Re: use mutex instead of semaphore in RocketPort driver

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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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