linux-kernel - Re: [PATCH v2] locking/mutex: Prevent lock starvation when spinning is enabled

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Date:	Wed, 10 Aug 2016 19:30:26 -0700
From:	Jason Low <jason.low2@....com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	jason.low2@....com, Ingo Molnar <mingo@...hat.com>,
	imre.deak@...el.com, linux-kernel@...r.kernel.org,
	Waiman Long <Waiman.Long@....com>,
	Davidlohr Bueso <dave@...olabs.net>,
	Tim Chen <tim.c.chen@...ux.intel.com>, terry.rudd@....com,
	"Paul E. McKenney" <paulmck@...ibm.com>
Subject: Re: [PATCH v2] locking/mutex: Prevent lock starvation when spinning
 is enabled

On Wed, 2016-08-10 at 11:44 -0700, Jason Low wrote:
> @@ -917,11 +976,12 @@ EXPORT_SYMBOL(mutex_trylock);
>  int __sched
>  __ww_mutex_lock(struct ww_mutex *lock, struct ww_acquire_ctx *ctx)
>  {
> -	int ret;
> +	int ret = 1;
>  
>  	might_sleep();
>  
> -	ret = __mutex_fastpath_lock_retval(&lock->base.count);
> +	if (!need_yield_to_waiter(lock))
> +		ret = __mutex_fastpath_lock_retval(&lock->base.count);
>  
>  	if (likely(!ret)) {
>  		ww_mutex_set_context_fastpath(lock, ctx);
> @@ -935,11 +995,12 @@ EXPORT_SYMBOL(__ww_mutex_lock);
>  int __sched
>  __ww_mutex_lock_interruptible(struct ww_mutex *lock, struct ww_acquire_ctx *ctx)
>  {
> -	int ret;
> +	int ret = 1;
>  
>  	might_sleep();
>  
> -	ret = __mutex_fastpath_lock_retval(&lock->base.count);
> +	if (!need_yield_to_waiter(lock))

And we would need to pass &lock->base instead of lock since lock is
struct ww_mutex * here.