From: "Christian König" <christian.koenig@amd.com> To: "Thomas Hellström" <thomas.hellstrom@linux.intel.com>, "Maarten Lankhorst" <maarten.lankhorst@linux.intel.com>, intel-gfx@lists.freedesktop.org, dri-devel@lists.freedesktop.org Cc: linaro-mm-sig@lists.linaro.org, matthew.auld@intel.com Subject: Re: [RFC PATCH 1/2] dma-fence: Avoid establishing a locking order between fence classes Date: Tue, 30 Nov 2021 14:26:20 +0100 [thread overview] Message-ID: <2551da4d-2e51-cc24-7d4a-84ae00a1547c@amd.com> (raw) In-Reply-To: <57df8b0b-1d65-155f-a9a6-8073bbd4f28f@linux.intel.com> Am 30.11.21 um 13:56 schrieb Thomas Hellström: > > On 11/30/21 13:42, Christian König wrote: >> Am 30.11.21 um 13:31 schrieb Thomas Hellström: >>> [SNIP] >>>> Other than that, I didn't investigate the nesting fails enough to >>>> say I can accurately review this. :) >>> >>> Basically the problem is that within enable_signaling() which is >>> called with the dma_fence lock held, we take the dma_fence lock of >>> another fence. If that other fence is a dma_fence_array, or a >>> dma_fence_chain which in turn tries to lock a dma_fence_array we hit >>> a splat. >> >> Yeah, I already thought that you constructed something like that. >> >> You get the splat because what you do here is illegal, you can't mix >> dma_fence_array and dma_fence_chain like this or you can end up in a >> stack corruption. > > Hmm. Ok, so what is the stack corruption, is it that the > enable_signaling() will end up with endless recursion? If so, wouldn't > it be more usable we break that recursion chain and allow a more > general use? The problem is that this is not easily possible for dma_fence_array containers. Just imagine that you drop the last reference to the containing fences during dma_fence_array destruction if any of the contained fences is another container you can easily run into recursion and with that stack corruption. That's one of the major reasons I came up with the dma_fence_chain container. This one you can chain any number of elements together without running into any recursion. > Also what are the mixing rules between these? Never use a > dma-fence-chain as one of the array fences and never use a > dma-fence-array as a dma-fence-chain fence? You can't add any other container to a dma_fence_array, neither other dma_fence_array instances nor dma_fence_chain instances. IIRC at least technically a dma_fence_chain can contain a dma_fence_array if you absolutely need that, but Daniel, Jason and I already had the same discussion a while back and came to the conclusion to avoid that as well if possible. Regards, Christian. > > /Thomas > > > > >> >> Regards, >> Christian. >> >>> >>> But I'll update the commit message with a typical splat. >>> >>> /Thomas >>
WARNING: multiple messages have this Message-ID (diff)
From: "Christian König" <christian.koenig@amd.com> To: "Thomas Hellström" <thomas.hellstrom@linux.intel.com>, "Maarten Lankhorst" <maarten.lankhorst@linux.intel.com>, intel-gfx@lists.freedesktop.org, dri-devel@lists.freedesktop.org Cc: linaro-mm-sig@lists.linaro.org, matthew.auld@intel.com Subject: Re: [Intel-gfx] [RFC PATCH 1/2] dma-fence: Avoid establishing a locking order between fence classes Date: Tue, 30 Nov 2021 14:26:20 +0100 [thread overview] Message-ID: <2551da4d-2e51-cc24-7d4a-84ae00a1547c@amd.com> (raw) In-Reply-To: <57df8b0b-1d65-155f-a9a6-8073bbd4f28f@linux.intel.com> Am 30.11.21 um 13:56 schrieb Thomas Hellström: > > On 11/30/21 13:42, Christian König wrote: >> Am 30.11.21 um 13:31 schrieb Thomas Hellström: >>> [SNIP] >>>> Other than that, I didn't investigate the nesting fails enough to >>>> say I can accurately review this. :) >>> >>> Basically the problem is that within enable_signaling() which is >>> called with the dma_fence lock held, we take the dma_fence lock of >>> another fence. If that other fence is a dma_fence_array, or a >>> dma_fence_chain which in turn tries to lock a dma_fence_array we hit >>> a splat. >> >> Yeah, I already thought that you constructed something like that. >> >> You get the splat because what you do here is illegal, you can't mix >> dma_fence_array and dma_fence_chain like this or you can end up in a >> stack corruption. > > Hmm. Ok, so what is the stack corruption, is it that the > enable_signaling() will end up with endless recursion? If so, wouldn't > it be more usable we break that recursion chain and allow a more > general use? The problem is that this is not easily possible for