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In order to be valid to dereference during the i915_fence_release, after
retiring the fence and releasing its refererences, we assume that
rq->engine can only be a real engine (that stay intact until the device
is shutdown after all fences have been flushed). However, due to a quirk
of preempt-to-busy, we may retire a request that still belongs to a
virtual engine and so eventually free it with rq->engine being invalid.
To avoid dereferencing that invalid engine, we look at the
execution_mask which if it indicates it may be executed on more than one
engine, we know it originated on a virtual engine and may still be on
one.
Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/1906
Fixes: 43acd6516c ("drm/i915: Keep a per-engine request pool")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200521140617.30015-2-chris@chris-wilson.co.uk
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.