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

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Message-ID: <a781481a0704242206i569e4768oe4b34272181f1ce@mail.gmail.com>
Date:	Wed, 25 Apr 2007 10:36:38 +0530
From:	"Satyam Sharma" <satyam.sharma@...il.com>
To:	"Robert Hancock" <hancockr@...w.ca>
Cc:	"Matthias Kaehlcke" <matthias.kaehlcke@...il.com>,
	"Oliver Neukum" <oneukum@...e.de>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] use mutex instead of semaphore in RocketPort driver

Hi Matthias,

On 4/25/07, Robert Hancock <hancockr@...w.ca> wrote:
> Matthias Kaehlcke wrote:
> > El Tue, Apr 24, 2007 at 07:53:04PM +0200 Oliver Neukum ha dit:
> >
> >> Am Dienstag, 24. April 2007 19:49 schrieb Matthias Kaehlcke:
> >>> @@ -1706,7 +1706,7 @@ static int rp_write(struct tty_struct *tty,
> >>>         if (count <= 0 || rocket_paranoia_check(info, "rp_write"))
> >>>                 return 0;
> >>>
> >>> -       down_interruptible(&info->write_sem);
> >>> +       mutex_lock_interruptible(&info->write_mtx);
> >> This is a bug. It is also present in the current code, but nevertheless
> >> it is a bug. If you use an interruptible lock, you must be ready to deal
> >> with interrupts, which are ignored by this code.
> > [...]
> > i'm a bit confused now about the interruptible locks, i thought using
> > them means that the process will be waked up when receiving a
> > signal. what role are playing interrupts when using interruptible locks?
>
> You are correct, interrupts aren't involved. However if the wait is
> interrupted by a signal, mutex_lock_interruptible will return a nonzero
> return code which needs to be checked for (and likely -ERESTARTSYS or
> -EINTR returned), otherwise the code will blindly continue as though it
> has locked the mutex even though it has not.

Think I'll elaborate Robert's explanation for your benefit :-) Unlike
mutex_lock() and down() that put the task to TASK_UNINTERRUPTIBLE
sleep if the lock can't be acquired immediately,
mutex_lock_interruptible() and down_interruptible() sleep in
TASK_INTERRUPTIBLE state. So the task _can_ be woken up (without even
acquiring the lock) by incoming signals. When that happens, we can't
just blindly go on ... so the return values of the _interruptible()
versions of the locking functions *must* be checked for success and if
not, the task should return with error.

Use -ERESTARTSYS if a previous intermediate caller checks this return
value and tries and restarts the whole operation. If no such previous
caller exists (and/or introducing it would involve a change in kernel
behaviour as seen from userspace), you can safely use -EINTR. The goal
is that userspace must not get to see -ERESTARTSYS.
-
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