[Intel-gfx] [PATCH 1/6] drm/i915/gt: Protect signaler walk with RCU

[Chris Wilson]

Quoting Matthew Auld (2020-02-20 12:47:28)
> On 20/02/2020 07:50, Chris Wilson wrote:
> > While we know that the waiters cannot disappear as we walk our list
> > (only that they might be added), the same cannot be said for our
> > signalers as they may be completed by the HW and retired as we process
> > this request. Ergo we need to use rcu to protect the list iteration and
> > remember to mark up the list_del_rcu.
> > 
> > v2: Mark the deps as safe-for-rcu
> > 
> > Fixes: 793c22617367 ("drm/i915/gt: Protect execlists_hold/unhold from new waiters")
> > Fixes: 32ff621fd744 ("drm/i915/gt: Allow temporary suspension of inflight requests")
> > Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> > Cc: Chris Wilson <chris at chris-wilson.co.uk>
> > Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> > Cc: Mika Kuoppala <mika.kuoppala at linux.intel.com>
> > Cc: Matthew Auld <matthew.auld at intel.com>
> > ---
> >   drivers/gpu/drm/i915/gt/intel_lrc.c   | 16 ++++++++++------
> >   drivers/gpu/drm/i915/i915_scheduler.c |  7 ++++---
> >   2 files changed, 14 insertions(+), 9 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/gt/intel_lrc.c b/drivers/gpu/drm/i915/gt/intel_lrc.c
> > index ba31cbe8c68e..47561dc29304 100644
> > --- a/drivers/gpu/drm/i915/gt/intel_lrc.c
> > +++ b/drivers/gpu/drm/i915/gt/intel_lrc.c
> > @@ -1668,9 +1668,9 @@ last_active(const struct intel_engine_execlists *execlists)
> >                                    wait_link)
> >   
> >   #define for_each_signaler(p__, rq__) \
> > -     list_for_each_entry_lockless(p__, \
> > -                                  &(rq__)->sched.signalers_list, \
> > -                                  signal_link)
> > +     list_for_each_entry_rcu(p__, \
> > +                             &(rq__)->sched.signalers_list, \
> > +                             signal_link)
> >   
> >   static void defer_request(struct i915_request *rq, struct list_head * const pl)
> >   {
> > @@ -2533,11 +2533,13 @@ static bool execlists_hold(struct intel_engine_cs *engine,
> >   static bool hold_request(const struct i915_request *rq)
> >   {
> >       struct i915_dependency *p;
> > +     bool result = false;
> >   
> >       /*
> >        * If one of our ancestors is on hold, we must also be on hold,
> >        * otherwise we will bypass it and execute before it.
> >        */
> > +     rcu_read_lock();
> >       for_each_signaler(p, rq) {
> >               const struct i915_request *s =
> >                       container_of(p->signaler, typeof(*s), sched);
> > @@ -2545,11 +2547,13 @@ static bool hold_request(const struct i915_request *rq)
> >               if (s->engine != rq->engine)
> >                       continue;
> >   
> > -             if (i915_request_on_hold(s))
> > -                     return true;
> > +             result = i915_request_on_hold(s);
> > +             if (result)
> > +                     break;
> >       }
> > +     rcu_read_unlock();
> >   
> > -     return false;
> > +     return result;
> >   }
> >   
> >   static void __execlists_unhold(struct i915_request *rq)
> > diff --git a/drivers/gpu/drm/i915/i915_scheduler.c b/drivers/gpu/drm/i915/i915_scheduler.c
> > index e19a37a83397..59f70b674665 100644
> > --- a/drivers/gpu/drm/i915/i915_scheduler.c
> > +++ b/drivers/gpu/drm/i915/i915_scheduler.c
> > @@ -486,7 +486,7 @@ void i915_sched_node_fini(struct i915_sched_node *node)
> >       list_for_each_entry_safe(dep, tmp, &node->signalers_list, signal_link) {
> >               GEM_BUG_ON(!list_empty(&dep->dfs_link));
> >   
> > -             list_del(&dep->wait_link);
> > +             list_del_rcu(&dep->wait_link);
> >               if (dep->flags & I915_DEPENDENCY_ALLOC)
> >                       i915_dependency_free(dep);
> >       }
> > @@ -497,7 +497,7 @@ void i915_sched_node_fini(struct i915_sched_node *node)
> >               GEM_BUG_ON(dep->signaler != node);
> >               GEM_BUG_ON(!list_empty(&dep->dfs_link));
> >   
> > -             list_del(&dep->signal_link);
> > +             list_del_rcu(&dep->signal_link);
> >               if (dep->flags & I915_DEPENDENCY_ALLOC)
> >                       i915_dependency_free(dep);
> >       }
> > @@ -526,7 +526,8 @@ static struct i915_global_scheduler global = { {
> >   int __init i915_global_scheduler_init(void)
> >   {
> >       global.slab_dependencies = KMEM_CACHE(i915_dependency,
> > -                                           SLAB_HWCACHE_ALIGN);
> > +                                           SLAB_HWCACHE_ALIGN |
> > +                                           SLAB_TYPESAFE_BY_RCU);
> 
> So, the claim is that we should be fine if the node is re-used and then 
> initialised, even though there might exist a minuscule window where 
> hold_request might still be able to see it, somehow?

Yes. That is my claim. The saving grace here is that for the on-hold
transitions we must go through the engine->active.lock which we are
holding. Ergo hold_request() is safe.
-Chris