On Mon, Aug 16, 2021 at 06:51:36AM -0700, Matthew Brost wrote:
> Lock the xarray and take ref to the context if needed.
>
> v2:
> (Checkpatch)
> - Add new line after declaration
>
> Signed-off-by: Matthew Brost <matthew.brost@xxxxxxxxx>
> ---
> .../gpu/drm/i915/gt/uc/intel_guc_submission.c | 84 ++++++++++++++++---
> 1 file changed, 73 insertions(+), 11 deletions(-)
>
> diff --git a/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c b/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c
> index ba19b99173fc..2ecb2f002bed 100644
> --- a/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c
> +++ b/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c
> @@ -599,8 +599,18 @@ static void scrub_guc_desc_for_outstanding_g2h(struct intel_guc *guc)
> unsigned long index, flags;
> bool pending_disable, pending_enable, deregister, destroyed, banned;
>
> + xa_lock_irqsave(&guc->context_lookup, flags);
> xa_for_each(&guc->context_lookup, index, ce) {
> - spin_lock_irqsave(&ce->guc_state.lock, flags);
> + /*
> + * Corner case where the ref count on the object is zero but and
> + * deregister G2H was lost. In this case we don't touch the ref
> + * count and finish the destroy of the context.
> + */
> + bool do_put = kref_get_unless_zero(&ce->ref);
This looks really scary, because in another loop below you have an
unconditional refcount increase. This means sometimes guc->context_lookup
xarray guarantees we hold a full reference on the context, sometimes we
don't. So we're right back in "protect the code" O(N^2) review complexity
instead of invariant rules about the datastructure, which is linear.
Essentially anytime you feel like you have to add a comment to explain
what's going on about concurrent stuff you're racing with, you're
protecting code, not data.
Since guc can't do a hole lot without the guc_id registered and all that,
I kinda expected you'd always have a full reference here. If there's
intermediate stages (e.g. around unregister) where this is currently not
always the case, then those should make sure a full reference is held.
Another option would be to threa ->context_lookup as a weak reference that
we lazily clean up when the context is finalized. That works too, but
probably not with a spinlock (since you most likely have to wait for all
pending guc transations to complete), but it's another option.
Either way I think standard process is needed here for locking design,
i.e.
1. come up with the right invariants ("we always have a full reference
when a context is ont he guc->context_lookup xarray")
2. come up with the locks. From the guc side the xa_lock is maybe good
enough, but from the context side this doesn't protect against a
re-registering racing against a deregistering. So probably needs more
rules on top, and then you have a nice lock inversion in a few places like
here.
3. document it and roll it out.
The other thing is that this is a very tricky iterator, and there's a few
copies of it. That is, if this is the right solution. As-is this should be
abstracted away into guc_context_iter_begin/next_end() helpers, e.g. like
we have for drm_connector_list_iter_begin/end_next as an example.
Cheers, Daniel
> +
> + xa_unlock(&guc->context_lookup);
> +
> + spin_lock(&ce->guc_state.lock);
>
> /*
> * Once we are at this point submission_disabled() is guaranteed
> @@ -616,7 +626,9 @@ static void scrub_guc_desc_for_outstanding_g2h(struct intel_guc *guc)
> banned = context_banned(ce);
> init_sched_state(ce);
>
> - spin_unlock_irqrestore(&ce->guc_state.lock, flags);
> + spin_unlock(&ce->guc_state.lock);
> +
> + GEM_BUG_ON(!do_put && !destroyed);
>
> if (pending_enable || destroyed || deregister) {
> atomic_dec(&guc->outstanding_submission_g2h);
> @@ -645,7 +657,12 @@ static void scrub_guc_desc_for_outstanding_g2h(struct intel_guc *guc)
>
> intel_context_put(ce);
> }
> +
> + if (do_put)
> + intel_context_put(ce);
> + xa_lock(&guc->context_lookup);
> }
> + xa_unlock_irqrestore(&guc->context_lookup, flags);
> }
>
> static inline bool
> @@ -866,16 +883,26 @@ void intel_guc_submission_reset(struct intel_guc *guc, bool stalled)
> {
> struct intel_context *ce;
> unsigned long index;
> + unsigned long flags;
>
> if (unlikely(!guc_submission_initialized(guc))) {
