From: Daniel Vetter <daniel@ffwll.ch>
To: Matthew Brost <matthew.brost@intel.com>
Cc: intel-gfx@lists.freedesktop.org, dri-devel@lists.freedesktop.org,
daniel.vetter@ffwll.ch
Subject: Re: [Intel-gfx] [PATCH 19/22] drm/i915/guc: Proper xarray usage for contexts_lookup
Date: Tue, 17 Aug 2021 12:27:29 +0200 [thread overview]
Message-ID: <YRuPEeq2e8qdKBc2@phenom.ffwll.local> (raw)
In-Reply-To: <20210816135139.10060-20-matthew.brost@intel.com>
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@intel.com>
> ---
> .../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
next prev parent reply other threads:[~2021-08-17 10:27 UTC|newest]
Thread overview: 56+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-08-16 13:51 [Intel-gfx] [PATCH 00/22] Clean up GuC CI failures, simplify locking, and kernel DOC Matthew Brost
2021-08-16 13:51 ` [Intel-gfx] [PATCH 01/22] drm/i915/guc: Fix blocked context accounting Matthew Brost
2021-08-16 13:51 ` [Intel-gfx] [PATCH 02/22] drm/i915/guc: Fix outstanding G2H accounting Matthew Brost
2021-08-17 9:39 ` Daniel Vetter
2021-08-17 18:17 ` Matthew Brost
2021-08-16 13:51 ` [Intel-gfx] [PATCH 03/22] drm/i915/guc: Unwind context requests in reverse order Matthew Brost
2021-08-16 13:51 ` [Intel-gfx] [PATCH 04/22] drm/i915/guc: Don't drop ce->guc_active.lock when unwinding context Matthew Brost
2021-08-16 13:51 ` [Intel-gfx] [PATCH 05/22] drm/i915/guc: Workaround reset G2H is received after schedule done G2H Matthew Brost
2021-08-17 9:32 ` Daniel Vetter
2021-08-17 15:03 ` Matthew Brost
2021-08-16 13:51 ` [Intel-gfx] [PATCH 06/22] drm/i915/execlists: Do not propagate errors to dependent fences Matthew Brost
2021-08-17 9:21 ` Daniel Vetter
2021-08-17 15:08 ` Matthew Brost
2021-08-17 15:49 ` Daniel Vetter
2021-08-16 13:51 ` [Intel-gfx] [PATCH 07/22] drm/i915/selftests: Add a cancel request selftest that triggers a reset Matthew Brost
2021-08-16 13:51 ` [Intel-gfx] [PATCH 08/22] drm/i915/guc: Don't enable scheduling on a banned context, guc_id invalid, not registered Matthew Brost
2021-08-17 9:47 ` Daniel Vetter
2021-08-17 9:57 ` Daniel Vetter
2021-08-17 16:44 ` Matthew Brost
2021-08-16 13:51 ` [Intel-gfx] [PATCH 09/22] drm/i915/selftests: Fix memory corruption in live_lrc_isolation Matthew Brost
2021-08-16 13:51 ` [Intel-gfx] [PATCH 10/22] drm/i915/selftests: Add initial GuC selftest for scrubbing lost G2H Matthew Brost
2021-08-16 13:51 ` [Intel-gfx] [PATCH 11/22] drm/i915/guc: Take context ref when cancelling request Matthew Brost
2021-08-16 13:51 ` [Intel-gfx] [PATCH 12/22] drm/i915/guc: Don't touch guc_state.sched_state without a lock Matthew Brost
2021-08-17 7:21 ` kernel test robot
2021-08-16 13:51 ` [Intel-gfx] [PATCH 13/22] drm/i915/guc: Reset LRC descriptor if register returns -ENODEV Matthew Brost
2021-08-16 13:51 ` [Intel-gfx] [PATCH 14/22] drm/i915: Allocate error capture in atomic context Matthew Brost
2021-08-17 10:06 ` Daniel Vetter
2021-08-17 16:12 ` Matthew Brost
2021-08-16 13:51 ` [Intel-gfx] [PATCH 15/22] drm/i915/guc: Flush G2H work queue during reset Matthew Brost
2021-08-17 10:06 ` Daniel Vetter
2021-08-16 13:51 ` [Intel-gfx] [PATCH 16/22] drm/i915/guc: Release submit fence from an IRQ Matthew Brost
2021-08-17 10:08 ` Daniel Vetter
2021-08-16 13:51 ` [Intel-gfx] [PATCH 17/22] drm/i915/guc: Move guc_blocked fence to struct guc_state Matthew Brost
2021-08-17 10:10 ` Daniel Vetter
2021-08-16 13:51 ` [Intel-gfx] [PATCH 18/22] drm/i915/guc: Rework and simplify locking Matthew Brost
2021-08-17 10:15 ` Daniel Vetter
2021-08-17 15:30 ` Matthew Brost
2021-08-16 13:51 ` [Intel-gfx] [PATCH 19/22] drm/i915/guc: Proper xarray usage for contexts_lookup Matthew Brost
2021-08-17 10:27 ` Daniel Vetter [this message]
2021-08-17 15:26 ` Matthew Brost
2021-08-17 17:13 ` Daniel Vetter
2021-08-17 17:13 ` Matthew Brost
2021-08-16 13:51 ` [Intel-gfx] [PATCH 20/22] drm/i915/guc: Drop pin count check trick between sched_disable and re-pin Matthew Brost
2021-08-16 13:51 ` [Intel-gfx] [PATCH 21/22] drm/i915/guc: Move GuC priority fields in context under guc_active Matthew Brost
2021-08-16 13:51 ` [Intel-gfx] [PATCH 22/22] drm/i915/guc: Add GuC kernel doc Matthew Brost
2021-08-17 11:11 ` Daniel Vetter
2021-08-17 16:36 ` Matthew Brost
2021-08-17 17:20 ` Daniel Vetter
2021-08-17 17:27 ` Michal Wajdeczko
2021-08-17 17:34 ` Daniel Vetter
2021-08-17 20:41 ` Michal Wajdeczko
2021-08-17 21:49 ` Daniel Vetter
2021-08-17 12:49 ` [Intel-gfx] ✗ Fi.CI.CHECKPATCH: warning for Clean up GuC CI failures, simplify locking, and kernel DOC (rev2) Patchwork
2021-08-17 12:51 ` [Intel-gfx] ✗ Fi.CI.SPARSE: " Patchwork
2021-08-17 13:22 ` [Intel-gfx] ✓ Fi.CI.BAT: success " Patchwork
2021-08-17 14:39 ` [Intel-gfx] ✗ Fi.CI.IGT: failure " Patchwork
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