Messages in this thread | | | Date | Mon, 27 Oct 2008 11:13:03 +0100 | From | Ingo Molnar <> | Subject | Re: [Bug 11824][PATCH] ieee1394: raw1394: fix possible deadlock in multithreaded clients |
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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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