linux-kernel - Re: [PATCH v3 2/2] locking: Implement an algorithm choice for Wound-Wait mutexes

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Date:   Fri, 15 Jun 2018 18:46:04 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Thomas Hellstrom <thellstrom@...are.com>
Cc:     dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        Ingo Molnar <mingo@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Gustavo Padovan <gustavo@...ovan.org>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Sean Paul <seanpaul@...omium.org>,
        David Airlie <airlied@...ux.ie>,
        Davidlohr Bueso <dave@...olabs.net>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Josh Triplett <josh@...htriplett.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Kate Stewart <kstewart@...uxfoundation.org>,
        Philippe Ombredanne <pombredanne@...b.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-doc@...r.kernel.org, linux-media@...r.kernel.org,
        linaro-mm-sig@...ts.linaro.org, stern@...land.harvard.edu
Subject: Re: [PATCH v3 2/2] locking: Implement an algorithm choice for
 Wound-Wait mutexes

On Fri, Jun 15, 2018 at 02:08:27PM +0200, Thomas Hellstrom wrote:

> @@ -772,6 +856,25 @@ __ww_mutex_add_waiter(struct mutex_waiter *waiter,
>  	}
>  
>  	list_add_tail(&waiter->list, pos);
> +	if (__mutex_waiter_is_first(lock, waiter))
> +		__mutex_set_flag(lock, MUTEX_FLAG_WAITERS);
> +
> +	/*
> +	 * Wound-Wait: if we're blocking on a mutex owned by a younger context,
> +	 * wound that such that we might proceed.
> +	 */
> +	if (!is_wait_die) {
> +		struct ww_mutex *ww = container_of(lock, struct ww_mutex, base);
> +
> +		/*
> +		 * See ww_mutex_set_context_fastpath(). Orders setting
> +		 * MUTEX_FLAG_WAITERS (atomic operation) vs the ww->ctx load,
> +		 * such that either we or the fastpath will wound @ww->ctx.
> +		 */
> +		smp_mb__after_atomic();
> +
> +		__ww_mutex_wound(lock, ww_ctx, ww->ctx);
> +	}

I think we want the smp_mb__after_atomic() in the same branch as
__mutex_set_flag(). So something like:

	if (__mutex_waiter_is_first()) {
		__mutex_set_flag();
		if (!is_wait_die)
			smp_mb__after_atomic();
	}

Or possibly even without the !is_wait_die. The rules for
smp_mb__*_atomic() are such that we want it unconditional after an
atomic, otherwise the semantics get too fuzzy.

Alan (rightfully) complained about that a while ago when he was auditing
users.