ieee1394: raw1394: fix possible deadlock in multithreaded clients
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>
diff --git a/drivers/ieee1394/raw1394.c b/drivers/ieee1394/raw1394.c
index 2cf4ae7..4bdfff0 100644
--- a/drivers/ieee1394/raw1394.c
+++ b/drivers/ieee1394/raw1394.c
@@ -2268,7 +2268,8 @@
return -EFAULT;
}
- mutex_lock(&fi->state_mutex);
+ if (!mutex_trylock(&fi->state_mutex))
+ return -EAGAIN;
switch (fi->state) {
case opened:
@@ -2548,7 +2549,8 @@
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 @@
break;
}
- mutex_lock(&fi->state_mutex);
+ if (!mutex_trylock(&fi->state_mutex))
+ return -EAGAIN;
switch (fi->iso_state) {
case RAW1394_ISO_INACTIVE: