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

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Message-ID: <20160818142735.GB10121@twins.programming.kicks-ass.net>
Date:	Thu, 18 Aug 2016 16:27:35 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Jason Low <jason.low2@....com>
Cc:	Ingo Molnar <mingo@...hat.com>, imre.deak@...el.com,
	linux-kernel@...r.kernel.org, Jason Low <jason.low2@...com>,
	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, Aug 10, 2016 at 11:44:08AM -0700, Jason Low wrote:
> @@ -556,8 +604,12 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
>  		 * other waiters. We only attempt the xchg if the count is
>  		 * non-negative in order to avoid unnecessary xchg operations:
>  		 */
> -		if (atomic_read(&lock->count) >= 0 &&
> +		if ((!need_yield_to_waiter(lock) || wakeups > 1) &&
> +		    atomic_read(&lock->count) >= 0 &&
>  		    (atomic_xchg_acquire(&lock->count, -1) == 1))
> +			if (wakeups > 1)
> +				clear_yield_to_waiter(lock);
> +
>  			break;
>  
>  		/*

There's some { } gone missing there...

Also, I think I'll change it to avoid that extra wakeups > 1 condition..