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![Chris Wilson](/assets/img/avatar_default.png)
By placing our idle-barriers in the i915_active fence tree, we expose those for reuse by other components that are issuing requests along the kernel_context. Reusing the proto-barrier active_node is perfectly fine as the new request implies a context-switch, and so an opportune point to run the idle-barrier. However, the proto-barrier is not equivalent to a normal active_node and care must be taken to avoid dereferencing the ERR_PTR used as its request marker. v2: Comment the more egregious cheek v3: A glossary! Reported-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com> Fixes:ce476c80b8
("drm/i915: Keep contexts pinned until after the next kernel context switch") Fixes:a9877da2d6
("drm/i915/oa: Reconfigure contexts on the fly") Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk> Cc: Lionel Landwerlin <lionel.g.landwerlin@intel.com> Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com> Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com> Link: https://patchwork.freedesktop.org/patch/msgid/20190802100015.1281-1-chris@chris-wilson.co.uk
418 lines
15 KiB
C
418 lines
15 KiB
C
/*
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* SPDX-License-Identifier: MIT
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*
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* Copyright © 2019 Intel Corporation
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*/
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#ifndef _I915_ACTIVE_H_
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#define _I915_ACTIVE_H_
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#include <linux/lockdep.h>
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#include "i915_active_types.h"
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#include "i915_request.h"
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/*
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* We treat requests as fences. This is not be to confused with our
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* "fence registers" but pipeline synchronisation objects ala GL_ARB_sync.
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* We use the fences to synchronize access from the CPU with activity on the
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* GPU, for example, we should not rewrite an object's PTE whilst the GPU
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* is reading them. We also track fences at a higher level to provide
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* implicit synchronisation around GEM objects, e.g. set-domain will wait
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* for outstanding GPU rendering before marking the object ready for CPU
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* access, or a pageflip will wait until the GPU is complete before showing
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* the frame on the scanout.
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*
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* In order to use a fence, the object must track the fence it needs to
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* serialise with. For example, GEM objects want to track both read and
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* write access so that we can perform concurrent read operations between
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* the CPU and GPU engines, as well as waiting for all rendering to
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* complete, or waiting for the last GPU user of a "fence register". The
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* object then embeds a #i915_active_request to track the most recent (in
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* retirement order) request relevant for the desired mode of access.
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* The #i915_active_request is updated with i915_active_request_set() to
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* track the most recent fence request, typically this is done as part of
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* i915_vma_move_to_active().
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*
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* When the #i915_active_request completes (is retired), it will
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* signal its completion to the owner through a callback as well as mark
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* itself as idle (i915_active_request.request == NULL). The owner
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* can then perform any action, such as delayed freeing of an active
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* resource including itself.
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*/
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void i915_active_retire_noop(struct i915_active_request *active,
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struct i915_request *request);
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/**
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* i915_active_request_init - prepares the activity tracker for use
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* @active - the active tracker
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* @rq - initial request to track, can be NULL
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* @func - a callback when then the tracker is retired (becomes idle),
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* can be NULL
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*
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* i915_active_request_init() prepares the embedded @active struct for use as
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* an activity tracker, that is for tracking the last known active request
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* associated with it. When the last request becomes idle, when it is retired
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* after completion, the optional callback @func is invoked.
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*/
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static inline void
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i915_active_request_init(struct i915_active_request *active,
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struct i915_request *rq,
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i915_active_retire_fn retire)
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{
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RCU_INIT_POINTER(active->request, rq);
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INIT_LIST_HEAD(&active->link);
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active->retire = retire ?: i915_active_retire_noop;
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}
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#define INIT_ACTIVE_REQUEST(name) i915_active_request_init((name), NULL, NULL)
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/**
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* i915_active_request_set - updates the tracker to watch the current request
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* @active - the active tracker
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* @request - the request to watch
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*
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* __i915_active_request_set() watches the given @request for completion. Whilst
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* that @request is busy, the @active reports busy. When that @request is
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* retired, the @active tracker is updated to report idle.
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*/
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static inline void
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__i915_active_request_set(struct i915_active_request *active,
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struct i915_request *request)
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{
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list_move(&active->link, &request->active_list);
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rcu_assign_pointer(active->request, request);
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}
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int __must_check
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i915_active_request_set(struct i915_active_request *active,
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struct i915_request *rq);
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/**
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* i915_active_request_set_retire_fn - updates the retirement callback
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* @active - the active tracker
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* @fn - the routine called when the request is retired
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* @mutex - struct_mutex used to guard retirements
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*
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* i915_active_request_set_retire_fn() updates the function pointer that
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* is called when the final request associated with the @active tracker
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* is retired.
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*/
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static inline void
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i915_active_request_set_retire_fn(struct i915_active_request *active,
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i915_active_retire_fn fn,
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struct mutex *mutex)
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{
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lockdep_assert_held(mutex);
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active->retire = fn ?: i915_active_retire_noop;
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}
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/**
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* i915_active_request_raw - return the active request
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* @active - the active tracker
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*
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* i915_active_request_raw() returns the current request being tracked, or NULL.
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* It does not obtain a reference on the request for the caller, so the caller
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* must hold struct_mutex.
