tree 07731a4be3b5ba66f52d3c7e4dd73c8c08e945cd
parent 9ea4feecc39dbed60496fa181d70fdb9d73c7250
author Tomas Elf <tomas.elf@intel.com> 1436452257 +0100
committer Daniel Vetter <daniel.vetter@ffwll.ch> 1436820159 +0200

drm/i915: Snapshot seqno of most recently submitted request.

The hang checker needs to inspect whether or not the ring request list is empty
as well as if the given engine has reached or passed the most recently
submitted request. The problem with this is that the hang checker cannot grab
the struct_mutex, which is required in order to safely inspect requests since
requests might be deallocated during inspection. In the past we've had kernel
panics due to this very unsynchronized access in the hang checker.

One solution to this problem is to not inspect the requests directly since
we're only interested in the seqno of the most recently submitted request - not
the request itself. Instead the seqno of the most recently submitted request is
stored separately, which the hang checker then inspects, circumventing the
issue of synchronization from the hang checker entirely.

This fixes a regression introduced in

commit 44cdd6d219bc64f6810b8ed0023a4d4db9e0fe68
Author: John Harrison <John.C.Harrison@Intel.com>
Date:   Mon Nov 24 18:49:40 2014 +0000

    drm/i915: Convert 'ring_idle()' to use requests not seqnos

v2 (Chris Wilson):
- Pass current engine seqno to ring_idle() from i915_hangcheck_elapsed() rather
than compute it over again.
- Remove extra whitespace.

Issue: VIZ-5998
Signed-off-by: Tomas Elf <tomas.elf@intel.com>
Cc: stable@vger.kernel.org
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
[danvet: Add regressing commit citation provided by Chris.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>