Re: [patch 5/9] mutex subsystem, core

[Oleg Nesterov <oleg@tv-sign.ru>]

Ingo Molnar wrote:
>
> +__mutex_lock_common(struct mutex *lock, struct mutex_waiter *waiter,
> +		    struct thread_info *ti,
> +		    unsigned long task_state __IP_DECL__)
> +{
> +	struct task_struct *task = ti->task;
> +	unsigned int old_val;
> +
> +	/*
> +	 * Lets try to take the lock again - this is needed even if
> +	 * we get here for the first time (shortly after failing to
> +	 * acquire the lock), to make sure that we get a wakeup once
> +	 * it's unlocked. Later on this is the operation that gives
> +	 * us the lock. If there are other waiters we need to xchg it
> +	 * to -1, so that when we release the lock, we properly wake
> +	 * up the other waiters:
> +	 */
> +	old_val = atomic_xchg(&lock->count, -1);
> +
> +	if (unlikely(old_val == 1)) {
> +		/*
> +		 * Got the lock - rejoice! But there's one small
> +		 * detail to fix up: above we have set the lock to -1,
> +		 * unconditionally. But what if there are no waiters?
> +		 * While it would work with -1 too, 0 is a better value
> +		 * in that case, because we wont hit the slowpath when
> +		 * we release the lock. We can simply use atomic_set()
> +		 * for this, because we are the owners of the lock now,
> +		 * and are still holding the wait_lock:
> +		 */
> +		if (likely(list_empty(&lock->wait_list)))
> +			atomic_set(&lock->count, 0);

This is a minor issue, but still I think it makes sense to optimize
for uncontended case:

	old_val = atomic_xchg(&lock->count, 0); // no sleepers

	if (old_val == 1) {
		// sleepers ?
		if (!list_empty(&lock->wait_list))
			// need to wakeup them
			atomic_set(&lock->count, -1);
		...
	}

Oleg.
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