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*/
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static inline struct i915_request *
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i915_active_request_raw(const struct i915_active_request *active,
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struct mutex *mutex)
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{
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return rcu_dereference_protected(active->request,
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lockdep_is_held(mutex));
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}
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/**
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* i915_active_request_peek - report the active request being monitored
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* @active - the active tracker
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*
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* i915_active_request_peek() returns the current request being tracked if
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* still active, or NULL. It does not obtain a reference on the request
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* for the caller, so the caller must hold struct_mutex.
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*/
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static inline struct i915_request *
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i915_active_request_peek(const struct i915_active_request *active,
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struct mutex *mutex)
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{
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struct i915_request *request;
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request = i915_active_request_raw(active, mutex);
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if (!request || i915_request_completed(request))
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return NULL;
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return request;
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}
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/**
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* i915_active_request_get - return a reference to the active request
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* @active - the active tracker
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*
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* i915_active_request_get() returns a reference to the active request, or NULL
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* if the active tracker is idle. The caller must hold struct_mutex.
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*/
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static inline struct i915_request *
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i915_active_request_get(const struct i915_active_request *active,
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struct mutex *mutex)
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{
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return i915_request_get(i915_active_request_peek(active, mutex));
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}
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/**
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* __i915_active_request_get_rcu - return a reference to the active request
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* @active - the active tracker
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*
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* __i915_active_request_get() returns a reference to the active request,
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* or NULL if the active tracker is idle. The caller must hold the RCU read
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* lock, but the returned pointer is safe to use outside of RCU.
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*/
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static inline struct i915_request *
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__i915_active_request_get_rcu(const struct i915_active_request *active)
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{
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/*
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* Performing a lockless retrieval of the active request is super
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* tricky. SLAB_TYPESAFE_BY_RCU merely guarantees that the backing
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* slab of request objects will not be freed whilst we hold the
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* RCU read lock. It does not guarantee that the request itself
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* will not be freed and then *reused*. Viz,
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*
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* Thread A Thread B
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*
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* rq = active.request
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* retire(rq) -> free(rq);
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* (rq is now first on the slab freelist)
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* active.request = NULL
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*
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* rq = new submission on a new object
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* ref(rq)
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*
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* To prevent the request from being reused whilst the caller
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* uses it, we take a reference like normal. Whilst acquiring
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* the reference we check that it is not in a destroyed state
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* (refcnt == 0). That prevents the request being reallocated
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* whilst the caller holds on to it. To check that the request
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* was not reallocated as we acquired the reference we have to
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* check that our request remains the active request across
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* the lookup, in the same manner as a seqlock. The visibility
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* of the pointer versus the reference counting is controlled
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* by using RCU barriers (rcu_dereference and rcu_assign_pointer).
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*
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* In the middle of all that, we inspect whether the request is
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* complete. Retiring is lazy so the request may be completed long
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* before the active tracker is updated. Querying whether the
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* request is complete is far cheaper (as it involves no locked
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* instructions setting cachelines to exclusive) than acquiring
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* the reference, so we do it first. The RCU read lock ensures the
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* pointer dereference is valid, but does not ensure that the
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* seqno nor HWS is the right one! However, if the request was
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* reallocated, that means the active tracker's request was complete.
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* If the new request is also complete, then both are and we can
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* just report the active tracker is idle. If the new request is
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* incomplete, then we acquire a reference on it and check that
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* it remained the active request.
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*
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* It is then imperative that we do not zero the request on
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* reallocation, so that we can chase the dangling pointers!
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* See i915_request_alloc().
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*/
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do {
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struct i915_request *request;
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request = rcu_dereference(active->request);
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if (!request || i915_request_completed(request))
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return NULL;
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/*
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* An especially silly compiler could decide to recompute the
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* result of i915_request_completed, more specifically
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* re-emit the load for request->fence.seqno. A race would catch
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* a later seqno value, which could flip the result from true to
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* false. Which means part of the instructions below might not
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* be executed, while later on instructions are executed. Due to
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* barriers within the refcounting the inconsistency can't reach
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* past the call to i915_request_get_rcu, but not executing
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* that while still executing i915_request_put() creates
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* havoc enough. Prevent this with a compiler barrier.
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*/
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barrier();
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request = i915_request_get_rcu(request);
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/*
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* What stops the following rcu_access_pointer() from occurring
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* before the above i915_request_get_rcu()? If we were
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* to read the value before pausing to get the reference to
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* the request, we may not notice a change in the active
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* tracker.
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*
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* The rcu_access_pointer() is a mere compiler barrier, which
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* means both the CPU and compiler are free to perform the
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* memory read without constraint. The compiler only has to
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* ensure that any operations after the rcu_access_pointer()
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* occur afterwards in program order. This means the read may
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* be performed earlier by an out-of-order CPU, or adventurous
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* compiler.
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*
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* The atomic operation at the heart of
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* i915_request_get_rcu(), see dma_fence_get_rcu(), is
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* atomic_inc_not_zero() which is only a full memory barrier
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* when successful. That is, if i915_request_get_rcu()
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* returns the request (and so with the reference counted
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* incremented) then the following read for rcu_access_pointer()
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* must occur after the atomic operation and so confirm
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* that this request is the one currently being tracked.
