Re: [PATCH v2 1/2] locking: Implement an algorithm choice for Wound-Wait mutexes

[Thomas Hellstrom]

Hi, Peter,

On 06/14/2018 02:41 PM, Peter Zijlstra wrote:
> On Thu, Jun 14, 2018 at 09:29:21AM +0200, Thomas Hellstrom wrote:
> > +static bool __ww_mutex_wound(struct mutex *lock,
> > +			     struct ww_acquire_ctx *ww_ctx,
> > +			     struct ww_acquire_ctx *hold_ctx)
> > +{
> > +	struct task_struct *owner = __mutex_owner(lock);
> > +
> > +	lockdep_assert_held(&lock->wait_lock);
> > +
> > +	if (owner && hold_ctx && __ww_ctx_stamp_after(hold_ctx, ww_ctx) &&
> > +	    ww_ctx->acquired > 0) {
> > +		hold_ctx->wounded = 1;
> > +
> > +		/*
> > +		 * wake_up_process() paired with set_current_state() inserts
> > +		 * sufficient barriers to make sure @owner either sees it's
> > +		 * wounded or has a wakeup pending to re-read the wounded
> > +		 * state.
> > +		 *
> > +		 * The value of hold_ctx->wounded in
> > +		 * __ww_mutex_lock_check_stamp();
> > +		 */
> > +		if (owner != current)
> > +			wake_up_process(owner);
> > +
> > +		return true;
> > +	}
> > +
> > +	return false;
> > +}
> > @@ -338,12 +377,18 @@ ww_mutex_set_context_fastpath(struct ww_mutex *lock, struct ww_acquire_ctx *ctx)
> >   	 * and keep spinning, or it will acquire wait_lock, add itself
> >   	 * to waiter list and sleep.
> >   	 */
> > -	smp_mb(); /* ^^^ */
> > +	smp_mb(); /* See comments above and below. */
> >   
> >   	/*
> > -	 * Check if lock is contended, if not there is nobody to wake up
> > +	 * Check if lock is contended, if not there is nobody to wake up.
> > +	 * We can use list_empty() unlocked here since it only compares a
> > +	 * list_head field pointer to the address of the list head
> > +	 * itself, similarly to how list_empty() can be considered RCU-safe.
> > +	 * The memory barrier above pairs with the memory barrier in
> > +	 * __ww_mutex_add_waiter and makes sure lock->ctx is visible before
> > +	 * we check for waiters.
> >   	 */
> > -	if (likely(!(atomic_long_read(&lock->base.owner) & MUTEX_FLAG_WAITERS)))
> > +	if (likely(list_empty(&lock->base.wait_list)))
> >   		return;
> >   
> OK, so what happens is that if we see !empty list, we take wait_lock,
> if we end up in __ww_mutex_wound() we must really have !empty wait-list.
>
> It can however still see !owner because __mutex_unlock_slowpath() can
> clear the owner field. But if owner is set, it must stay valid because
> FLAG_WAITERS and we're holding wait_lock.

If __ww_mutex_wound() is called from ww_mutex_set_context_fastpath() 
owner is the current process so we can never see !owner. However if 
__ww_mutex_wound() is called from __ww_mutex_add_waiter() then the above 
is true.

> So the wake_up_process() is in fact safe.
>
> Let me put that in a comment.


Thanks,

Thomas