Date: Mon, 27 Oct 2008 11:13:03 +0100
From: Ingo Molnar <mingo@elte.hu>
To: Stefan Richter <stefanr@s5r6.in-berlin.de>
Cc: linux1394-devel@lists.sourceforge.net, bugme-daemon@bugzilla.kernel.org,
       linux-kernel@vger.kernel.org, Dan Dennedy <dan@dennedy.org>,
       Johannes Weiner <hannes@saeurebad.de>,
       Peter Zijlstra <a.p.zijlstra@chello.nl>
Subject: Re: [Bug 11824][PATCH] ieee1394: raw1394: fix possible deadlock in
	multithreaded clients
Message-ID: <20081027101303.GI8116@elte.hu>
References: <bug-11824-4803@http.bugzilla.kernel.org/> <4902F41E.5070306@s5r6.in-berlin.de> <tkrat.94e673551b87c230@s5r6.in-berlin.de>
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* Stefan Richter <stefanr@s5r6.in-berlin.de> wrote:

> Regression in 2.6.28-rc1:  When I added the new state_mutex which
> prevents corruption of raw1394's internal state when accessed by
> multithreaded client applications, the following possible though
> highly unlikely deadlock slipped in:
> 
> Thread A:                  Thread B:
>  - acquire mmap_sem         - raw1394_write() or raw1394_ioctl()
>  - raw1394_mmap()           - acquire state_mutex
>  - acquire state_mutex      - copy_to/from_user(), possible page fault:
>                               acquire mmap_sem
> 
> The simplest fix is to use mutex_trylock() instead of mutex_lock() in
> raw1394_mmap().  This changes the behavior under contention in a way
> which is visible to userspace clients.  However, since multithreaded
> access was entirely buggy before state_mutex was added and libraw1394's
> documentation advised application programmers to use a handle only in a
> single thread, this change in behaviour should not be an issue in
> practice at all.
> 
> Since we have to use mutex_trylock() in raw1394_mmap() regardless
> whether /dev/raw1394 was opened with O_NONBLOCK or not, we now use
> mutex_trylock() unconditionally everywhere for state_mutex, just to have
> consistent behavior.
> 
> Reported-by: Johannes Weiner <hannes@saeurebad.de>
> Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>
> ---
> 
> Background:  That new state_mutex went only in because raw1394_ioctl()
> already head some weak protection by the Big Kernel Lock, which I
> removed for the general reasons pro BKL removal (get better performance
> with local locks; make the locking clearer, easier to debug, more
> reliable).
> 
>  drivers/ieee1394/raw1394.c |    9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
> 
> Index: linux/drivers/ieee1394/raw1394.c
> ===================================================================
> --- linux.orig/drivers/ieee1394/raw1394.c
> +++ linux/drivers/ieee1394/raw1394.c
> @@ -2268,7 +2268,8 @@ static ssize_t raw1394_write(struct file
>  		return -EFAULT;
>  	}
>  
> -	mutex_lock(&fi->state_mutex);
> +	if (!mutex_trylock(&fi->state_mutex))
> +		return -EAGAIN;
>  
>  	switch (fi->state) {
>  	case opened:
> @@ -2548,7 +2549,8 @@ static int raw1394_mmap(struct file *fil
>  	struct file_info *fi = file->private_data;
>  	int ret;
>  
> -	mutex_lock(&fi->state_mutex);
> +	if (!mutex_trylock(&fi->state_mutex))
> +		return -EAGAIN;
>  
>  	if (fi->iso_state == RAW1394_ISO_INACTIVE)
>  		ret = -EINVAL;
> @@ -2669,7 +2671,8 @@ static long raw1394_ioctl(struct file *f
>  		break;
>  	}
>  
> -	mutex_lock(&fi->state_mutex);
> +	if (!mutex_trylock(&fi->state_mutex))
> +		return -EAGAIN;

So we can return a spurious -EAGAIN to user-space, if the state_mutex 
is taken briefly by some other context?

	Ingo
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