linux-kernel - Re: [PATCH v2 04/11] locking/ww_mutex: Set use_ww

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Date:   Tue, 6 Dec 2016 16:25:37 +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 04/11] locking/ww_mutex: Set use_ww_ctx even when
 locking without a context

On Thu, Dec 01, 2016 at 03:06:47PM +0100, Nicolai Hähnle wrote:

> @@ -640,10 +640,11 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
>  	struct mutex_waiter waiter;
>  	unsigned long flags;
>  	bool first = false;
> -	struct ww_mutex *ww;
>  	int ret;
>  
> -	if (use_ww_ctx) {
> +	if (use_ww_ctx && ww_ctx) {
> +		struct ww_mutex *ww;
> +
>  		ww = container_of(lock, struct ww_mutex, base);
>  		if (unlikely(ww_ctx == READ_ONCE(ww->ctx)))
>  			return -EALREADY;

So I don't see the point of removing *ww from the function scope, we can
still compute that container_of() even if !ww_ctx, right? That would
safe a ton of churn below, adding all those struct ww_mutex declarations
and container_of() casts.

(and note that the container_of() is a fancy NO-OP because base is the
first member).

> @@ -656,8 +657,12 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
>  	    mutex_optimistic_spin(lock, ww_ctx, use_ww_ctx, false)) {
>  		/* got the lock, yay! */
>  		lock_acquired(&lock->dep_map, ip);
> -		if (use_ww_ctx)
> +		if (use_ww_ctx && ww_ctx) {
> +			struct ww_mutex *ww;
> +
> +			ww = container_of(lock, struct ww_mutex, base);
>  			ww_mutex_set_context_fastpath(ww, ww_ctx);
> +		}
>  		preempt_enable();
>  		return 0;
>  	}
> @@ -702,7 +707,7 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
>  			goto err;
>  		}
>  
> -		if (use_ww_ctx && ww_ctx->acquired > 0) {
> +		if (use_ww_ctx && ww_ctx && ww_ctx->acquired > 0) {
>  			ret = __ww_mutex_lock_check_stamp(lock, ww_ctx);
>  			if (ret)
>  				goto err;
> @@ -742,8 +747,12 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
>  	/* got the lock - cleanup and rejoice! */
>  	lock_acquired(&lock->dep_map, ip);
>  
> -	if (use_ww_ctx)
> +	if (use_ww_ctx && ww_ctx) {
> +		struct ww_mutex *ww;
> +
> +		ww = container_of(lock, struct ww_mutex, base);
>  		ww_mutex_set_context_slowpath(ww, ww_ctx);
> +	}
>  
>  	spin_unlock_mutex(&lock->wait_lock, flags);
>  	preempt_enable();

All that then reverts to:

-	if (use_ww_ctx)
+	if (use_ww_ctx && ww_ctx)