• Chris Wilson's avatar
    drm/i915: Mark i915_request.timeline as a volatile, rcu pointer · d19d71fc
    Chris Wilson authored
    The request->timeline is only valid until the request is retired (i.e.
    before it is completed). Upon retiring the request, the context may be
    unpinned and freed, and along with it the timeline may be freed. We
    therefore need to be very careful when chasing rq->timeline that the
    pointer does not disappear beneath us. The vast majority of users are in
    a protected context, either during request construction or retirement,
    where the timeline->mutex is held and the timeline cannot disappear. It
    is those few off the beaten path (where we access a second timeline) that
    need extra scrutiny -- to be added in the next patch after first adding
    the warnings about dangerous access.
    
    One complication, where we cannot use the timeline->mutex itself, is
    during request submission onto hardware (under spinlocks). Here, we want
    to check on the timeline to finalize the breadcrumb, and so we need to
    impose a second rule to ensure that the request->timeline is indeed
    valid. As we are submitting the request, it's context and timeline must
    be pinned, as it will be used by the hardware. Since it is pinned, we
    know the request->timeline must still be valid, and we cannot submit the
    idle barrier until after we release the engine->active.lock, ergo while
    submitting and holding that spinlock, a second thread cannot release the
    timeline.
    
    v2: Don't be lazy inside selftests; hold the timeline->mutex for as long
    as we need it, and tidy up acquiring the timeline with a bit of
    refactoring (i915_active_add_request)
    Signed-off-by: default avatarChris Wilson <chris@chris-wilson.co.uk>
    Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
    Reviewed-by: default avatarTvrtko Ursulin <tvrtko.ursulin@intel.com>
    Link: https://patchwork.freedesktop.org/patch/msgid/20190919111912.21631-1-chris@chris-wilson.co.uk
    d19d71fc
i915_active.h 14.9 KB