[Intel-gfx] [PATCH] drm/i915: Fix timeout with missed interrupts in __wait_seqno

[Daniel Vetter]

On Thu, Dec 12, 2013 at 02:48:47PM +0200, Ville Syrjälä wrote:
> On Tue, Dec 10, 2013 at 05:02:43PM +0200, Mika Kuoppala wrote:
> > Commit 094f9a54e355 ("drm/i915: Fix __wait_seqno to use true infinite
> > timeouts") added support for __wait_seqno to detect missing interrupts and
> > go around them by polling. As there is also timeout detection in
> > __wait_seqno, the polling and timeout detection were done with the same
> > timer.
> > 
> > When there has been missed interrupts and polling is needed, the timer is
> > set to trigger in (now + 1) jiffies in future, instead of the caller
> > specified timeout.
> > 
> > Now when io_schedule() returns, we calculate the jiffies left to timeout
> > using the timer expiration value. As the current jiffies is now bound to be
> > always equal or greater than the expiration value, the timeout_jiffies will
> > become zero or negative and we return -ETIME to caller even tho the
> > timeout was never reached.
> > 
> > Fix this by decoupling timeout calculation from timer expiration.
> > 
> > v2: Commit message with some sense in it (Chris Wilson)
> > 
> > v3: add parenthesis on timeout_expire calculation
> > 
> > v4: don't read jiffies without timeout (Chris Wilson)
> > 
> > Signed-off-by: Mika Kuoppala <mika.kuoppala at intel.com>
> > ---
> >  drivers/gpu/drm/i915/i915_gem.c |   14 ++++++--------
> >  1 file changed, 6 insertions(+), 8 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> > index 92149bc..6d2e786 100644
> > --- a/drivers/gpu/drm/i915/i915_gem.c
> > +++ b/drivers/gpu/drm/i915/i915_gem.c
> > @@ -1017,7 +1017,7 @@ static int __wait_seqno(struct intel_ring_buffer *ring, u32 seqno,
> >  	drm_i915_private_t *dev_priv = ring->dev->dev_private;
> >  	struct timespec before, now;
> >  	DEFINE_WAIT(wait);
> > -	long timeout_jiffies;
> > +	unsigned long timeout_expire;
> >  	int ret;
> >  
> >  	WARN(dev_priv->pc8.irqs_disabled, "IRQs disabled\n");
> > @@ -1025,7 +1025,7 @@ static int __wait_seqno(struct intel_ring_buffer *ring, u32 seqno,
> >  	if (i915_seqno_passed(ring->get_seqno(ring, true), seqno))
> >  		return 0;
> >  
> > -	timeout_jiffies = timeout ? timespec_to_jiffies_timeout(timeout) : 1;
> > +	timeout_expire = timeout ? jiffies + timespec_to_jiffies_timeout(timeout) : 0;
> >  
> >  	if (dev_priv->info->gen >= 6 && can_wait_boost(file_priv)) {
> >  		gen6_rps_boost(dev_priv);
> > @@ -1044,7 +1044,6 @@ static int __wait_seqno(struct intel_ring_buffer *ring, u32 seqno,
> >  	getrawmonotonic(&before);
> >  	for (;;) {
> >  		struct timer_list timer;
> > -		unsigned long expire;
> >  
> >  		prepare_to_wait(&ring->irq_queue, &wait,
> >  				interruptible ? TASK_INTERRUPTIBLE : TASK_UNINTERRUPTIBLE);
> > @@ -1070,23 +1069,22 @@ static int __wait_seqno(struct intel_ring_buffer *ring, u32 seqno,
> >  			break;
> >  		}
> >  
> > -		if (timeout_jiffies <= 0) {
> > +		if (timeout && time_after_eq(jiffies, timeout_expire)) {
> >  			ret = -ETIME;
> >  			break;
> >  		}
> >  
> >  		timer.function = NULL;
> >  		if (timeout || missed_irq(dev_priv, ring)) {
> > +			unsigned long expire;
> > +
> >  			setup_timer_on_stack(&timer, fake_irq, (unsigned long)current);
> > -			expire = jiffies + (missed_irq(dev_priv, ring) ? 1: timeout_jiffies);
> > +			expire = missed_irq(dev_priv, ring) ? jiffies + 1 : timeout_expire;
> 
> I guess we have very small race here if we get called w/ timeout==NULL, and
> missed_irq() was true above but is no longer true here. At that point we would
> set expire=0 and might end up waiting for quite a while. But that issue was
> present already in the code before this patch and otherwise it all
> looks good to me, so:

We shouldn't ever reset misseq_irq in normal operations, so this should be
ok.

> Reviewed-by: Ville Syrjälä <ville.syrjala at linux.intel.com>

Queued for -next, thanks for the patch.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch