Re: [PATCH] drm: fb helper should avoid sleeping in panic context

[rui wang]

BTW, the impact of this bug is that the panic cannot reboot the
machine and it prints an infinite stream of the error messages. The
panic goes on forever because there's a for(;;) loop in
__mutex_lock_common(), so we enter __schedule() again and again.

Thanks
Rui

On 12/4/14, ruiv.wang@xxxxxxxxx <ruiv.wang@xxxxxxxxx> wrote:
> From: Rui Wang <rui.y.wang@xxxxxxxxx>
>
> There are still some places in the fb helper that need to avoid
> sleeping in panic context. Here's an example:
>
> [   65.615496] bad: scheduling from the idle thread!
> [   65.620747] CPU: 92 PID: 0 Comm: swapper/92 Tainted: G   M        E
> 3.18.0-rc4-7-default+ #20
>
> [   65.630364] Hardware name: Intel Corporation BRICKLAND/BRICKLAND, BIOS
> BRHSXSD1.86B.0056.R01.1409242327 09/24/2014
> [   65.641923]  ffff88087f693d80 ffff88087f689878 ffffffff81566db9
> 0000000000000000
> [   65.650226]  ffff88087f693d80 ffff88087f689898 ffffffff810871ff
> ffff88046eb3e0d0
> [   65.658527]  ffff88087f693d80 ffff88087f6898c8 ffffffff8107c1fa
> 000000017f6898b8
> [   65.666830] Call Trace:
> [   65.669557]  <#MC>  [<ffffffff81566db9>] dump_stack+0x46/0x58
> [   65.675994]  [<ffffffff810871ff>] dequeue_task_idle+0x2f/0x40
> [   65.682412]  [<ffffffff8107c1fa>] dequeue_task+0x5a/0x80
> [   65.688345]  [<ffffffff810804f3>] deactivate_task+0x23/0x30
> [   65.694569]  [<ffffffff81569050>] __schedule+0x580/0x7f0
> [   65.700502]  [<ffffffff81569739>] schedule_preempt_disabled+0x29/0x70
> [   65.707696]  [<ffffffff8156abb6>] __ww_mutex_lock_slowpath+0xb8/0x162
> [   65.714891]  [<ffffffff8156acb3>] __ww_mutex_lock+0x53/0x85
> [   65.721125]  [<ffffffffa00b3a5d>] drm_modeset_lock+0x3d/0x110 [drm]
> [   65.728132]  [<ffffffffa00b3c2a>] __drm_modeset_lock_all+0x8a/0x120
> [drm]
> [   65.735721]  [<ffffffffa00b3cd0>] drm_modeset_lock_all+0x10/0x30 [drm]
> [   65.743015]  [<ffffffffa01af8bf>] drm_fb_helper_pan_display+0x2f/0xf0
> [drm_kms_helper]
> [   65.751857]  [<ffffffff8132bd21>] fb_pan_display+0xd1/0x1a0
> [   65.758081]  [<ffffffff81326010>] bit_update_start+0x20/0x50
> [   65.764400]  [<ffffffff813259f2>] fbcon_switch+0x3a2/0x550
> [   65.770528]  [<ffffffff813a01c9>] redraw_screen+0x189/0x240
> [   65.776750]  [<ffffffff81322f8a>] fbcon_blank+0x20a/0x2d0
> [   65.782778]  [<ffffffff8137d359>] ? erst_writer+0x209/0x330
> [   65.789002]  [<ffffffff810ba2f3>] ? internal_add_timer+0x63/0x80
> [   65.795710]  [<ffffffff810bc137>] ? mod_timer+0x127/0x1e0
> [   65.801740]  [<ffffffff813a0cd8>] do_unblank_screen+0xa8/0x1d0
> [   65.808255]  [<ffffffff813a0e10>] unblank_screen+0x10/0x20
> [   65.814381]  [<ffffffff812ca0d9>] bust_spinlocks+0x19/0x40
> [   65.820508]  [<ffffffff81561ca7>] panic+0x106/0x1f5
> [   65.825955]  [<ffffffff8102336c>] mce_panic+0x2ac/0x2e0
> [   65.831789]  [<ffffffff812c796a>] ? delay_tsc+0x4a/0x80
> [   65.837625]  [<ffffffff81024e1f>] do_machine_check+0xbaf/0xbf0
> [   65.844138]  [<ffffffff813365d7>] ? intel_idle+0xc7/0x150
> [   65.850166]  [<ffffffff8156f03f>] machine_check+0x1f/0x30
> [   65.856195]  [<ffffffff813365d7>] ? intel_idle+0xc7/0x150
> [   65.862222]  <<EOE>>  [<ffffffff814283d5>]
> cpuidle_enter_state+0x55/0x170
> [   65.869823]  [<ffffffff814285a7>] cpuidle_enter+0x17/0x20
> [   65.875852]  [<ffffffff81097b08>] cpu_startup_entry+0x2d8/0x370
> [   65.882467]  [<ffffffff8102fe29>] start_secondary+0x159/0x180
>
> There's __drm_modeset_lock_all() which Daniel Vetter introduced for this
> purpose. We can leverage that without reinventing anything. This patch
> works with the latest kernel.
>
> Signed-off-by: Rui Wang <rui.y.wang@xxxxxxxxx>
> ---
>  drivers/gpu/drm/drm_fb_helper.c |    8 ++++++--
>  1 files changed, 6 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/gpu/drm/drm_fb_helper.c
> b/drivers/gpu/drm/drm_fb_helper.c
> index 0c0c39b..70dd2f4 100644
> --- a/drivers/gpu/drm/drm_fb_helper.c
> +++ b/drivers/gpu/drm/drm_fb_helper.c
> @@ -732,7 +732,9 @@ int drm_fb_helper_setcmap(struct fb_cmap *cmap, struct
> fb_info *info)
>  	int i, j, rc = 0;
>  	int start;
>
> -	drm_modeset_lock_all(dev);
> +	if (__drm_modeset_lock_all(dev, !!oops_in_progress)) {
> +		return -EBUSY;
> +	}
>  	if (!drm_fb_helper_is_bound(fb_helper)) {
>  		drm_modeset_unlock_all(dev);
>  		return -EBUSY;
> @@ -910,7 +912,9 @@ int drm_fb_helper_pan_display(struct fb_var_screeninfo
> *var,
>  	int ret = 0;
>  	int i;
>
> -	drm_modeset_lock_all(dev);
> +	if (__drm_modeset_lock_all(dev, !!oops_in_progress)) {
> +		return -EBUSY;
> +	}
>  	if (!drm_fb_helper_is_bound(fb_helper)) {
>  		drm_modeset_unlock_all(dev);
>  		return -EBUSY;
> --
> 1.7.5.4
>
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