> /* Reset called during driver load? GuC not yet initialised! */
> return;
> }
>
> - xa_for_each(&guc->context_lookup, index, ce)
> + xa_lock_irqsave(&guc->context_lookup, flags);
> + xa_for_each(&guc->context_lookup, index, ce) {
> + intel_context_get(ce);
> + xa_unlock(&guc->context_lookup);
> +
> if (intel_context_is_pinned(ce))
> __guc_reset_context(ce, stalled);
>
> + intel_context_put(ce);
> + xa_lock(&guc->context_lookup);
> + }
> + xa_unlock_irqrestore(&guc->context_lookup, flags);
> +
> /* GuC is blown away, drop all references to contexts */
> xa_destroy(&guc->context_lookup);
> }
> @@ -950,11 +977,21 @@ void intel_guc_submission_cancel_requests(struct intel_guc *guc)
> {
> struct intel_context *ce;
> unsigned long index;
> + unsigned long flags;
> +
> + xa_lock_irqsave(&guc->context_lookup, flags);
> + xa_for_each(&guc->context_lookup, index, ce) {
> + intel_context_get(ce);
> + xa_unlock(&guc->context_lookup);
>
> - xa_for_each(&guc->context_lookup, index, ce)
> if (intel_context_is_pinned(ce))
> guc_cancel_context_requests(ce);
>
> + intel_context_put(ce);
> + xa_lock(&guc->context_lookup);
> + }
> + xa_unlock_irqrestore(&guc->context_lookup, flags);
> +
> guc_cancel_sched_engine_requests(guc->sched_engine);
>
> /* GuC is blown away, drop all references to contexts */
> @@ -2848,21 +2885,26 @@ void intel_guc_find_hung_context(struct intel_engine_cs *engine)
> struct intel_context *ce;
> struct i915_request *rq;
> unsigned long index;
> + unsigned long flags;
>
> /* Reset called during driver load? GuC not yet initialised! */
> if (unlikely(!guc_submission_initialized(guc)))
> return;
>
> + xa_lock_irqsave(&guc->context_lookup, flags);
> xa_for_each(&guc->context_lookup, index, ce) {
> + intel_context_get(ce);
> + xa_unlock(&guc->context_lookup);
> +
> if (!intel_context_is_pinned(ce))
> - continue;
> + goto next;
>
> if (intel_engine_is_virtual(ce->engine)) {
> if (!(ce->engine->mask & engine->mask))
> - continue;
> + goto next;
> } else {
> if (ce->engine != engine)
> - continue;
> + goto next;
> }
>
> list_for_each_entry(rq, &ce->guc_active.requests, sched.link) {
> @@ -2872,9 +2914,17 @@ void intel_guc_find_hung_context(struct intel_engine_cs *engine)
> intel_engine_set_hung_context(engine, ce);
>
> /* Can only cope with one hang at a time... */
> - return;
> + intel_context_put(ce);
> + xa_lock(&guc->context_lookup);
> + goto done;
> }
> +next:
> + intel_context_put(ce);
> + xa_lock(&guc->context_lookup);
> +
> }
> +done:
> + xa_unlock_irqrestore(&guc->context_lookup, flags);
> }
>
> void intel_guc_dump_active_requests(struct intel_engine_cs *engine,
> @@ -2890,23 +2940,32 @@ void intel_guc_dump_active_requests(struct intel_engine_cs *engine,
> if (unlikely(!guc_submission_initialized(guc)))
> return;
>
> + xa_lock_irqsave(&guc->context_lookup, flags);
> xa_for_each(&guc->context_lookup, index, ce) {
> + intel_context_get(ce);
> + xa_unlock(&guc->context_lookup);
> +
> if (!intel_context_is_pinned(ce))
> - continue;
> + goto next;
>
> if (intel_engine_is_virtual(ce->engine)) {
> if (!(ce->engine->mask & engine->mask))
> - continue;
> + goto next;
> } else {
> if (ce->engine != engine)
> - continue;
> + goto next;
> }
>
> spin_lock_irqsave(&ce->guc_active.lock, flags);
> intel_engine_dump_active_requests(&ce->guc_active.requests,
> hung_rq, m);
> spin_unlock_irqrestore(&ce->guc_active.lock, flags);
> +
> +next:
> + intel_context_put(ce);
> + xa_lock(&guc->context_lookup);
> }
> + xa_unlock_irqrestore(&guc->context_lookup, flags);
> }
>
> void intel_guc_submission_print_info(struct intel_guc *guc,
> @@ -2960,7 +3019,9 @@ void intel_guc_submission_print_context_info(struct intel_guc *guc,
> {
> struct intel_context *ce;
> unsigned long index;
> + unsigned long flags;
>
> + xa_lock_irqsave(&guc->context_lookup, flags);
> xa_for_each(&guc->context_lookup, index, ce) {
> drm_printf(p, "GuC lrc descriptor %u:\n", ce->guc_id);
> drm_printf(p, "\tHW Context Desc: 0x%08x\n", ce->lrc.lrca);
> @@ -2979,6 +3040,7 @@ void intel_guc_submission_print_context_info(struct intel_guc *guc,
>
> guc_log_context_priority(p, ce);
> }
> + xa_unlock_irqrestore(&guc->context_lookup, flags);
> }
>
> static struct intel_context *
> --
> 2.32.0
>
--
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
