linux-kernel - Re: Question regarding mutex locking

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Date:	Wed, 28 Nov 2007 23:56:07 +0100
From:	Jarek Poplawski <jarkao2@...pl>
To:	Jarek Poplawski <jarkao2@...pl>
CC:	Larry Finger <larry.finger@...inger.net>,
	Andreas Schwab <schwab@...e.de>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: Question regarding mutex locking

Jarek Poplawski wrote, On 11/28/2007 11:45 PM:

> Larry Finger wrote, On 11/28/2007 04:41 PM:
> 
>> Andreas Schwab wrote:
>>> Larry Finger <Larry.Finger@...inger.net> writes:
>>>
>>>> If a particular routine needs to lock a mutex, but it may be entered with that mutex already locked,
>>>> would the following code be SMP safe?
>>>>
>>>> hold_lock = mutex_trylock()
>>>>
>>>> ...
>>>>
>>>> if (hold_lock)
>>>> 	mutex_unlock()
>>> When two CPUs may enter the critical region at the same time, what is
>>> the point of the mutex?  Also, the first CPU may unlock the mutex while
>>> the second one is still inside the critical region.
>> Thank you for that answer. I think that I'm finally beginning to understand.
> 
> Probably it would be faster without these "...", which look like
> no man's land...
> 
> hold_lock = mutex_trylock()
> if (hold_lock) {
> 	/* SMP safe */
> 	...
>  	mutex_unlock()
> } else {
> 	/* SMP unsafe */
> 	...
> 	/* maybe try again after some break or check */


OOPS! Of course, since it can be called with this lock held,
any break is not enough: we can only check if there is a
possibility that another thread is holding the lock.

> }
> 
> Regards,
> Jarek P.


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