We need to hold the dev->mode_config.mutex whilst detecting the output
status. But we also need to drop it for the call into
drm_fb_helper_single_fb_probe(), which indirectly acquires the lock when
attaching the fbcon.
Failure to do so exposes a race with normal output probing. Detected by
adding some warnings that the mutex is held to the backend detect routines:
[ 17.772456] WARNING: at drivers/gpu/drm/i915/intel_crt.c:471 intel_crt_detect+0x3e/0x373 [i915]()
[ 17.772458] Hardware name: Latitude E6400
[ 17.772460] Modules linked in: ....
[ 17.772582] Pid: 11, comm: kworker/0:1 Tainted: G W 2.6.38.4-custom.2 #8
[ 17.772584] Call Trace:
[ 17.772591] [<ffffffff81046af5>] ? warn_slowpath_common+0x78/0x8c
[ 17.772603] [<ffffffffa03f3e5c>] ? intel_crt_detect+0x3e/0x373 [i915]
[ 17.772612] [<ffffffffa0355d49>] ? drm_helper_probe_single_connector_modes+0xbf/0x2af [drm_kms_helper]
[ 17.772619] [<ffffffffa03534d5>] ? drm_fb_helper_probe_connector_modes+0x39/0x4d [drm_kms_helper]
[ 17.772625] [<ffffffffa0354760>] ? drm_fb_helper_hotplug_event+0xa5/0xc3 [drm_kms_helper]
[ 17.772633] [<ffffffffa035577f>] ? output_poll_execute+0x146/0x17c [drm_kms_helper]
[ 17.772638] [<ffffffff81193c01>] ? cfq_init_queue+0x247/0x345
[ 17.772644] [<ffffffffa0355639>] ? output_poll_execute+0x0/0x17c [drm_kms_helper]
[ 17.772648] [<ffffffff8105b540>] ? process_one_work+0x193/0x28e
[ 17.772652] [<ffffffff8105c6bc>] ? worker_thread+0xef/0x172
[ 17.772655] [<ffffffff8105c5cd>] ? worker_thread+0x0/0x172
[ 17.772658] [<ffffffff8105c5cd>] ? worker_thread+0x0/0x172
[ 17.772663] [<ffffffff8105f767>] ? kthread+0x7a/0x82
[ 17.772668] [<ffffffff8100a724>] ? kernel_thread_helper+0x4/0x10
[ 17.772671] [<ffffffff8105f6ed>] ? kthread+0x0/0x82
[ 17.772674] [<ffffffff8100a720>] ? kernel_thread_helper+0x0/0x10
Reported-by: Frederik Himpe <fhimpe@telenet.be>
References: https://bugs.freedesktop.org/show_bug.cgi?id=36394
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Dave Airlie <airlied@redhat.com>
752d2635eb