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>
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@ -2268,7 +2268,8 @@ static ssize_t raw1394_write(struct file *file, const char __user * buffer,
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return -EFAULT;
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}
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mutex_lock(&fi->state_mutex);
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if (!mutex_trylock(&fi->state_mutex))
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return -EAGAIN;
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switch (fi->state) {
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case opened:
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@ -2548,7 +2549,8 @@ static int raw1394_mmap(struct file *file, struct vm_area_struct *vma)
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struct file_info *fi = file->private_data;
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int ret;
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mutex_lock(&fi->state_mutex);
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if (!mutex_trylock(&fi->state_mutex))
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return -EAGAIN;
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if (fi->iso_state == RAW1394_ISO_INACTIVE)
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ret = -EINVAL;
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@ -2669,7 +2671,8 @@ static long raw1394_ioctl(struct file *file, unsigned int cmd,
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break;
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}
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mutex_lock(&fi->state_mutex);
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if (!mutex_trylock(&fi->state_mutex))
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return -EAGAIN;
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switch (fi->iso_state) {
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case RAW1394_ISO_INACTIVE:
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