tree 4b05a8e120a297db144b79bbcda8c409ee0a51bf
parent f4af65958a6ea987ff61504ad9f053f8ba8da674
author Hans de Goede <hdegoede@redhat.com> 1318162606 -0300
committer Mauro Carvalho Chehab <mchehab@redhat.com> 1325853857 -0200

[media] pwc: Rework locking

While testing gtk-v4l's new ctrl event code, I hit the following deadlock
in the pwc driver:

Thread 1:
-Does a VIDIOC_G_CTRL
-video2_ioctl takes the modlock
-video2_ioctl calls v4l2_g_ctrl
-v4l2_g_ctrl takes the ctrl_handler lock
-v4l2_g_ctrl calls pwc_g_volatile_ctrl
-pwc_g_volatile_ctrl releases the modlock as the usb transfer can take a
 significant amount of time and we don't want to block DQBUF / QBUF too long
Thread 2:
-Does a VIDIOC_FOO_CTRL
-video2_ioctl takes the modlock
-video2_ioctl calls v4l2_foo_ctrl
-v4l2_foo_ctrl blocks while trying to take the ctrl_handler lock
Thread 1:
-Blocks while trying to re-take the modlock, as its caller will eventually
 unlock that

Now we have thread 1 waiting for the modlock while holding the ctrl_handler
lock and thread 2 waiting for the ctrl_handler lock while holding the
modlock -> deadlock.

Conclusion:
1) We cannot unlock modlock from pwc_s_ctrl / pwc_g_volatile_ctrl,
   but this can cause QBUF / DQBUF to block for up to a full second
2) After evaluating various option I came to the conclusion that pwc should
   stop using the v4l2 core locking, and instead do its own locking

Thus this patch stops pwc using the v4l2 core locking, and replaces that with
it doing its own locking where necessary.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>