[Intel-gfx] [PATCH] drm/i915: Fix syncmap memory leak

[Matthew Brost]

On Fri, Aug 06, 2021 at 11:23:06AM -0700, John Harrison wrote:
> On 7/30/2021 12:53, Matthew Brost wrote:
> > A small race exists between intel_gt_retire_requests_timeout and
> > intel_timeline_exit which could result in the syncmap not getting
> > free'd. Rather than work to hard to seal this race, simply cleanup the
> free'd -> freed
> 

Sure.

> > syncmap on fini.
> > 
> > unreferenced object 0xffff88813bc53b18 (size 96):
> >    comm "gem_close_race", pid 5410, jiffies 4294917818 (age 1105.600s)
> >    hex dump (first 32 bytes):
> >      01 00 00 00 00 00 00 00 00 00 00 00 0a 00 00 00  ................
> >      00 00 00 00 00 00 00 00 6b 6b 6b 6b 06 00 00 00  ........kkkk....
> >    backtrace:
> >      [<00000000120b863a>] __sync_alloc_leaf+0x1e/0x40 [i915]
> >      [<00000000042f6959>] __sync_set+0x1bb/0x240 [i915]
> >      [<0000000090f0e90f>] i915_request_await_dma_fence+0x1c7/0x400 [i915]
> >      [<0000000056a48219>] i915_request_await_object+0x222/0x360 [i915]
> >      [<00000000aaac4ee3>] i915_gem_do_execbuffer+0x1bd0/0x2250 [i915]
> >      [<000000003c9d830f>] i915_gem_execbuffer2_ioctl+0x405/0xce0 [i915]
> >      [<00000000fd7a8e68>] drm_ioctl_kernel+0xb0/0xf0 [drm]
> >      [<00000000e721ee87>] drm_ioctl+0x305/0x3c0 [drm]
> >      [<000000008b0d8986>] __x64_sys_ioctl+0x71/0xb0
> >      [<0000000076c362a4>] do_syscall_64+0x33/0x80
> >      [<00000000eb7a4831>] entry_SYSCALL_64_after_hwframe+0x44/0xa9
> > 
> > Signed-off-by: Matthew Brost <matthew.brost at intel.com>
> > Fixes: 531958f6f357 ("drm/i915/gt: Track timeline activeness in enter/exit")
> > Cc: <stable at vger.kernel.org>
> > ---
> >   drivers/gpu/drm/i915/gt/intel_timeline.c | 9 +++++++++
> >   1 file changed, 9 insertions(+)
> > 
> > diff --git a/drivers/gpu/drm/i915/gt/intel_timeline.c b/drivers/gpu/drm/i915/gt/intel_timeline.c
> > index c4a126c8caef..1257f4f11e66 100644
> > --- a/drivers/gpu/drm/i915/gt/intel_timeline.c
> > +++ b/drivers/gpu/drm/i915/gt/intel_timeline.c
> > @@ -127,6 +127,15 @@ static void intel_timeline_fini(struct rcu_head *rcu)
> >   	i915_vma_put(timeline->hwsp_ggtt);
> >   	i915_active_fini(&timeline->active);
> > +
> > +	/*
> > +	 * A small race exists between intel_gt_retire_requests_timeout and
> > +	 * intel_timeline_exit which could result in the syncmap not getting
> > +	 * free'd. Rather than work to hard to seal this race, simply cleanup
> > +	 * the syncmap on fini.
> What is the race? I'm going round in circles just trying to work out how
> intel_gt_retire_requests_timeout is supposed to get to intel_timeline_exit
> in the first place.
> 

intel_gt_retire_requests_timeout increments tl->active_count, active_count == 2
intel_timeline_exit is called, returns on atomic_add_unless, active_count == 1
intel_gt_retire_requests_timeout decrements tl->active_count, active_count == 0
i915_syncmap_free is never called, memory leak

Matt

> Also, free'd -> freed.
> 
> John.
> 
> 
> > +	 */
> > +	i915_syncmap_free(&timeline->sync);
> > +
> >   	kfree(timeline);
> >   }
>