linux-kernel - Re: Question regarding mutex locking

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Date:	Wed, 28 Nov 2007 23:45:13 +0100
From:	Jarek Poplawski <jarkao2@...pl>
To:	Larry Finger <larry.finger@...inger.net>
CC:	Andreas Schwab <schwab@...e.de>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: Question regarding mutex locking

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 */
}

Regards,
Jarek P.
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