It looks like there is a race condition between unbinding a framebuffer on a hotplug event and user space trying to flip:

BUG: unable to handle kernel NULL pointer dereference at 0000000000000058 IP: [<ffffffffa008c7d3>] intel_crtc_page_flip+0xc9/0x39c [i915] PGD 114724067 PUD 1145bd067 PMD 0 Oops: 0000 [#1] SMP Pid: 10954, comm: X Not tainted 2.6.35-rc5_stable_20100714+ #1 P5Q-EM/P5Q-EM RIP: 0010:[<ffffffffa008c7d3>] [<ffffffffa008c7d3>] intel_crtc_page_flip+0xc9/0x39c [i915] RSP: 0018:ffff880114927cc8 EFLAGS: 00010282 RAX: 0000000000000000 RBX: ffff88012df48320 RCX: ffff88010c945600 RDX: ffff880001a109c8 RSI: ffff88010c945840 RDI: ffff88012df48320 RBP: ffff880114927d18 R08: ffff88012df48280 R09: ffff88012df48320 R10: 0000000003c2e0b0 R11: 0000000000003246 R12: ffff88010c945840 R13: ffff88012df48000 R14: 0000000000000060 R15: ffff88012dbb8000 FS: 00007f9e6078e830(0000) GS:ffff880001a00000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 000000008005003b CR2: 0000000000000058 CR3: 00000001177a8000 CR4: 00000000000406f0 DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000 DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400 Process X (pid: 10954, threadinfo ffff880114926000, task ffff88012a4a1690) Stack: ffff88010c945600 ffff880115b176c0 ffff88012db10000 0000000000000246 <0> fffffff40006101c ffff88010c945600 00000000ffffffea ffff88010c945600 <0> ffff88012df48320 ffff88011b4b6780 ffff880114927d78 ffffffffa003bd0e Call Trace: [<ffffffffa003bd0e>] drm_mode_page_flip_ioctl+0x1bc/0x214 [drm] [<ffffffffa00311fc>] drm_ioctl+0x25e/0x35e [drm] [<ffffffffa003bb52>] ? drm_mode_page_flip_ioctl+0x0/0x214 [drm] [<ffffffff810f1c3c>] vfs_ioctl+0x2a/0x9e [<ffffffff810f227e>] do_vfs_ioctl+0x531/0x565 [<ffffffff810f2307>] sys_ioctl+0x55/0x77 [<ffffffff810e56d6>] ? sys_read+0x47/0x6f [<ffffffff81002a2b>] system_call_fastpath+0x16/0x1b Code: 45 d4 f4 ff ff ff 0f 84 e0 02 00 00 48 8b 4d b0 49 8d 9d 20 03 00 00 48 89 df 49 89 4c 24 38 49 8b 07 49 89 44 24 20 49 8b 47 20 <48> 8b 40 58 49 c7 04 24 00 00 00 00 49 c7 44 24 18 a9 a5 08 a0 RIP [<ffffffffa008c7d3>] intel_crtc_page_flip+0xc9/0x39c [i915] RSP <ffff880114927cc8> CR2: 0000000000000058

References:

Bug 28811 - [page-flipping] GPU hang when modeset after unplugging another monitor (under compiz) https://bugs.freedesktop.org/show_bug.cgi?id=28811

Signed-off-by: Chris Wilson chris@chris-wilson.co.uk Cc: Jesse Barnes jbarnes@virtuousgeek.org Cc: Dave Airlie airlied@redhat.com --- drivers/gpu/drm/drm_crtc.c | 9 +++++++++ 1 files changed, 9 insertions(+), 0 deletions(-)

diff --git a/drivers/gpu/drm/drm_crtc.c b/drivers/gpu/drm/drm_crtc.c index 0371a2f..d9d5e9f 100644 --- a/drivers/gpu/drm/drm_crtc.c +++ b/drivers/gpu/drm/drm_crtc.c @@ -2610,6 +2610,15 @@ int drm_mode_page_flip_ioctl(struct drm_device *dev, goto out; crtc = obj_to_crtc(obj);

+ if (crtc->fb == NULL) { + /* The framebuffer is currently unbound, presumably + * due to a hotplug event, that userspace has not + * yet discovered. + */ + ret = -EBUSY; + goto out; + } + if (crtc->funcs->page_flip == NULL) goto out;