[Intel-gfx] ✗ Fi.CI.IGT: failure for drm/i915/userptr: Wrap mmu_notifier inside its own rw_semaphore

[Chris Wilson]

Quoting Daniel Vetter (2018-03-27 07:48:00)
> On Mon, Mar 26, 2018 at 11:38:55PM +0100, Chris Wilson wrote:
> > Quoting Chris Wilson (2018-03-26 21:08:33)
> > > Quoting Patchwork (2018-03-26 17:53:44)
> > > > Test gem_userptr_blits:
> > > >         Subgroup coherency-unsync:
> > > >                 pass       -> INCOMPLETE (shard-hsw)
> > > 
> > > Forgot that obj->userptr.mn may not exist.
> > > 
> > > >         Subgroup dmabuf-sync:
> > > >                 pass       -> DMESG-WARN (shard-hsw)
> > > 
> > > But this is the tricky lockdep one, warning of the recursion from gup
> > > into mmu_invalidate_range, i.e.
> > > 
> > > down_read(&i915_mmu_notifier->sem);
> > > down_read(&mm_struct->mmap_sem);
> > >         gup();
> > >                 down_write(&i915_mmut_notifier->sem);
> > > 
> > > That seems a genuine deadlock... So I wonder how we managed to get a
> > > lockdep splat and not a dead machine. Maybe gup never triggers the
> > > recursion for our set of flags? Hmm.
> > 
> > In another universe, CI found
> > 
> > [  255.666496] ======================================================
> > [  255.666498] WARNING: possible circular locking dependency detected
> > [  255.666500] 4.16.0-rc6-CI-Trybot_1944+ #1 Tainted: G     U  W       
> > [  255.666502] ------------------------------------------------------
> > [  255.666503] gem_userptr_bli/4794 is trying to acquire lock:
> > [  255.666505]  (fs_reclaim){+.+.}, at: [<00000000e1b95c73>] fs_reclaim_acquire.part.12+0x0/0x30
> > [  255.666510] 
> >                but task is already holding lock:
> > [  255.666512]  (&mn->sem){+.+.}, at: [<000000007c59ba79>] i915_gem_userptr_mn_invalidate_range_start+0x3e/0x1a0 [i915]
> > [  255.666553] 
> >                which lock already depends on the new lock.
> > 
> > [  255.666555] 
> >                the existing dependency chain (in reverse order) is:
> > [  255.666557] 
> >                -> #2 (&mn->sem){+.+.}:
> > [  255.666578]        i915_gem_userptr_mn_invalidate_range_start+0x3e/0x1a0 [i915]
> > [  255.666581]        __mmu_notifier_invalidate_range_start+0x73/0xb0
> > [  255.666584]        zap_page_range_single+0xcc/0xe0
> > [  255.666586]        unmap_mapping_pages+0xd4/0x110
> > [  255.666606]        i915_vma_revoke_mmap+0x7e/0x1c0 [i915]
> > [  255.666625]        i915_vma_unbind+0x60a/0xa10 [i915]
> > [  255.666644]        i915_gem_object_set_tiling+0xf6/0x5b0 [i915]
> > [  255.666662]        i915_gem_set_tiling_ioctl+0x262/0x2f0 [i915]
> > [  255.666665]        drm_ioctl_kernel+0x60/0xa0
> > [  255.666667]        drm_ioctl+0x27e/0x320
> > [  255.666669]        do_vfs_ioctl+0x8a/0x670
> > [  255.666670]        SyS_ioctl+0x36/0x70
> > [  255.666672]        do_syscall_64+0x65/0x1a0
> > [  255.666675]        entry_SYSCALL_64_after_hwframe+0x42/0xb7
> > [  255.666676] 
> >                -> #1 (&mapping->i_mmap_rwsem){++++}:
> > [  255.666680]        unmap_mapping_pages+0x3d/0x110
> > [  255.666698]        i915_vma_revoke_mmap+0x7e/0x1c0 [i915]
> > [  255.666716]        i915_vma_unbind+0x60a/0xa10 [i915]
> > [  255.666734]        i915_gem_object_unbind+0xa0/0x130 [i915]
> > [  255.666751]        i915_gem_shrink+0x2d1/0x5d0 [i915]
> > [  255.666767]        i915_drop_caches_set+0x92/0x190 [i915]
> > [  255.666770]        simple_attr_write+0xab/0xc0
> > [  255.666772]        full_proxy_write+0x4b/0x70
> > [  255.666774]        __vfs_write+0x1e/0x130
> > [  255.666776]        vfs_write+0xbd/0x1b0
> > [  255.666778]        SyS_write+0x40/0xa0
> > [  255.666779]        do_syscall_64+0x65/0x1a0
> > [  255.666781]        entry_SYSCALL_64_after_hwframe+0x42/0xb7
> > [  255.666783] 
> >                -> #0 (fs_reclaim){+.+.}:
> > [  255.666786]        fs_reclaim_acquire.part.12+0x24/0x30
> > [  255.666788]        __alloc_pages_nodemask+0x1f1/0x11d0
> > [  255.666790]        __get_free_pages+0x9/0x40
> > [  255.666792]        __pud_alloc+0x25/0xb0
> > [  255.666794]        copy_page_range+0xa75/0xaf0
> > [  255.666796]        copy_process.part.7+0x1267/0x1d90
> > [  255.666798]        _do_fork+0xc0/0x6b0
> > [  255.666800]        do_syscall_64+0x65/0x1a0
> > [  255.666801]        entry_SYSCALL_64_after_hwframe+0x42/0xb7
> > [  255.666803] 
> >                other info that might help us debug this:
> > 
> > [  255.666805] Chain exists of:
> >                  fs_reclaim --> &mapping->i_mmap_rwsem --> &mn->sem
> > 
> > [  255.666809]  Possible unsafe locking scenario:
> > 
> > [  255.666811]        CPU0                    CPU1
> > [  255.666812]        ----                    ----
> > [  255.666814]   lock(&mn->sem);
> > [  255.666815]                                lock(&mapping->i_mmap_rwsem);
> > [  255.666817]                                lock(&mn->sem);
> > [  255.666819]   lock(fs_reclaim);
> > [  255.666821] 
> > 
> > So a shrinker deadlock. That doesn't look easy to wriggle out of, as we
> > have a random chunk of code that's between invalidate_range_start and
> > invalidate_range_end.
> 
> Christian König said something like "with this design you can't allocate
> anything while holding locks you might need from the mmu notifier".
> Because reclaim eats into the mmu notifiers.

Oh, we aren't allocating from under the locks. That's the first thing I
double checked. Afaict, the only window is the code in the caller that's
between range_start/range_end. If that also can't touch fs_reclaim, then
this is just a red herring...
-Chris