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drm/i915/gt: Unlock engine-pm after queuing the kernel context switch 

In commit a79ca656b6 ("drm/i915: Push the wakeref->count deferral to
the backend"), I erroneously concluded that we last modify the engine
inside __i915_request_commit() meaning that we could enable concurrent
submission for userspace as we enqueued this request. However, this
falls into a trap with other users of the engine->kernel_context waking
up and submitting their request before the idle-switch is queued, with
the result that the kernel_context is executed out-of-sequence most
likely upsetting the GPU and certainly ourselves when we try to retire
the out-of-sequence requests.

As such we need to hold onto the effective engine->kernel_context mutex
lock (via the engine pm mutex proxy) until we have finish queuing the
request to the engine.

v2: Serialise against concurrent intel_gt_retire_requests()
v3: Describe the hairy locking scheme with intel_gt_retire_requests()
for future reference.
v4: Combine timeline->lock and engine pm release; it's hairy.

Fixes: a79ca656b6 ("drm/i915: Push the wakeref->count deferral to the backend")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191120165514.3955081-2-chris@chris-wilson.co.uk
This commit is contained in:
Chris Wilson 2019-11-20 16:55:14 +00:00
parent a6edbca74b
commit 5cba288466
1 changed files with 40 additions and 7 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

47
drivers/gpu/drm/i915/gt/intel_engine_pm.c
View File

@ -73,8 +73,25 @@ static inline void __timeline_mark_unlock(struct intel_context *ce,
#endif /* !IS_ENABLED(CONFIG_LOCKDEP) */
static void
__intel_timeline_enter_and_release_pm(struct intel_timeline *tl,
struct intel_engine_cs *engine)
{
struct intel_gt_timelines *timelines = &engine->gt->timelines;
spin_lock(&timelines->lock);
if (!atomic_fetch_inc(&tl->active_count))
list_add_tail(&tl->link, &timelines->active_list);
__intel_wakeref_defer_park(&engine->wakeref);
spin_unlock(&timelines->lock);
}
static bool switch_to_kernel_context(struct intel_engine_cs *engine)
{
struct intel_context *ce = engine->kernel_context;
struct i915_request *rq;
unsigned long flags;
bool result = true;
@ -98,16 +115,31 @@ static bool switch_to_kernel_context(struct intel_engine_cs *engine)
* This should hold true as we can only park the engine after
* retiring the last request, thus all rings should be empty and
* all timelines idle.
*
* For unlocking, there are 2 other parties and the GPU who have a
* stake here.
*
* A new gpu user will be waiting on the engine-pm to start their
* engine_unpark. New waiters are predicated on engine->wakeref.count
* and so intel_wakeref_defer_park() acts like a mutex_unlock of the
* engine->wakeref.
*
* The other party is intel_gt_retire_requests(), which is walking the
* list of active timelines looking for completions. Meanwhile as soon
* as we call __i915_request_queue(), the GPU may complete our request.
* Ergo, if we put ourselves on the timelines.active_list
* (se intel_timeline_enter()) before we increment the
* engine->wakeref.count, we may see the request completion and retire
* it causing an undeflow of the engine->wakeref.
*/
flags = __timeline_mark_lock(engine->kernel_context);
flags = __timeline_mark_lock(ce);
GEM_BUG_ON(atomic_read(&ce->timeline->active_count) < 0);
rq = __i915_request_create(engine->kernel_context, GFP_NOWAIT);
rq = __i915_request_create(ce, GFP_NOWAIT);
if (IS_ERR(rq))
/* Context switch failed, hope for the best! Maybe reset? */
goto out_unlock;
intel_timeline_enter(i915_request_timeline(rq));
/* Check again on the next retirement. */
engine->wakeref_serial = engine->serial + 1;
i915_request_add_active_barriers(rq);
@ -116,13 +148,14 @@ static bool switch_to_kernel_context(struct intel_engine_cs *engine)
rq->sched.attr.priority = I915_PRIORITY_BARRIER;
__i915_request_commit(rq);
/* Release our exclusive hold on the engine */
__intel_wakeref_defer_park(&engine->wakeref);
__i915_request_queue(rq, NULL);
/* Expose ourselves to intel_gt_retire_requests() and new submission */
__intel_timeline_enter_and_release_pm(ce->timeline, engine);
result = false;
out_unlock:
__timeline_mark_unlock(engine->kernel_context, flags);
__timeline_mark_unlock(ce, flags);
return result;
}