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Message-ID: <46535AD3.9060904@simon.arlott.org.uk>
Date: Tue, 22 May 2007 22:04:19 +0100
From: Simon Arlott <simon@...e.lp0.eu>
To: Matthias Kaehlcke <matthias.kaehlcke@...il.com>,
Arjan van de Ven <arjan@...radead.org>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: use mutex instead of semaphore in RocketPort driver
On 22/05/07 21:06, Matthias Kaehlcke wrote:
> El Tue, May 22, 2007 at 09:59:01AM -0700 Arjan van de Ven ha dit:
>
>>>
Please provide context when quoting a patch, git grep takes a while...
>>> - 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
>
No. At least one user of tty_operations/tty_driver's write function
doesn't check the return value so it would never be retried, mutex_lock
should be used instead.
All of the _interruptible and functions that return -ERESTARTSYS should
probably use __must_check...
--
Simon Arlott
-
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