Date:	Thu, 15 May 2014 18:13:06 +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 17:58, schrieb Maarten Lankhorst:
> op 15-05-14 17:48, Christian König schreef:
>> Am 15.05.2014 16:18, schrieb Maarten Lankhorst:
>>> op 15-05-14 15:19, Christian König schreef:
>>>> Am 15.05.2014 15:04, schrieb Maarten Lankhorst:
>>>>> op 15-05-14 11:42, Christian König schreef:
>>>>>> 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. 
>>>>>
>>>>> Does this modification look sane?
>>>> Adding the timeout is on my todo list for quite some time as well, 
>>>> so this part makes sense.
>>>>
>>>>> +static long __radeon_fence_wait(struct fence *f, bool intr, long 
>>>>> timeout)
>>>>> +{
>>>>> +    struct radeon_fence *fence = to_radeon_fence(f);
>>>>> +    u64 target_seq[RADEON_NUM_RINGS] = {};
>>>>> +
>>>>> +    target_seq[fence->ring] = fence->seq;
>>>>> +    return radeon_fence_wait_seq_timeout(fence->rdev, target_seq, 
>>>>> intr, timeout);
>>>>> +}
>>>> When this call is comming from outside the radeon driver you need 
>>>> to lock rdev->exclusive_lock here to make sure not to interfere 
>>>> with a possible reset.
>>> Ah thanks, I'll add that.
>>>
>>>>>      .get_timeline_name = radeon_fence_get_timeline_name,
>>>>>      .enable_signaling = radeon_fence_enable_signaling,
>>>>>      .signaled = __radeon_fence_signaled,
>>>> Do we still need those callback when we implemented the wait callback?
>>> .get_timeline_name is used for debugging (trace events).
>>> .signaled is the non-blocking call to check if the fence is signaled 
>>> or not.
>>> .enable_signaling is used for adding callbacks upon fence 
>>> completion, the default 'fence_default_wait' uses it, so
>>> when it works no separate implementation is needed unless you want 
>>> to do more than just waiting.
>>> It's also used when fence_add_callback is called. i915 can be 
>>> patched to use it. ;-)
>>
>> I just meant enable_signaling, the other ones are fine with me. The 
>> problem with enable_signaling is that it's called with a spin lock 
>> held, so we can't sleep.
>>
>> While resetting the GPU could be moved out into a timer the problem 
>> here is that I can't lock rdev->exclusive_lock in such situations.
>>
>> This means when i915 would call into radeon to enable signaling for a 
>> fence we can't make sure that there is not GPU reset running on 
>> another CPU. And touching the IRQ registers while a reset is going on 
>> is a really good recipe to lockup the whole system.
> If you increase the irq counter on all rings before doing a gpu reset, 
> adjust the state and call sw_irq_put when done this race could never 
> happen. Or am I missing something?
>
Beside that's being extremely ugly in the case of a hard PCI reset even 
touching any register or just accessing VRAM in this moment can crash 
the box. Just working around the enable/disable of the interrupt here 
won't help us much.

Adding another spin lock won't work so well either, because the reset 
function itself wants to sleep as well.

The only solution I see off hand is making the critical reset code path 
work in atomic context as well, but that's not really doable cause AFAIK 
we need to work with functions from the PCI subsystem and spinning on a 
lock for up to a second is not really desirable also.

Christian.

> ~Maarten
>

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