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SubjectRe: [Bug 11824][PATCH] ieee1394: raw1394: fix possible deadlock in multithreaded clients

* 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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