Hi Rodrigo,
Thank you for review.
On Monday, 22 January 2024 22:09:38 CET Rodrigo Vivi wrote:
> On Mon, Jan 22, 2024 at 03:04:42PM +0100, Janusz Krzysztofik wrote:
> > Object debugging tools were sporadically reporting illegal attempts to
> > free a still active i915 VMA object when parking a GPU tile 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. In case of single-tile 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-tile platforms, a request may use a VMA from a
> > tile 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 this issue on 2-tile Meteor Lake by acquiring
>
> please do not confuse the terminology here. MTL is 1-tile platform,
> with multiple GTs (1 for Render/Compute and 1 for Media).
I didn't realize that "tile" is not the same as "GT". I can review the whole
description and replace all occurrences of "tile" with "GT".
>
> Also you could probably avoid mentioning the other case here when
> you are actively trying to resolve the RKL's single GT case.
OK, but let me keep that part of commit description in the cover letter then,
because:
- historically, the issue was more frequently reproduced in CI on MTL than on
other platforms, and that was the initial scope I started working on,
- the full description as is better reflects phases of my process of
reproduction and root cause analysis of the issue,
- the MTL case was specifically addressed by the former insufficient
workaround which now I'm now proposing to drop.
> > 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, it 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.
> >
> > OTOH, CI reports indicate that single-tile platforms also suffer
> > 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 tile 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.
>
> okay, this explains the first block of the patch below, but I'm afraid
> that it doesn't explain why:
>
> - if (flags & PIN_GLOBAL)
That's explained in v2 changelog below, I believe. I can add that information
to the body of commit description as well.
Thanks,
Janusz
> >
> > 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.
> >
> > Fixes: d93939730347 ("drm/i915: Remove the vma refcount")
> > Closes: https://gitlab.freedesktop.org/drm/intel/issues/8875
> > Signed-off-by: Janusz Krzysztofik <janusz.krzyszto...@linux.intel.com>
> > Cc: Thomas Hellström <thomas.hellst...@linux.intel.com>
> > Cc: Nirmoy Das <nirmoy....@intel.com>
> > Cc: Andi Shyti <andi.sh...@linux.intel.com>
> > Cc: sta...@vger.kernel.org # v5.19+
> > ---
> > drivers/gpu/drm/i915/i915_vma.c | 26 +++++++++++++++++++-------
> > drivers/gpu/drm/i915/i915_vma_types.h | 1 +
> > 2 files changed, 20 insertions(+), 7 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/i915/i915_vma.c
> > b/drivers/gpu/drm/i915/i915_vma.c
> > index d09aad34ba37f..604d420b9e1fd 100644
> > --- a/drivers/gpu/drm/i915/i915_vma.c
> > +++ b/drivers/gpu/drm/i915/i915_vma.c
> > @@ -34,6 +34,7 @@
> > #include "gt/intel_engine.h"
> > #include "gt/intel_engine_heartbeat.h"
> > #include "gt/intel_gt.h"
> > +#include "gt/intel_gt_pm.h"
> > #include "gt/intel_gt_requests.h"
> > #include "gt/intel_tlb.h"
> >
> > @@ -103,12 +104,25 @@ static inline struct i915_vma *active_to_vma(struct
> > i915_active *ref)
> >
> > static int __i915_vma_active(struct i915_active *ref)
> > {
> > - return i915_vma_tryget(active_to_vma(ref)) ? 0 : -ENOENT;
> > + struct i915_vma *vma = active_to_vma(ref);
> > +
> > + if (!i915_vma_tryget(vma))
> > + return -ENOENT;
> > +
> > + if (!i915_vma_is_ggtt(vma))
> > + vma->wakeref = intel_gt_pm_get(vma->vm->gt);
> > +
> > + return 0;
> > }
> >
> > static void __i915_vma_retire(struct i915_active *ref)
> > {
> > - i915_vma_put(active_to_vma(ref));
> > + struct i915_vma *vma = active_to_vma(ref);
> > +
> > + if (!i915_vma_is_ggtt(vma))
> > + intel_gt_pm_put_async(vma->vm->gt, vma->wakeref);
> > +
> > + i915_vma_put(vma);
> > }
> >
> > static struct i915_vma *
> > @@ -1404,7 +1418,7 @@ int i915_vma_pin_ww(struct i915_vma *vma, struct
> > i915_gem_ww_ctx *ww,
> > struct i915_vma_work *work = NULL;
> > struct dma_fence *moving = NULL;
> > struct i915_vma_resource *vma_res = NULL;
> > - intel_wakeref_t wakeref = 0;
> > + intel_wakeref_t wakeref;
> > unsigned int bound;
> > int err;
> >
> > @@ -1424,8 +1438,7 @@ int i915_vma_pin_ww(struct i915_vma *vma, struct
> > i915_gem_ww_ctx *ww,
> > if (err)
> > return err;
> >
> > - if (flags & PIN_GLOBAL)
> > - wakeref = intel_runtime_pm_get(&vma->vm->i915->runtime_pm);
> > + wakeref = intel_runtime_pm_get(&vma->vm->i915->runtime_pm);
> >
> > if (flags & vma->vm->bind_async_flags) {
> > /* lock VM */
> > @@ -1561,8 +1574,7 @@ int i915_vma_pin_ww(struct i915_vma *vma, struct
> > i915_gem_ww_ctx *ww,
> > if (work)
> > dma_fence_work_commit_imm(&work->base);
> > err_rpm:
> > - if (wakeref)
> > - intel_runtime_pm_put(&vma->vm->i915->runtime_pm, wakeref);
> > + intel_runtime_pm_put(&vma->vm->i915->runtime_pm, wakeref);
> >
> > if (moving)
> > dma_fence_put(moving);
> > diff --git a/drivers/gpu/drm/i915/i915_vma_types.h
> > b/drivers/gpu/drm/i915/i915_vma_types.h
> > index 64472b7f0e770..f0086fadff4d3 100644
> > --- a/drivers/gpu/drm/i915/i915_vma_types.h
> > +++ b/drivers/gpu/drm/i915/i915_vma_types.h
> > @@ -264,6 +264,7 @@ struct i915_vma {
> > #define I915_VMA_SCANOUT ((int)BIT(I915_VMA_SCANOUT_BIT))
> >
> > struct i915_active active;
> > + intel_wakeref_t wakeref;
> >
> > #define I915_VMA_PAGES_BIAS 24
> > #define I915_VMA_PAGES_ACTIVE (BIT(24) | 1)
>
