Comment 4 for bug 912863

dmesg extract:

[ 1421.209814] Process Xorg (pid: 1255, threadinfo ffff880412086000, task ffff880421851710)
[ 1421.211153] Stack:
[ 1421.212475] ffff880412087498 ffffffff81650886 3830000000000000 ffffffffa00bc1a7
[ 1421.213818] ffffffffa00b5284 ffff880412087500 ffffffffa00b5284 ffffffffa00bc1c0
[ 1421.215160] ffff8804120874e8 ffff8804218cd820 ffff8804218cd800 ffff8803b253a180
[ 1421.216496] Call Trace:
[ 1421.217823] [<ffffffff81650886>] __mutex_lock_slowpath+0x46/0x150
[ 1421.219154] [<ffffffff8165052a>] mutex_lock+0x2a/0x50
[ 1421.220476] [<ffffffffa00f9b13>] intel_pipe_set_base+0xa3/0x2f0 [i915]
[ 1421.221793] [<ffffffff81311fd4>] ? snprintf+0x34/0x40
[ 1421.223112] [<ffffffffa006ef78>] drm_crtc_helper_set_config+0x758/0xaf0 [drm_kms_helper]
[ 1421.224443] [<ffffffff81351fb1>] ? fb_set_var+0x1a1/0x310
[ 1421.225763] [<ffffffffa006b786>] drm_fb_helper_pan_display+0x86/0xd0 [drm_kms_helper]
[ 1421.227092] [<ffffffff813516ed>] fb_pan_display+0xbd/0x170
[ 1421.228420] [<ffffffff813624f9>] bit_update_start+0x29/0x60
[ 1421.229744] [<ffffffff81360df2>] fbcon_switch+0x3b2/0x560
[ 1421.231071] [<ffffffff813ca919>] redraw_screen+0x189/0x2a0
[ 1421.232336] [<ffffffff8135f4da>] fbcon_blank+0x21a/0x2d0
[ 1421.233532] [<ffffffff8108e176>] ? down_trylock+0x36/0x50
[ 1421.234730] [<ffffffff8106584c>] ? console_trylock+0x1c/0x70
[ 1421.235926] [<ffffffff8103bb29>] ? default_spin_lock_flags+0x9/0x10
[ 1421.237122] [<ffffffff81651cae>] ? _raw_spin_lock_irqsave+0x2e/0x40
[ 1421.238321] [<ffffffff810751d8>] ? lock_timer_base.isra.29+0x38/0x70
[ 1421.239526] [<ffffffff81076593>] ? mod_timer+0x143/0x2f0
[ 1421.240723] [<ffffffff813cb61c>] do_unblank_screen.part.20+0x9c/0x170
[ 1421.241911] [<ffffffff813cb738>] do_unblank_screen+0x48/0x80
[ 1421.243092] [<ffffffff813cb780>] unblank_screen+0x10/0x20
[ 1421.244260] [<ffffffff8131532d>] bust_spinlocks+0x1d/0x40
[ 1421.245425] [<ffffffff81652b2f>] oops_end+0x3f/0xf0
[ 1421.246587] [<ffffffff81016858>] die+0x58/0x90
[ 1421.247741] [<ffffffff81652514>] do_trap+0xc4/0x170
[ 1421.248890] [<ffffffff81013f15>] do_invalid_op+0x95/0xb0
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