linux-kernel - Re: [PATCH v2 1/7] locking/rwsem: check for active writer/spinner before wakeup

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Date:	Fri, 08 Aug 2014 12:50:12 -0700
From:	Jason Low <jason.low2@...com>
To:	Davidlohr Bueso <davidlohr@...com>
Cc:	Waiman Long <Waiman.Long@...com>, Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, Scott J Norton <scott.norton@...com>,
	aswin@...com
Subject: Re: [PATCH v2 1/7] locking/rwsem: check for active writer/spinner
 before wakeup


>  __visible __used noinline
> @@ -730,6 +744,23 @@ __mutex_unlock_common_slowpath(struct mutex *lock, int nested)
>  	if (__mutex_slowpath_needs_to_unlock())
>  		atomic_set(&lock->count, 1);
>  
> +/*
> + * Skipping the mutex_has_owner() check when DEBUG, allows us to
> + * avoid taking the wait_lock in order to do not call mutex_release()
> + * and debug_mutex_unlock() when !DEBUG. This can otherwise result in
> + * deadlocks when another task enters the lock's slowpath in mutex_lock().
> + */
> +#ifndef CONFIG_DEBUG_MUTEXES
> +	/*
> +	 * Abort the wakeup operation if there is an another mutex owner, as the
> +	 * lock was stolen. mutex_unlock() should have cleared the owner field
> +	 * before calling this function. If that field is now set, another task
> +	 * must have acquired the mutex.
> +	 */
> +	if (mutex_has_owner(lock))
> +		return;

Would we need the mutex lock count to eventually get set to a negative
value if there are waiters? An optimistic spinner can get the lock and
set lock->count to 0. Then the lock count might remain 0 since a waiter
might not get waken up here to try-lock and set lock->count to -1 if it
goes back to sleep in the lock path.


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