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In

commit ebc0808f
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Tue Oct 18 13:02:51 2016 +0100

    drm/i915: Restrict pagefault disabling to just around copy_from_user()

we entirely missed that there's a slow path call to eb_relocate_entry
(or i915_gem_execbuffer_relocate_entry as it was called back then)
which was left fully wrapped by pagefault_disable/enable() calls.
Previously any issues with blocking calls where handled by the
following code:

	/* we can't wait for rendering with pagefaults disabled */
	if (pagefault_disabled() && !object_is_idle(obj))
		return -EFAULT;

Now at this point the prefaulting was still around, which means in
normal applications it was very hard to hit this bug. No idea why the
regressions in igts weren't caught.

Now this all changed big time with 2 patches merged closely together.

First

commit 2889caa9
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Fri Jun 16 15:05:19 2017 +0100

    drm/i915: Eliminate lots of iterations over the execobjects array

removes the prefaulting from the first relocation path, pushing it into
the first slowpath (of which this patch added a total of 3 escalation
levels). This would have really quickly uncovered the above bug, were
it not for immediate adding a duct-tape on top with

commit 7dd4f672
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Fri Jun 16 15:05:24 2017 +0100

    drm/i915: Async GPU relocation processing

by pushing all all the relocation patching to the gpu if the buffer
was busy, which avoided all the possible blocking calls.

The entire slowpath was then furthermore ditched in

commit 7dc8f114
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Wed Mar 11 16:03:10 2020 +0000

        drm/i915/gem: Drop relocation slowpath

and resurrected in

commit fd1500fc


Author: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Date:   Wed Aug 19 16:08:43 2020 +0200

        Revert "drm/i915/gem: Drop relocation slowpath".

but this did not further impact what's going on.

Since pagefault_disable/enable is an atomic section, any sleeping in
there is prohibited, and we definitely do that without gpu relocations
since we have to wait for the gpu usage to finish before we can patch
up the relocations.
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Jon Bloomfield <jon.bloomfield@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: "Thomas Hellström" <thomas.hellstrom@linux.intel.com>
Cc: Matthew Auld <matthew.auld@intel.com>
Cc: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Cc: Dave Airlie <airlied@redhat.com>
Cc: Jason Ekstrand <jason@jlekstrand.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20210618214503.1773805-1-daniel.vetter@ffwll.ch