Hi Nirmoy, On Monday, 29 January 2024 10:24:07 CET Nirmoy Das wrote: > Hi Janusz, > > There seems to be a regression in CI related to this: > > https://intel-gfx-ci.01.org/tree/drm-tip/Patchwork_129026v2/bat-dg1-7/ igt@gem_lmem_swapping@random-engines@xxxxxxxxxx#dmesg-warnings1053 > > Please have a look. Yes, that's a problem, the series can't be merged in its current shape. However, I'm not sure if that's really a regression, or rather an exposure of another, already existing issue. It looks like a race among two concurrent calls to our __active_retire() used in DMA fence callbacks. I'm going to verify some ideas for a fix on trybot. Thanks, Janusz > > > Regards, > > Nirmoy > > On 1/24/2024 6:13 PM, Janusz Krzysztofik wrote: > > Object debugging tools were sporadically reporting illegal attempts to > > free a still active i915 VMA object when parking a GT believed to be idle. > > > > [161.359441] ODEBUG: free active (active state 0) object: ffff88811643b958 object type: i915_active hint: __i915_vma_active+0x0/0x50 [i915] > > [161.360082] WARNING: CPU: 5 PID: 276 at lib/debugobjects.c:514 debug_print_object+0x80/0xb0 > > ... > > [161.360304] CPU: 5 PID: 276 Comm: kworker/5:2 Not tainted 6.5.0-rc1- CI_DRM_13375-g003f860e5577+ #1 > > [161.360314] Hardware name: Intel Corporation Rocket Lake Client Platform/ RocketLake S UDIMM 6L RVP, BIOS RKLSFWI1.R00.3173.A03.2204210138 04/21/2022 > > [161.360322] Workqueue: i915-unordered __intel_wakeref_put_work [i915] > > [161.360592] RIP: 0010:debug_print_object+0x80/0xb0 > > ... > > [161.361347] debug_object_free+0xeb/0x110 > > [161.361362] i915_active_fini+0x14/0x130 [i915] > > [161.361866] release_references+0xfe/0x1f0 [i915] > > [161.362543] i915_vma_parked+0x1db/0x380 [i915] > > [161.363129] __gt_park+0x121/0x230 [i915] > > [161.363515] ____intel_wakeref_put_last+0x1f/0x70 [i915] > > > > That has been tracked down to be happening when another thread is > > deactivating the VMA inside __active_retire() helper, after the VMA's > > active counter has been already decremented to 0, but before deactivation > > of the VMA's object is reported to the object debugging tool. > > > > We could prevent from that race by serializing i915_active_fini() with > > __active_retire() via ref->tree_lock, but that wouldn't stop the VMA from > > being used, e.g. from __i915_vma_retire() called at the end of > > __active_retire(), after that VMA has been already freed by a concurrent > > i915_vma_destroy() on return from the i915_active_fini(). Then, we should > > rather fix the issue at the VMA level, not in i915_active. > > > > Since __i915_vma_parked() is called from __gt_park() on last put of the > > GT's wakeref, the issue could be addressed by holding the GT wakeref long > > enough for __active_retire() to complete before that wakeref is released > > and the GT parked. > > > > A VMA associated with a request doesn't acquire a GT wakeref by itself. > > Instead, it depends on a wakeref held directly by the request's active > > intel_context for a GT associated with its VM, and indirectly on that > > intel_context's engine wakeref if the engine belongs to the same GT as the > > VMA's VM. Those wakerefs are released asynchronously to VMA deactivation. > > > > In case of single-GT platforms, at least one of those wakerefs is usually > > held long enough for the request's VMA to be deactivated on time, before > > it is destroyed on last put of its VM GT wakeref. However, on multi-GT > > platforms, a request may use a VMA from a GT other than the one that hosts > > the request's engine, then it is protected only with the intel_context's > > VM GT wakeref. > > > > There was an attempt to fix the issue on 2-GT Meteor Lake by acquiring an > > extra wakeref for a Primary GT from i915_gem_do_execbuffer() -- see commit > > f56fe3e91787 ("drm/i915: Fix a VMA UAF for multi-gt platform"). However, > > that fix occurred insufficient -- the issue was still reported by CI. > > That wakeref was released on exit from i915_gem_do_execbuffer(), then > > potentially before completion of the request and deactivation of its > > associated VMAs. Moreover, CI reports indicated that single-GT platforms > > also suffered sporadically from the same race. > > > > I believe the issue was introduced by commit d93939730347 ("drm/i915: > > Remove the vma refcount") which moved a call to i915_active_fini() from > > a dropped i915_vma_release(), called on last put of the removed VMA kref, > > to i915_vma_parked() processing path called on last put of a GT wakeref. > > However, its visibility to the object debugging tool was suppressed by a > > bug in i915_active that was fixed two weeks later with commit e92eb246feb9 > > ("drm/i915/active: Fix missing debug object activation"). > > > > Fix the issue by getting a wakeref for the VMA's GT when activating it, > > and putting that wakeref only after the VMA is deactivated. However, > > exclude global GTT from that processing path, otherwise the GPU never goes > > idle. Since __i915_vma_retire() may be called from atomic contexts, use > > async variant of wakeref put. Also, to avoid circular locking dependency, > > take care of acquiring the wakeref before VM mutex when both are needed. > > > > Having that fixed, stop explicitly acquiring the extra GT0 wakeref from > > inside i915_gem_do_execbuffer(), and also drop an extra call to > > i915_active_wait(), introduced by commit 7a2280e8dcd2 ("drm/i915: Wait for > > active retire before i915_active_fini()") as another insufficient fix for > > this UAF race. > > > > v5: Replace "tile" with "GT" across commit descriptions (Rodrigo), > > - reword commit message and description of patch 2 reusing relevant > > chunks moved there from commit description of patch 1 (Rodrigo), > > - explain why we take a temporary wakeref unconditionally inside > > i915_vma_pin_ww() (Rodrigo). > > v4: Refresh on top of commit 5e4e06e4087e ("drm/i915: Track gt pm > > wakerefs") (Andi), > > - for more easy backporting, split out removal of former insufficient > > workarounds and move them to separate patches (Nirmoy). > > - clean up commit message and description a bit. > > v3: Identify root cause more precisely, and a commit to blame, > > - identify and drop former workarounds, > > - update commit message and description. > > v2: Get the wakeref before VM mutex to avoid circular locking dependency, > > - drop questionable Fixes: tag. > > > > Janusz Krzysztofik (3): > > drm/i915/vma: Fix UAF on destroy against retire race > > drm/i915: Remove extra multi-gt pm-references > > Revert "drm/i915: Wait for active retire before i915_active_fini()" > > > > .../gpu/drm/i915/gem/i915_gem_execbuffer.c | 17 ----------- > > drivers/gpu/drm/i915/i915_vma.c | 28 +++++++++++++------ > > drivers/gpu/drm/i915/i915_vma_types.h | 1 + > > 3 files changed, 20 insertions(+), 26 deletions(-) > > >
