Commit f36a75ab authored by Jesus Narvaez's avatar Jesus Narvaez Committed by John Harrison
Browse files

drm/i915/guc: Handle race condition where wakeref count drops below 0 



There is a rare race condition when preparing for a reset where
guc_lrc_desc_unpin() could be in the process of deregistering a context
while a different thread is scrubbing outstanding contexts and it alters
the context state and does a wakeref put. Then, if there is a failure
with deregister_context(), a second wakeref put could occur. As a result
the wakeref count could drop below 0 and fail an INTEL_WAKEREF_BUG_ON()
check.

Therefore if there is a failure with deregister_context(), undo the
context state changes and do a wakeref put only if the context was set
to be destroyed earlier.

v2: Expand comment to better explain change. (Daniele)
v3: Removed addition to the original comment. (Daniele)

Fixes: 2f2cc53b ("drm/i915/guc: Close deregister-context race against CT-loss")
Signed-off-by: default avatarJesus Narvaez <jesus.narvaez@intel.com>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Alan Previn <alan.previn.teres.alexis@intel.com>
Cc: Anshuman Gupta <anshuman.gupta@intel.com>
Cc: Mousumi Jana <mousumi.jana@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: default avatarDaniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Signed-off-by: default avatarJohn Harrison <John.C.Harrison@Intel.com>
Link: https://lore.kernel.org/r/20250528230551.1855177-1-jesus.narvaez@intel.com
parent a6a26786

drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c

+14 −3
Original line number Diff line number Diff line
@@ -3443,17 +3443,28 @@ static inline int guc_lrc_desc_unpin(struct intel_context *ce) 
	 * GuC is active, lets destroy this context, but at this point we can still be racing

	 * with suspend, so we undo everything if the H2G fails in deregister_context so

	 * that GuC reset will find this context during clean up.

	 *

	 * There is a race condition where the reset code could have altered

	 * this context's state and done a wakeref put before we try to

	 * deregister it here. So check if the context is still set to be

	 * destroyed before undoing earlier changes, to avoid two wakeref puts

	 * on the same context.

	 */

	ret = deregister_context(ce, ce->guc_id.id);

	if (ret) {

		bool pending_destroyed;

		spin_lock_irqsave(&ce->guc_state.lock, flags);

		pending_destroyed = context_destroyed(ce);

		if (pending_destroyed) {

			set_context_registered(ce);

			clr_context_destroyed(ce);

		}

		spin_unlock_irqrestore(&ce->guc_state.lock, flags);

		/*

		 * As gt-pm is awake at function entry, intel_wakeref_put_async merely decrements

		 * the wakeref immediately but per function spec usage call this after unlock.

		 */

		if (pending_destroyed)

			intel_wakeref_put_async(&gt->wakeref);

	}