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  1. 31 Jul, 2023 1 commit
    • Janusz Krzysztofik's avatar
      drm/i915: Fix premature release of request's reusable memory · 946e047a
      Janusz Krzysztofik authored 
      Infinite waits for completion of GPU activity have been observed in CI,
mostly inside __i915_active_wait(), triggered by igt@gem_barrier_race or
igt@perf@stress-open-close.  Root cause analysis, based of ftrace dumps
generated with a lot of extra trace_printk() calls added to the code,
revealed loops of request dependencies being accidentally built,
preventing the requests from being processed, each waiting for completion
of another one's activity.

After we substitute a new request for a last active one tracked on a
timeline, we set up a dependency of our new request to wait on completion
of current activity of that previous one.  While doing that, we must take
care of keeping the old request still in memory until we use its
attributes for setting up that await dependency, or we can happen to set
up the await dependency on an unrelated request that already reuses the
memory previously allocated to the old one, already released.  Combined
with perf adding consecutive kernel context remote requests to different
user context timelines, unresolvable loops of await dependencies can be
built, leading do infinite waits.

We obtain a pointer to the previous request to wait upon when we
substitute it with a pointer to our new request in an active tracker,
e.g. in intel_timeline.last_request.  In some processing paths we protect
that old request from being freed before we use it by getting a reference
to it under RCU protection, but in others, e.g.  __i915_request_commit()
-> __i915_request_add_to_timeline() -> __i915_request_ensure_ordering(),
we don't.  But anyway, since the requests' memory is SLAB_FAILSAFE_BY_RCU,
that RCU protection is not sufficient against reuse of memory.

We could protect i915_request's memory from being prematurely reused by
calling its release function via call_rcu() and using rcu_read_lock()
consequently, as proposed in v1.  However, that approach leads to
significant (up to 10 times) increase of SLAB utilization by i915_request
SLAB cache.  Another potential approach is to take a reference to the
previous active fence.

When updating an active fence tracker, we first lock the new fence,
substitute a pointer of the current active fence with the new one, then we
lock the substituted fence.  With this approach, there is a time window
after the substitution and before the lock when the request can be
concurrently released by an interrupt handler and its memory reused, then
we may happen to lock and return a new, unrelated request.

Always get a reference to the current active fence first, before
replacing it with a new one.  Having it protected from premature release
and reuse, lock it and then replace with the new one but only if not
yet signalled via a potential concurrent interrupt nor replaced with
another one by a potential concurrent thread, otherwise retry, starting
from getting a reference to the new current one.  Adjust users to not
get a reference to the previous active fence themselves and always put the
reference got by __i915_active_fence_set() when no longer needed.

v3: Fix lockdep splat reports and other issues caused by incorrect use of
    try_cmpxchg() (use (cmpxchg() != prev) instead)
v2: Protect request's memory by getting a reference to it in favor of
    delegating its release to call_rcu() (Chris)

Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/8211
Fixes: df9f85d8 ("drm/i915: Serialise i915_active_fence_set() with itself")
Suggested-by: default avatarChris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: default avatarJanusz Krzysztofik <janusz.krzysztofik@linux.intel.com>
Cc: <stable@vger.kernel.org> # v5.6+
Reviewed-by: default avatarAndi Shyti <andi.shyti@linux.intel.com>
Signed-off-by: default avatarAndi Shyti <andi.shyti@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230720093543.832147-2-janusz.krzysztofik@linux.intel.com
      946e047a
  3. 27 Jul, 2023 1 commit
    • Alan Previn's avatar
      drm/i915/selftest/gsc: Ensure GSC Proxy init completes before selftests · b1cef13e
      Alan Previn authored 
      On MTL, if the GSC Proxy init flows haven't completed, submissions to the
GSC engine will fail. Those init flows are dependent on the mei's
gsc_proxy component that is loaded in parallel with i915 and a
worker that could potentially start after i915 driver init is done.

That said, all subsytems that access the GSC engine today does check
for such init flow completion before using the GSC engine. However,
selftests currently don't wait on anything before starting.

To fix this, add a waiter function at the start of __run_selftests
that waits for gsc-proxy init flows to complete. Selftests shouldn't
care if the proxy-init failed as that should be flagged elsewhere.

Difference from prior versions:
   v7: - Change the fw status to INTEL_UC_FIRMWARE_LOAD_FAIL if the
         proxy-init fails so that intel_gsc_uc_fw_proxy_get_status
         catches it. (Daniele)
   v6: - Add a helper that returns something more than a boolean
         so we selftest can stop waiting if proxy-init hadn't
         completed but failed (Daniele).
   v5: - Move the call to __wait_gsc_proxy_completed from common
         __run_selftests dispatcher to the group-level selftest
         function (Trvtko).
       - change the pr_info to pr_warn if we hit the timeout.
   v4: - Remove generalized waiters function table framework (Tvrtko).
       - Remove mention of CI-framework-timeout from comments (Tvrtko).
   v3: - Rebase to latest drm-tip.
   v2: - Based on internal testing, increase the timeout for gsc-proxy
         specific case to 8 seconds.
Signed-off-by: default avatarAlan Previn <alan.previn.teres.alexis@intel.com>
Reviewed-by: default avatarDaniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Signed-off-by: default avatarDaniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230720230126.375566-1-alan.previn.teres.alexis@intel.com
      b1cef13e
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