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

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Date:	Thu, 15 May 2014 03:06:37 +0200
From:	Maarten Lankhorst <maarten.lankhorst@...onical.com>
To:	Christian König <deathsimple@...afone.de>,
	airlied@...ux.ie
CC:	nouveau@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
	dri-devel@...ts.freedesktop.org
Subject: Re: [RFC PATCH v1 08/16] drm/radeon: use common fence implementation
 for fences

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
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