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SubjectRe: rcu_read_lock lost its compiler barrier
On Mon, Jun 03, 2019 at 01:26:26PM +0800, Herbert Xu wrote:
> On Sun, Jun 02, 2019 at 08:47:07PM -0700, Paul E. McKenney wrote:
> >
> > 1. These guarantees are of full memory barriers, -not- compiler
> > barriers.
>
> What I'm saying is that wherever they are, they must come with
> compiler barriers. I'm not aware of any synchronisation mechanism
> in the kernel that gives a memory barrier without a compiler barrier.
>
> > 2. These rules don't say exactly where these full memory barriers
> > go. SRCU is at one extreme, placing those full barriers in
> > srcu_read_lock() and srcu_read_unlock(), and !PREEMPT Tree RCU
> > at the other, placing these barriers entirely within the callback
> > queueing/invocation, grace-period computation, and the scheduler.
> > Preemptible Tree RCU is in the middle, with rcu_read_unlock()
> > sometimes including a full memory barrier, but other times with
> > the full memory barrier being confined as it is with !PREEMPT
> > Tree RCU.
>
> The rules do say that the (full) memory barrier must precede any
> RCU read-side that occur after the synchronize_rcu and after the
> end of any RCU read-side that occur before the synchronize_rcu.
>
> All I'm arguing is that wherever that full mb is, as long as it
> also carries with it a barrier() (which it must do if it's done
> using an existing kernel mb/locking primitive), then we're fine.
>
> > Interleaving and inserting full memory barriers as per the rules above:
> >
> > CPU1: WRITE_ONCE(a, 1)
> > CPU1: synchronize_rcu
> > /* Could put a full memory barrier here, but it wouldn't help. */
>
> CPU1: smp_mb();
> CPU2: smp_mb();
>
> Let's put them in because I think they are critical. smp_mb() also
> carries with it a barrier().
>
> > CPU2: rcu_read_lock();
> > CPU1: b = 2;
> > CPU2: if (READ_ONCE(a) == 0)
> > CPU2: if (b != 1) /* Weakly ordered CPU moved this up! */
> > CPU2: b = 1;
> > CPU2: rcu_read_unlock
> >
> > In fact, CPU2's load from b might be moved up to race with CPU1's store,
> > which (I believe) is why the model complains in this case.
>
> Let's put aside my doubt over how we're even allowing a compiler
> to turn
>
> b = 1
>
> into
>
> if (b != 1)
> b = 1
>
> Since you seem to be assuming that (a == 0) is true in this case

I think Paul's example assuming (a == 0) is false, and maybe
speculative writes (by compilers) needs to added into consideration?
Please consider the following case (I add a few smp_mb()s), the case may
be a little bit crasy, you have been warned ;-)

CPU1: WRITE_ONCE(a, 1)
CPU1: synchronize_rcu called

CPU1: smp_mb(); /* let assume there is one here */

CPU2: rcu_read_lock();
CPU2: smp_mb(); /* let assume there is one here */

/* "if (b != 1) b = 1" reordered */
CPU2: r0 = b; /* if (b != 1) reordered here, r0 == 0 */
CPU2: if (r0 != 1) /* true */
CPU2: b = 1; /* b == 1 now, this is a speculative write
by compiler
*/

CPU1: b = 2; /* b == 2 */

CPU2: if (READ_ONCE(a) == 0) /* false */
CPU2: ...
CPU2 else /* undo the speculative write */
CPU2: b = r0; /* b == 0 */

CPU2: smp_mb();
CPU2: read_read_unlock();

I know this is too crasy for us to think a compiler like this, but this
might be the reason why the model complain about this.

Paul, did I get this right? Or you mean something else?

Regards,
Boqun



> (as the assignment b = 1 is carried out), then because of the
> presence of the full memory barrier, the RCU read-side section
> must have started prior to the synchronize_rcu. This means that
> synchronize_rcu is not allowed to return until at least the end
> of the grace period, or at least until the end of rcu_read_unlock.
>
> So it actually should be:
>
> CPU1: WRITE_ONCE(a, 1)
> CPU1: synchronize_rcu called
> /* Could put a full memory barrier here, but it wouldn't help. */
>
> CPU1: smp_mb();
> CPU2: smp_mb();
>
> CPU2: grace period starts
> ...time passes...
> CPU2: rcu_read_lock();
> CPU2: if (READ_ONCE(a) == 0)
> CPU2: if (b != 1) /* Weakly ordered CPU moved this up! */
> CPU2: b = 1;
> CPU2: rcu_read_unlock
> ...time passes...
> CPU2: grace period ends
>
> /* This full memory barrier is also guaranteed by RCU. */
> CPU2: smp_mb();
>
> CPU1 synchronize_rcu returns
> CPU1: b = 2;
>
> Cheers,
> --
> Email: Herbert Xu <herbert@gondor.apana.org.au>
> Home Page: http://gondor.apana.org.au/~herbert/
> PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt
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