Re: [PATCH v2 2/3] locking: Clarify requirements for smp_mb__after_spinlock()

[Andrea Parri <andrea.parri@amarulasolutions.com>]

On Mon, Jul 02, 2018 at 05:37:35PM +0200, Peter Zijlstra wrote:
> On Mon, Jul 02, 2018 at 05:11:55PM +0200, Andrea Parri wrote:
> >  /*
> > + * smp_mb__after_spinlock() provides the equivalent of a full memory barrier
> > + * between program-order earlier lock acquisitions and program-order later
> > + * memory accesses.
> >   *
> > + * This guarantees that the following two properties hold:
> >   *
> > + *   1) Given the snippet:
> >   *
> > + *	  { X = 0;  Y = 0; }
> >   *
> > + *	  CPU0				CPU1
> >   *
> > + *	  WRITE_ONCE(X, 1);		WRITE_ONCE(Y, 1);
> > + *	  spin_lock(S);			smp_mb();
> > + *	  smp_mb__after_spinlock();	r1 = READ_ONCE(X);
> > + *	  r0 = READ_ONCE(Y);
> > + *	  spin_unlock(S);
> >   *
> > + *      it is forbidden that CPU0 does not observe CPU1's store to Y (r0 = 0)
> > + *      and CPU1 does not observe CPU0's store to X (r1 = 0); see the comments
> > + *      preceding the call to smp_mb__after_spinlock() in __schedule() and in
> > + *      try_to_wake_up().
> > + *
> > + *   2) Given the snippet:
> > + *
> > + *  { X = 0;  Y = 0; }
> > + *
> > + *  CPU0		CPU1				CPU2
> > + *
> > + *  spin_lock(S);	spin_lock(S);			r1 = READ_ONCE(Y);
> > + *  WRITE_ONCE(X, 1);	smp_mb__after_spinlock();	smp_rmb();
> > + *  spin_unlock(S);	r0 = READ_ONCE(X);		r2 = READ_ONCE(X);
> > + *			WRITE_ONCE(Y, 1);
> > + *			spin_unlock(S);
> > + *
> > + *      it is forbidden that CPU0's critical section executes before CPU1's
> > + *      critical section (r0 = 1), CPU2 observes CPU1's store to Y (r1 = 1)
> > + *      and CPU2 does not observe CPU0's store to X (r2 = 0); see the comments
> > + *      preceding the calls to smp_rmb() in try_to_wake_up() for similar
> > + *      snippets but "projected" onto two CPUs.
> 
> Maybe explicitly note that 2) is the RCsc lock upgrade.

Yes, I'll do a respin to add this note and the below Ack shortly.

Thanks,
  Andrea


> 
> 
> >   * Since most load-store architectures implement ACQUIRE with an smp_mb() after
> >   * the LL/SC loop, they need no further barriers. Similarly all our TSO
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index da8f12119a127..ec9ef0aec71ac 100644
> > +++ b/kernel/sched/core.c
> > @@ -1999,21 +1999,20 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
> >  	 * be possible to, falsely, observe p->on_rq == 0 and get stuck
> >  	 * in smp_cond_load_acquire() below.
> >  	 *
> > +	 * sched_ttwu_pending()			try_to_wake_up()
> > +	 *   STORE p->on_rq = 1			  LOAD p->state
> > +	 *   UNLOCK rq->lock
> > +	 *
> > +	 * __schedule() (switch to task 'p')
> > +	 *   LOCK rq->lock			  smp_rmb();
> > +	 *   smp_mb__after_spinlock();
> > +	 *   UNLOCK rq->lock
> >  	 *
> >  	 * [task p]
> > +	 *   STORE p->state = UNINTERRUPTIBLE	  LOAD p->on_rq
> >  	 *
> > +	 * Pairs with the LOCK+smp_mb__after_spinlock() on rq->lock in
> > +	 * __schedule().  See the comment for smp_mb__after_spinlock().
> >  	 */
> >  	smp_rmb();
> >  	if (p->on_rq && ttwu_remote(p, wake_flags))
> > @@ -2027,15 +2026,17 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
> >  	 * One must be running (->on_cpu == 1) in order to remove oneself
> >  	 * from the runqueue.
> >  	 *
> > +	 * __schedule() (switch to task 'p')	try_to_wake_up()
> > +	 *   STORE p->on_cpu = 1		  LOAD p->on_rq
> > +	 *   UNLOCK rq->lock
> > +	 *
> > +	 * __schedule() (put 'p' to sleep)
> > +	 *   LOCK rq->lock			  smp_rmb();
> > +	 *   smp_mb__after_spinlock();
> > +	 *   STORE p->on_rq = 0			  LOAD p->on_cpu
> >  	 *
> > +	 * Pairs with the LOCK+smp_mb__after_spinlock() on rq->lock in
> > +	 * __schedule().  See the comment for smp_mb__after_spinlock().
> >  	 */
> >  	smp_rmb();
> 
> Ah yes, good.
> 
> Ack!
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