linux-kernel - Re: Question regarding mutex locking

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Message-ID: <474DF4CB.4090606@o2.pl>
Date:	Thu, 29 Nov 2007 00:07:55 +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 11:56 PM:

> 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 */


...But, not for sure! If our caller holds the lock and we can
check this...

>> 	...
>> 	/* 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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