linux-kernel - Re: [PATCH v2 02/11] locking/ww_mutex: Re-check ww->ctx in the inner optimistic spin loop

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Date:   Tue, 6 Dec 2016 16:06:20 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Nicolai Hähnle <nhaehnle@...il.com>
Cc:     linux-kernel@...r.kernel.org,
        Nicolai Hähnle <Nicolai.Haehnle@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Maarten Lankhorst <dev@...ankhorst.nl>,
        Daniel Vetter <daniel@...ll.ch>,
        Chris Wilson <chris@...is-wilson.co.uk>,
        dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH v2 02/11] locking/ww_mutex: Re-check ww->ctx in the inner
 optimistic spin loop

On Thu, Dec 01, 2016 at 03:06:45PM +0100, Nicolai Hähnle wrote:
> +++ b/kernel/locking/mutex.c
> @@ -350,7 +350,8 @@ ww_mutex_set_context_slowpath(struct ww_mutex *lock,
>   * access and not reliable.
>   */
>  static noinline
> -bool mutex_spin_on_owner(struct mutex *lock, struct task_struct *owner)
> +bool mutex_spin_on_owner(struct mutex *lock, struct task_struct *owner,
> +			 bool use_ww_ctx, struct ww_acquire_ctx *ww_ctx)
>  {
>  	bool ret = true;
>  
> @@ -373,6 +374,28 @@ bool mutex_spin_on_owner(struct mutex *lock, struct task_struct *owner)
>  			break;
>  		}
>  
> +		if (use_ww_ctx && ww_ctx->acquired > 0) {
> +			struct ww_mutex *ww;
> +
> +			ww = container_of(lock, struct ww_mutex, base);
> +
> +			/*
> +			 * If ww->ctx is set the contents are undefined, only
> +			 * by acquiring wait_lock there is a guarantee that
> +			 * they are not invalid when reading.
> +			 *
> +			 * As such, when deadlock detection needs to be
> +			 * performed the optimistic spinning cannot be done.
> +			 *
> +			 * Check this in every inner iteration because we may
> +			 * be racing against another thread's ww_mutex_lock.
> +			 */
> +			if (READ_ONCE(ww->ctx)) {
> +				ret = false;
> +				break;
> +			}
> +		}
> +
>  		cpu_relax();
>  	}
>  	rcu_read_unlock();

Aside from the valid question about mutex_can_spin_on_owner() there's
another 'problem' here, mutex_spin_on_owner() is marked noinline, so all
the use_ww_ctx stuff doesn't 'work' here.

As is, I think we're missing an __always_inline on
mutex_optimistic_spin, I'll have to go see what that does for code
generation, but given both @use_ww_ctx and @waiter there that makes
sense.