linux-kernel - Re: use mutex instead of semaphore in RocketPort driver

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Date:	Tue, 22 May 2007 22:06:02 +0200
From:	Matthias Kaehlcke <matthias.kaehlcke@...il.com>
To:	Arjan van de Ven <arjan@...radead.org>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: use mutex instead of semaphore in RocketPort driver

El Tue, May 22, 2007 at 09:59:01AM -0700 Arjan van de Ven ha dit:

> >  
> > -	down_interruptible(&info->write_sem);
> > +	mutex_lock_interruptible(&info->write_mtx);
> >  
> >  #ifdef ROCKET_DEBUG_WRITE
> >  	printk(KERN_INFO "rp_write %d chars...", count);
> > @@ -1773,7 +1776,7 @@ end:
> >  		wake_up_interruptible(&tty->poll_wait);
> >  #endif
> >  	}
> > -	up(&info->write_sem);
> > +	mutex_unlock(&info->write_mtx);
> >  	return retval
> 
> this code is very very buggy.

more buggy than with the use of a semaphore?
 
> mutex_lock_interruptible() may not get the mutex in case a signal
> happens... and yet you unlox the mutex unconditionally!!!

as far as i understand only the thread that locked the mutex can
unlock it (as opposed to semaphores, which can be released by any
thread/process). obviously this doesn't make the code be more
correct. what i don't know is how the kernel behaves when
trying to unlock a mutex the thread doesn't own. another and possibly
more important problem of the code is that in case of being
interrupted by a signal the data that should be protected by the
mutex/semaphore can be accessed/changed by two threads at the same
time.

would the following resolve the problem?

if(mutex_lock_interruptible(&info->write_mtx)) 
        return -ERESTARTSYS

thanks for your comments

-- 
Matthias Kaehlcke
Linux Application Developer
Barcelona

            Nothing is more despicable than respect based on fear
                              (Albert Camus)
                                                                 .''`.
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