netdev - Re: rcu_read_lock lost its compiler barrier

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Date:   Thu, 6 Jun 2019 10:16:57 +0200
From:   Andrea Parri <andrea.parri@...rulasolutions.com>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     "Paul E. McKenney" <paulmck@...ux.ibm.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Fengguang Wu <fengguang.wu@...el.com>, LKP <lkp@...org>,
        LKML <linux-kernel@...r.kernel.org>,
        Netdev <netdev@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Luc Maranget <luc.maranget@...ia.fr>,
        Jade Alglave <j.alglave@....ac.uk>
Subject: Re: rcu_read_lock lost its compiler barrier

> This example really does point out a weakness in the LKMM's handling of 
> data races.  Herbert's litmus test is a great starting point:
> 
> 
> C xu
> 
> {}
> 
> P0(int *a, int *b)
> {
> 	WRITE_ONCE(*a, 1);
> 	synchronize_rcu();
> 	*b = 2;
> }
> 
> P1(int *a, int *b)
> {
> 	rcu_read_lock();
> 	if (READ_ONCE(*a) == 0)
> 		*b = 1;
> 	rcu_read_unlock();
> }
> 
> exists (~b=2)
> 
> 
> Currently the LKMM says the test is allowed and there is a data race, 
> but this answer clearly is wrong since it would violate the RCU 
> guarantee.
> 
> The problem is that the LKMM currently requires all ordering/visibility
> of plain accesses to be mediated by marked accesses.  But in this case,
> the visibility is mediated by RCU.  Technically, we need to add a
> relation like
> 
> 	([M] ; po ; rcu-fence ; po ; [M])
> 
> into the definitions of ww-vis, wr-vis, and rw-xbstar.  Doing so
> changes the litmus test's result to "not allowed" and no data race.  
> However, I'm not certain that this single change is the entire fix;  
> more thought is needed.

This seems a sensible change to me: looking forward to seeing a patch,
on top of -rcu/dev, for further review and testing!

We could also add (to LKMM) the barrier() for rcu_read_{lock,unlock}()
discussed in this thread (maybe once the RCU code and the informal doc
will have settled in such direction).

It seems worth stressing the fact that _neither_ of these changes will
prevent the test below from being racy, considered the two accesses to
"a" happen concurrently / without synchronization.

Thanks,
  Andrea

C xu-2

{}

P0(int *a, int *b)
{
	*a = 1;
	synchronize_rcu();
	WRITE_ONCE(*b, 2);
}
 
P1(int *a, int *b)
{
	rcu_read_lock();
	if (*a == 0)
		WRITE_ONCE(*b, 1);
	rcu_read_unlock();
}
 
exists (~b=2)