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

[Ville Syrjälä]

On Thu, Oct 20, 2016 at 03:36:54PM +0200, Takashi Iwai wrote:
> On Thu, 20 Oct 2016 15:28:14 +0200,
> Ville Syrjälä wrote:
> > 
> > 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?
> 
> Before the first call, it clears the clip and passes the copied clip
> to the callback.  Then the second call will be with the cleared and
> untouched clip, i.e. with x1=~0.  This confuses
> qxl_framebuffer_dirty().
> 
> Of course, we can filter out in the callback side by checking the
> clip.  It was actually my first version.  But basically it's a race
> and should be covered better in the caller side.

The race is still there AFAICS. The worker may already be executing but
not yet in the critical section, at which point drm_fb_helper_dirty()
will expand the dirty rectangle, and schedule another work. So the first
worker will already see the expanded rectangle, and second worker will
get zilch.

I think the only good fix is to have the worker validate the dirty
rectangle before calling the driver.

> 
> 
> thanks,
> 
> Takashi
> 
> > 
> > > 
> > > 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@xxxxxxx>
> > > ---
> > >  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
> > 

-- 
Ville Syrjälä
Intel OTC