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

[Peter Zijlstra]

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.


>   * 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!