[PATCH] drm/fb-helper: Fix race between deferred_io worker and dirty updater

[Ville Syrjälä]

On Thu, Oct 20, 2016 at 03:20:55PM +0200, Takashi Iwai wrote:
> Since 4.7 kernel, we've seen the error messages like
> 
>  kernel: [TTM] Buffer eviction failed
>  kernel: qxl 0000:00:02.0: object_init failed for (4026540032, 0x00000001)
>  kernel: [drm:qxl_alloc_bo_reserved [qxl]] *ERROR* failed to allocate VRAM BO
> 
> on QXL when switching and accessing on VT.  The culprit was the generic
> deferred_io code (qxl driver switched to it since 4.7).  There is a
> race between the dirty clip update and the call of callback.
> 
> In drm_fb_helper_dirty(), the dirty clip is updated in the spinlock,
> while it kicks off the update worker outside the spinlock.  Meanwhile
> the update worker clears the dirty clip in the spinlock, too.  Thus,
> when drm_fb_helper_dirty() is called concurrently, schedule_work() is
> called after the clip is cleared in the first worker call.

Why does that matter? The first worker should have done all the
necessary work already, no?

> 
> The fix is simply moving schedule_work() inside the spinlock.
> 
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=98322
> Bugzilla: https://bugzilla.suse.com/show_bug.cgi?id=1003298
> Fixes: eaa434defaca ('drm/fb-helper: Add fb_deferred_io support')
> Signed-off-by: Takashi Iwai <tiwai at suse.de>
> ---
>  drivers/gpu/drm/drm_fb_helper.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
> index 03414bde1f15..bae392dea2cc 100644
> --- a/drivers/gpu/drm/drm_fb_helper.c
> +++ b/drivers/gpu/drm/drm_fb_helper.c
> @@ -861,9 +861,8 @@ static void drm_fb_helper_dirty(struct fb_info *info, u32 x, u32 y,
>  	clip->y1 = min_t(u32, clip->y1, y);
>  	clip->x2 = max_t(u32, clip->x2, x + width);
>  	clip->y2 = max_t(u32, clip->y2, y + height);
> -	spin_unlock_irqrestore(&helper->dirty_lock, flags);
> -
>  	schedule_work(&helper->dirty_work);
> +	spin_unlock_irqrestore(&helper->dirty_lock, flags);
>  }
>  
>  /**
> -- 
> 2.10.1
> 
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-- 
Ville Syrjälä
Intel OTC