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Since we allow removing the timeline map at runtime, there is a risk that rq->hwsp points into a stale page. To control that risk, we hold the RCU read lock while reading *rq->hwsp, but we missed a couple of important barriers. First, the unpinning / removal of the timeline map must be after all RCU readers into that map are complete, i.e. after an rcu barrier (in this case courtesy of call_rcu()). Secondly, we must make sure that the rq->hwsp we are about to dereference under the RCU lock is valid. In this case, we make the rq->hwsp pointer safe during i915_request_retire() and so we know that rq->hwsp may become invalid only after the request has been signaled. Therefore is the request is not yet signaled when we acquire rq->hwsp under the RCU, we know that rq->hwsp will remain valid for the duration of the RCU read lock. This is a very small window that may lead to either considering the request not completed (causing a delay until the request is checked again, any wait for the request is not affected) or dereferencing an invalid pointer. Fixes:3adac4689f("drm/i915: Introduce concept of per-timeline (context) HWSP") Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk> Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com> Cc: <stable@vger.kernel.org> # v5.1+ Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com> Link: https://patchwork.freedesktop.org/patch/msgid/20201218122421.18344-1-chris@chris-wilson.co.uk (cherry picked from commit9bb36cf660) Signed-off-by: Jani Nikula <jani.nikula@intel.com> Link: https://patchwork.freedesktop.org/patch/msgid/20210118101755.476744-1-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.