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

Maarten Lankhorst
2014-05-15T01:06:37Z

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.

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.

~Maarten