Message-ID: <474DF207.2090705@o2.pl>
Date: Wed, 28 Nov 2007 23:56:07 +0100
From: Jarek Poplawski <jarkao2@o2.pl>
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To: Jarek Poplawski <jarkao2@o2.pl>
CC: Larry Finger <larry.finger@lwfinger.net>, Andreas Schwab <schwab@suse.de>,
       LKML <linux-kernel@vger.kernel.org>
Subject: Re: Question regarding mutex locking
References: <474CF06C.8020208@lwfinger.net> <jemysy2y3y.fsf@sykes.suse.de> <474D8C22.7010902@lwfinger.net> <474DEF79.1080900@o2.pl>
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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@lwfinger.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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