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*
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* The corresponding write barrier is part of
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* rcu_assign_pointer().
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*/
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if (!request || request == rcu_access_pointer(active->request))
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return rcu_pointer_handoff(request);
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i915_request_put(request);
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} while (1);
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}
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/**
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* i915_active_request_get_unlocked - return a reference to the active request
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* @active - the active tracker
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*
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* i915_active_request_get_unlocked() returns a reference to the active request,
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* or NULL if the active tracker is idle. The reference is obtained under RCU,
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* so no locking is required by the caller.
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*
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* The reference should be freed with i915_request_put().
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*/
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static inline struct i915_request *
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i915_active_request_get_unlocked(const struct i915_active_request *active)
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{
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struct i915_request *request;
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rcu_read_lock();
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request = __i915_active_request_get_rcu(active);
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rcu_read_unlock();
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return request;
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}
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/**
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* i915_active_request_isset - report whether the active tracker is assigned
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* @active - the active tracker
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*
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* i915_active_request_isset() returns true if the active tracker is currently
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* assigned to a request. Due to the lazy retiring, that request may be idle
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* and this may report stale information.
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*/
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static inline bool
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i915_active_request_isset(const struct i915_active_request *active)
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{
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return rcu_access_pointer(active->request);
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}
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/**
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* i915_active_request_retire - waits until the request is retired
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* @active - the active request on which to wait
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*
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* i915_active_request_retire() waits until the request is completed,
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* and then ensures that at least the retirement handler for this
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* @active tracker is called before returning. If the @active
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* tracker is idle, the function returns immediately.
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*/
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static inline int __must_check
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i915_active_request_retire(struct i915_active_request *active,
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struct mutex *mutex)
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{
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struct i915_request *request;
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long ret;
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request = i915_active_request_raw(active, mutex);
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if (!request)
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return 0;
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ret = i915_request_wait(request,
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I915_WAIT_INTERRUPTIBLE,
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MAX_SCHEDULE_TIMEOUT);
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if (ret < 0)
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return ret;
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list_del_init(&active->link);
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RCU_INIT_POINTER(active->request, NULL);
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active->retire(active, request);
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return 0;
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}
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/*
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* GPU activity tracking
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*
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* Each set of commands submitted to the GPU compromises a single request that
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* signals a fence upon completion. struct i915_request combines the
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* command submission, scheduling and fence signaling roles. If we want to see
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* if a particular task is complete, we need to grab the fence (struct
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* i915_request) for that task and check or wait for it to be signaled. More
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* often though we want to track the status of a bunch of tasks, for example
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* to wait for the GPU to finish accessing some memory across a variety of
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* different command pipelines from different clients. We could choose to
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* track every single request associated with the task, but knowing that
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* each request belongs to an ordered timeline (later requests within a
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* timeline must wait for earlier requests), we need only track the
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* latest request in each timeline to determine the overall status of the
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* task.
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*
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* struct i915_active provides this tracking across timelines. It builds a
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* composite shared-fence, and is updated as new work is submitted to the task,
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* forming a snapshot of the current status. It should be embedded into the
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* different resources that need to track their associated GPU activity to
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* provide a callback when that GPU activity has ceased, or otherwise to
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* provide a serialisation point either for request submission or for CPU
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* synchronisation.
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*/
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void __i915_active_init(struct drm_i915_private *i915,
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struct i915_active *ref,
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int (*active)(struct i915_active *ref),
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void (*retire)(struct i915_active *ref),
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struct lock_class_key *key);
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#define i915_active_init(i915, ref, active, retire) do { \
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static struct lock_class_key __key; \
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\
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__i915_active_init(i915, ref, active, retire, &__key); \
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} while (0)
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int i915_active_ref(struct i915_active *ref,
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u64 timeline,
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struct i915_request *rq);
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int i915_active_wait(struct i915_active *ref);
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int i915_request_await_active(struct i915_request *rq,
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struct i915_active *ref);
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int i915_request_await_active_request(struct i915_request *rq,
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struct i915_active_request *active);
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int i915_active_acquire(struct i915_active *ref);
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void i915_active_release(struct i915_active *ref);
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void __i915_active_release_nested(struct i915_active *ref, int subclass);
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bool i915_active_trygrab(struct i915_active *ref);
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void i915_active_ungrab(struct i915_active *ref);
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static inline bool
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i915_active_is_idle(const struct i915_active *ref)
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{
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return !atomic_read(&ref->count);
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}
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#if IS_ENABLED(CONFIG_DRM_I915_DEBUG_GEM)
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void i915_active_fini(struct i915_active *ref);
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#else
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static inline void i915_active_fini(struct i915_active *ref) { }
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#endif
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int i915_active_acquire_preallocate_barrier(struct i915_active *ref,
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struct intel_engine_cs *engine);
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void i915_active_acquire_barrier(struct i915_active *ref);
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void i915_request_add_active_barriers(struct i915_request *rq);
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#endif /* _I915_ACTIVE_H_ */
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