dma_fence_array containers. Just imagine that you drop the last reference to the containing fences during dma_fence_array destruction if any of the contained fences is another container you can easily run into recursion and with that stack corruption. That's one of the major reasons I came up with the dma_fence_chain container. This one you can chain any number of elements together without running into any recursion. > Also what are the mixing rules between these? Never use a > dma-fence-chain as one of the array fences and never use a > dma-fence-array as a dma-fence-chain fence? You can't add any other container to a dma_fence_array, neither other dma_fence_array instances nor dma_fence_chain instances. IIRC at least technically a dma_fence_chain can contain a dma_fence_array if you absolutely need that, but Daniel, Jason and I already had the same discussion a while back and came to the conclusion to avoid that as well if possible. Regards, Christian. > > /Thomas > > > > >> >> Regards, >> Christian. >> >>> >>> But I'll update the commit message with a typical splat. >>> >>> /Thomas >>
next prev parent reply other threads:[~2021-11-30 13:26 UTC|newest] Thread overview: 65+ messages / expand[flat|nested] mbox.gz Atom feed top 2021-11-30 12:19 [RFC PATCH 0/2] Attempt to avoid dma-fence-[chain|array] lockdep splats Thomas Hellström 2021-11-30 12:19 ` [Intel-gfx] " Thomas Hellström 2021-11-30 12:19 ` [RFC PATCH 1/2] dma-fence: Avoid establishing a locking order between fence classes Thomas Hellström 2021-11-30 12:19 ` [Intel-gfx] " Thomas Hellström 2021-11-30 12:25 ` Maarten Lankhorst 2021-11-30 12:25 ` [Intel-gfx] " Maarten Lankhorst 2021-11-30 12:31 ` Thomas Hellström 2021-11-30 12:31 ` [Intel-gfx] " Thomas Hellström 2021-11-30 12:42 ` Christian König 2021-11-30 12:42 ` [Intel-gfx] " Christian König 2021-11-30 12:56 ` Thomas Hellström 2021-11-30 12:56 ` [Intel-gfx] " Thomas Hellström 2021-11-30 13:26 ` Christian König [this message] 2021-11-30 13:26 ` Christian König 2021-11-30 14:35 ` Thomas Hellström 2021-11-30 14:35 ` [Intel-gfx] " Thomas Hellström 2021-11-30 15:02 ` Christian König 2021-11-30 15:02 ` [Intel-gfx] " Christian König 2021-11-30 18:12 ` Thomas Hellström 2021-11-30 18:12 ` Thomas Hellström 2021-11-30 19:27 ` Thomas Hellström 2021-11-30 19:27 ` [Intel-gfx] " Thomas Hellström 2021-12-01 7:05 ` Christian König 2021-12-01 7:05 ` [Intel-gfx] " Christian König 2021-12-01 8:23 ` [Linaro-mm-sig] " Thomas Hellström (Intel) 2021-12-01 8:23 ` [Intel-gfx] " Thomas Hellström (Intel) 2021-12-01 8:36 ` Christian König 2021-12-01 8:36 ` [Intel-gfx] " Christian König 2021-12-01 10:15 ` Thomas Hellström (Intel) 2021-12-01 10:15 ` [Intel-gfx] " Thomas Hellström (Intel) 2021-12-01 10:32 ` Christian König 2021-12-01 10:32 ` [Intel-gfx] " Christian König 2021-12-01 11:04 ` Thomas Hellström (Intel) 2021-12-01 11:04 ` [Intel-gfx] " Thomas Hellström (Intel) 2021-12-01 11:25 ` Christian König 2021-12-01 11:25 ` [Intel-gfx] " Christian König 2021-12-01 12:16 ` Thomas Hellström (Intel) 2021-12-01 12:16 ` Thomas Hellström (Intel) 2021-12-03 13:08 ` Christian König 2021-12-03 13:08 ` [Intel-gfx] " Christian König 2021-12-03 14:18 ` Thomas Hellström 2021-12-03 14:18 ` [Intel-gfx] " Thomas Hellström 2021-12-03 14:26 ` Christian König 2021-12-03 14:26 ` [Intel-gfx] " Christian König 2021-12-03 14:50 ` Thomas Hellström 2021-12-03 14:50 ` [Intel-gfx] " Thomas Hellström 2021-12-03 15:00 ` Christian König 2021-12-03 15:00 ` [Intel-gfx] " Christian König 2021-12-03 15:13 ` Thomas Hellström (Intel) 2021-12-03 15:13 ` [Intel-gfx] " Thomas Hellström (Intel) 2021-12-07 18:08 ` Daniel Vetter 2021-12-07 18:08 ` Daniel Vetter 2021-12-07 20:46 ` Thomas Hellström 2021-12-07 20:46 ` Thomas Hellström 2021-12-20 9:37 ` Daniel Vetter 2021-12-20 9:37 ` Daniel Vetter 2021-11-30 12:32 ` Thomas Hellström 2021-11-30 12:32 ` [Intel-gfx] " Thomas Hellström 2021-11-30 12:19 ` [RFC PATCH 2/2] dma-fence: Avoid excessive recursive fence locking from enable_signaling() callbacks Thomas Hellström 2021-11-30 12:19 ` [Intel-gfx] " Thomas Hellström 2021-11-30 12:36 ` [RFC PATCH 0/2] Attempt to avoid dma-fence-[chain|array] lockdep splats Christian König 2021-11-30 12:36 ` [Intel-gfx] " Christian König 2021-11-30 13:05 ` [Intel-gfx] ✗ Fi.CI.SPARSE: warning for " Patchwork 2021-11-30 13:48 ` [Intel-gfx] ✓ Fi.CI.BAT: success " Patchwork 2021-11-30 17:47 ` [Intel-gfx] ✓ Fi.CI.IGT: " Patchwork
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