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

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Message-ID: <53748BFD.1050608@vodafone.de>
Date:	Thu, 15 May 2014 11:42:21 +0200
From:	Christian König <deathsimple@...afone.de>
To:	Maarten Lankhorst <maarten.lankhorst@...onical.com>,
	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

Am 15.05.2014 11:38, schrieb Maarten Lankhorst:
> op 15-05-14 11:21, Christian König schreef:
>> 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.
> Correct. :-)
>>> 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.
> You could set up a hangcheck timer like intel does, and end up with a 
> reliable hangcheck detection that doesn't depend on cpu waits. :-) Or 
> override the default wait function and restore the old behavior.

Overriding the default wait function sounds better, please implement it 
this way.

Thanks,
Christian.

>
> ~Maarten
>

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