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Fri, 11 Apr 2025 11:05:07 +0000 Content-Type: multipart/alternative; boundary="------------M0ol96SFa6ecnoacdIKMfCh4" Message-ID: <81a70ba6-94b1-4bb3-a0b2-9e8890f90b33@amd.com> Date: Fri, 11 Apr 2025 13:05:02 +0200 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 1/3] drm/nouveau: Prevent signaled fences in pending list To: phasta@kernel.org, Lyude Paul , Danilo Krummrich , David Airlie , Simona Vetter , Sabrina Dubroca , Sumit Semwal Cc: dri-devel@lists.freedesktop.org, nouveau@lists.freedesktop.org, linux-kernel@vger.kernel.org, netdev@vger.kernel.org, linux-media@vger.kernel.org, linaro-mm-sig@lists.linaro.org, stable@vger.kernel.org References: <20250410092418.135258-2-phasta@kernel.org> <20250410092418.135258-3-phasta@kernel.org> <8583665a-6886-4245-be49-fd8839cfe212@amd.com> <50c9530d-e274-4f89-8620-16afe0981239@amd.com> <1a73e5fe4350d6ee4b7d807612264eb637c4f2a9.camel@mailbox.org> Content-Language: en-US From: =?UTF-8?Q?Christian_K=C3=B6nig?= In-Reply-To: X-ClientProxiedBy: FR3P281CA0180.DEUP281.PROD.OUTLOOK.COM (2603:10a6:d10:a0::13) To PH7PR12MB5685.namprd12.prod.outlook.com (2603:10b6:510:13c::22) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: PH7PR12MB5685:EE_|DM6PR12MB4316:EE_ X-MS-Office365-Filtering-Correlation-Id: 6eede885-e744-4ce9-8f11-08dd78e8b802 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0; 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charset=UTF-8 Content-Transfer-Encoding: 8bit Am 11.04.25 um 11:29 schrieb Philipp Stanner: > [SNIP] > It could be, however, that at the same moment nouveau_fence_signal() is > removing that entry, holding the appropriate lock. > > So we have a race. Again. Ah, yes of course. If signaled is called with or without the lock is actually undetermined. > You see, fixing things in Nouveau is difficult :) > It gets more difficult if you want to clean it up "properly", so it > conforms to rules such as those from dma_fence. > > I have now provided two fixes that both work, but you are not satisfied > with from the dma_fence-maintainer's perspective. I understand that, > but please also understand that it's actually not my primary task to > work on Nouveau. I just have to fix this bug to move on with my > scheduler work. Well I'm happy with whatever solution as long as it works, but as far as I can see the approach with the callback simply doesn't. You just can't drop the fence reference for the list from the callback. > So if you have another idea, feel free to share it. But I'd like to > know how we can go on here. Well the fence code actually works, doesn't it? The problem is rather that setting the error throws a warning because it doesn't expect signaled fences on the pending list. Maybe we should fix that instead. > I'm running out of ideas. What I'm wondering if we couldn't just remove > performance hacky fastpath functions such as > nouveau_fence_is_signaled() completely. It seems redundant to me. That would work for me as well. > > Or we might add locking to it, but IDK what was achieved with RCU here. > In any case it's definitely bad that Nouveau has so many redundant and > half-redundant mechanisms. Yeah, agree messing with the locks even more won't help us here. Regards, Christian. > > > P. > >> >> P. >> >>> Regards, >>> Christian. >>> >>>> P. >>>> >>>> >>>> >>>>> Regards, >>>>> Christian. >>>>> >>>>>> Replace the call to dma_fence_is_signaled() with >>>>>> nouveau_fence_base_is_signaled(). >>>>>> >>>>>> Cc: # 4.10+, precise commit not to >>>>>> be >>>>>> determined >>>>>> Signed-off-by: Philipp Stanner >>>>>> --- >>>>>>  drivers/gpu/drm/nouveau/nouveau_fence.c | 2 +- >>>>>>  1 file changed, 1 insertion(+), 1 deletion(-) >>>>>> >>>>>> diff --git a/drivers/gpu/drm/nouveau/nouveau_fence.c >>>>>> b/drivers/gpu/drm/nouveau/nouveau_fence.c >>>>>> index 7cc84472cece..33535987d8ed 100644 >>>>>> --- a/drivers/gpu/drm/nouveau/nouveau_fence.c >>>>>> +++ b/drivers/gpu/drm/nouveau/nouveau_fence.c >>>>>> @@ -274,7 +274,7 @@ nouveau_fence_done(struct nouveau_fence >>>>>> *fence) >>>>>>   nvif_event_block(&fctx->event); >>>>>>   spin_unlock_irqrestore(&fctx->lock, flags); >>>>>>   } >>>>>> - return dma_fence_is_signaled(&fence->base); >>>>>> + return test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence- >>>>>>> base.flags); >>>>>>  } >>>>>>   >>>>>>  static long --------------M0ol96SFa6ecnoacdIKMfCh4 Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: 8bit Am 11.04.25 um 11:29 schrieb Philipp Stanner:
[SNIP] 
It could be, however, that at the same moment nouveau_fence_signal() is
removing that entry, holding the appropriate lock.

So we have a race. Again.

Ah, yes of course. If signaled is called with or without the lock is actually undetermined.

You see, fixing things in Nouveau is difficult :)
It gets more difficult if you want to clean it up "properly", so it
conforms to rules such as those from dma_fence.

I have now provided two fixes that both work, but you are not satisfied
with from the dma_fence-maintainer's perspective. I understand that,
but please also understand that it's actually not my primary task to
work on Nouveau. I just have to fix this bug to move on with my
scheduler work.

Well I'm happy with whatever solution as long as it works, but as far as I can see the approach with the callback simply doesn't.

You just can't drop the fence reference for the list from the callback.

So if you have another idea, feel free to share it. But I'd like to
know how we can go on here.

Well the fence code actually works, doesn't it? The problem is rather that setting the error throws a warning because it doesn't expect signaled fences on the pending list.

Maybe we should fix that instead.

I'm running out of ideas. What I'm wondering if we couldn't just remove
performance hacky fastpath functions such as
nouveau_fence_is_signaled() completely. It seems redundant to me.

That would work for me as well.


Or we might add locking to it, but IDK what was achieved with RCU here.
In any case it's definitely bad that Nouveau has so many redundant and
half-redundant mechanisms.

Yeah, agree messing with the locks even more won't help us here.

Regards,
Christian.



P.



P.


Regards,
Christian.


P.




Regards,
Christian.


Replace the call to dma_fence_is_signaled() with
nouveau_fence_base_is_signaled().

Cc: <stable@vger.kernel.org> # 4.10+, precise commit not to
be
determined
Signed-off-by: Philipp Stanner <phasta@kernel.org>
---
 drivers/gpu/drm/nouveau/nouveau_fence.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/nouveau/nouveau_fence.c
b/drivers/gpu/drm/nouveau/nouveau_fence.c
index 7cc84472cece..33535987d8ed 100644
--- a/drivers/gpu/drm/nouveau/nouveau_fence.c
+++ b/drivers/gpu/drm/nouveau/nouveau_fence.c
@@ -274,7 +274,7 @@ nouveau_fence_done(struct nouveau_fence
*fence)
 			nvif_event_block(&fctx->event);
 		spin_unlock_irqrestore(&fctx->lock, flags);
 	}
-	return dma_fence_is_signaled(&fence->base);
+	return test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence-

base.flags);

 }
 
 static long


      

      

    

    

  


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