Re: [PATCH] drm/i915: Prevent recursive deadlock on releasing a busy userptr

Chris Wilson
2014-09-02T09:39:00Z

On Tue, Sep 02, 2014 at 11:35:00AM +0200, Daniel Vetter wrote:
> On Mon, Sep 01, 2014 at 02:23:17PM +0100, Tvrtko Ursulin wrote:
> >
> > On 08/07/2014 02:20 PM, Chris Wilson wrote:
> > >During release of the GEM object we hold the struct_mutex. As the
> > >object may be holding onto the last reference for the task->mm,
> > >calling mmput() may trigger exit_mmap() which close the vma
> > >which will call drm_gem_vm_close() and attempt to reacquire
> > >the struct_mutex. In order to avoid that recursion, we have
> > >to defer the mmput() until after we drop the struct_mutex,
> > >i.e. we need to schedule a worker to do the clean up. A further issue
> > >spotted by Tvrtko was caused when we took a GTT mmapping of a userptr
> > >buffer object. In that case, we would never call mmput as the object
> > >would be cyclically referenced by the GTT mmapping and not freed upon
> > >process exit - keeping the entire process mm alive after the process
> > >task was reaped. The fix employed is to replace the mm_users/mmput()
> > >reference handling to mm_count/mmdrop() for the shared i915_mm_struct.
> > >
> > > INFO: task test_surfaces:1632 blocked for more than 120 seconds.
> > > Tainted: GF O 3.14.5+ #1
> > > "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> > > test_surfaces D 0000000000000000 0 1632 1590 0x00000082
> > > ffff88014914baa8 0000000000000046 0000000000000000 ffff88014914a010
> > > 0000000000012c40 0000000000012c40 ffff8800a0058210 ffff88014784b010
> > > ffff88014914a010 ffff880037b1c820 ffff8800a0058210 ffff880037b1c824
> > > Call Trace:
> > > [<ffffffff81582499>] schedule+0x29/0x70
> > > [<ffffffff815825fe>] schedule_preempt_disabled+0xe/0x10
> > > [<ffffffff81583b93>] __mutex_lock_slowpath+0x183/0x220
> > > [<ffffffff81583c53>] mutex_lock+0x23/0x40
> > > [<ffffffffa005c2a3>] drm_gem_vm_close+0x33/0x70 [drm]
> > > [<ffffffff8115a483>] remove_vma+0x33/0x70
> > > [<ffffffff8115a5dc>] exit_mmap+0x11c/0x170
> > > [<ffffffff8104d6eb>] mmput+0x6b/0x100
> > > [<ffffffffa00f44b9>] i915_gem_userptr_release+0x89/0xc0 [i915]
> > > [<ffffffffa00e6706>] i915_gem_free_object+0x126/0x250 [i915]
> > > [<ffffffffa005c06a>] drm_gem_object_free+0x2a/0x40 [drm]
> > > [<ffffffffa005cc32>] drm_gem_object_handle_unreference_unlocked+0xe2/0x120 [drm]
> > > [<ffffffffa005ccd4>] drm_gem_object_release_handle+0x64/0x90 [drm]
> > > [<ffffffff8127ffeb>] idr_for_each+0xab/0x100
> > > [<ffffffffa005cc70>] ? drm_gem_object_handle_unreference_unlocked+0x120/0x120 [drm]
> > > [<ffffffff81583c46>] ? mutex_lock+0x16/0x40
> > > [<ffffffffa005c354>] drm_gem_release+0x24/0x40 [drm]
> > > [<ffffffffa005b82b>] drm_release+0x3fb/0x480 [drm]
> > > [<ffffffff8118d482>] __fput+0xb2/0x260
> > > [<ffffffff8118d6de>] ____fput+0xe/0x10
> > > [<ffffffff8106f27f>] task_work_run+0x8f/0xf0
> > > [<ffffffff81052228>] do_exit+0x1a8/0x480
> > > [<ffffffff81052551>] do_group_exit+0x51/0xc0
> > > [<ffffffff810525d7>] SyS_exit_group+0x17/0x20
> > > [<ffffffff8158e092>] system_call_fastpath+0x16/0x1b
> > >
> > >v2: Incorporate feedback from Tvrtko and remove the unnessary mm
> > >referencing when creating the i915_mm_struct and improve some of the
> > >function names and comments.
> >
> > [snip]
> >
> > I reviewed this some weeks back and did not spot any issues.
> >
> > Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
> Queued for -next, thanks for the patch. Aside: Is there other userptr
> stuff outstanding? I've definitely lost track of them :(

The one I posted with the extra r-b and t-b has a minor tweak to kill
the unused error code during init. but otherwise this is the last
outstanding patch.
-Chris

--
Chris Wilson, Intel Open Source Technology Centre