kernel.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
> >