Re: [RFC PATCH v1 08/16] drm/radeon: use common fence implementation for fences

[Christian König]

Am 15.05.2014 03:06, schrieb Maarten Lankhorst:
> op 14-05-14 17:29, Christian König schreef:
> > > +    /* did fence get signaled after we enabled the sw irq? */
> > > +    if 
> > > (atomic64_read(&fence->rdev->fence_drv[fence->ring].last_seq) >= 
> > > fence->seq) {
> > > +        radeon_irq_kms_sw_irq_put(fence->rdev, fence->ring);
> > > +        return false;
> > > +    }
> > > +
> > > +    fence->fence_wake.flags = 0;
> > > +    fence->fence_wake.private = NULL;
> > > +    fence->fence_wake.func = radeon_fence_check_signaled;
> > > +    __add_wait_queue(&fence->rdev->fence_queue, &fence->fence_wake);
> > > +    fence_get(f);
> > That looks like a race condition to me. The fence needs to be added 
> > to the wait queue before the check, not after.
> >
> > Apart from that the whole approach looks like a really bad idea to 
> > me. How for example is lockup detection supposed to happen with this? 
> It's not a race condition because fence_queue.lock is held when this 
> function is called.
Ah, I see. That's also the reason why you moved the wake_up_all out of 
the processing function.

> Lockup's a bit of a weird problem, the changes wouldn't allow core ttm 
> code to handle the lockup any more,
> but any driver specific wait code would still handle this. I did this 
> by design, because in future patches the wait
> function may be called from outside of the radeon driver. The official 
> wait function takes a timeout parameter,
> so lockups wouldn't be fatal if the timeout is set to something like 
> 30*HZ for example, it would still return
> and report that the function timed out.
Timeouts help with the detection of the lockup, but not at all with the 
handling of them.

What we essentially need is a wait callback into the driver that is 
called in non atomic context without any locks held.

This way we can block for the fence to become signaled with a timeout 
and can then also initiate the reset handling if necessary.

The way you designed the interface now means that the driver never gets 
a chance to wait for the hardware to become idle and so never has the 
opportunity to the reset the whole thing.

Christian.

> ~Maarten

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