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Message-Id: <20161026191810.12275-4-dh.herrmann@gmail.com>
Date: Wed, 26 Oct 2016 21:17:59 +0200
From: David Herrmann <dh.herrmann@...il.com>
To: linux-kernel@...r.kernel.org
Cc: Andy Lutomirski <luto@...capital.net>,
Jiri Kosina <jikos@...nel.org>, Greg KH <greg@...ah.com>,
Hannes Reinecke <hare@...e.com>,
Steven Rostedt <rostedt@...dmis.org>,
Arnd Bergmann <arnd@...db.de>, Tom Gundersen <teg@...m.no>,
David Herrmann <dh.herrmann@...il.com>,
Josh Triplett <josh@...htriplett.org>,
Linus Torvalds <torvalds@...ux-foundation.org>,
Andrew Morton <akpm@...ux-foundation.org>
Subject: [RFC v1 03/14] bus1: util - active reference utility library
From: Tom Gundersen <teg@...m.no>
The bus1_active object implements active references. They work
similarly to plain object reference counters, but allow disabling
any new references from being taken.
Each bus1_active object goes through a set of states:
NEW: Initial state, no active references can be acquired
ACTIVE: Live state, active references can be acquired
DRAINING: Deactivated but lingering, no active references
can be acquired
DRAINED: Deactivated and all active references were dropped
RELEASED: Fully drained and synchronously released
Initially, all bus1_active objects are in state NEW. As soon as they're
activated, they enter ACTIVE and active references can be acquired.
This is the normal, live state. Once the object is deactivated, it
enters state DRAINING. No new active references can be acquired, but
some threads might still own active references. Once all those are
dropped, the object enters state DRAINED. Now the object can be
released a *single* time, before it enters state RELEASED and is
finished. It cannot be re-used anymore.
Active-references are very useful to track threads that invoke callbacks
on an object. As long as a callback is running, an active reference is
held, and as such the object is usually protected from being destroyed.
The destructor of the object needs to deactivate *and* drain the object,
before releasing resources.
Active references will be used heavy by the upcoming bus1_peer object.
Whenever a peer operates on a remote peer, it must acquire and hold an
active reference on that remote peer. This guarantees that the remote
peer will wait for this operation to finish before possibly
disconnecting from the bus.
In concept, active-references can be seen as rw-locks. However, they
have much more strict state-transitions. Prior art can be seen in
super-blocks ('atomic_t s_active'), and kernfs ('atomic_t active').
Signed-off-by: Tom Gundersen <teg@...m.no>
Signed-off-by: David Herrmann <dh.herrmann@...il.com>
---
ipc/bus1/Makefile | 3 +-
ipc/bus1/util/active.c | 419 +++++++++++++++++++++++++++++++++++++++++++++++++
ipc/bus1/util/active.h | 154 ++++++++++++++++++
3 files changed, 575 insertions(+), 1 deletion(-)
create mode 100644 ipc/bus1/util/active.c
create mode 100644 ipc/bus1/util/active.h
diff --git a/ipc/bus1/Makefile b/ipc/bus1/Makefile
index d3a4491..9e491691 100644
--- a/ipc/bus1/Makefile
+++ b/ipc/bus1/Makefile
@@ -1,5 +1,6 @@
bus1-y := \
- main.o
+ main.o \
+ util/active.o
obj-$(CONFIG_BUS1) += bus1.o
diff --git a/ipc/bus1/util/active.c b/ipc/bus1/util/active.c
new file mode 100644
index 0000000..5f5fdaa
--- /dev/null
+++ b/ipc/bus1/util/active.c
@@ -0,0 +1,419 @@
+/*
+ * Copyright (C) 2013-2016 Red Hat, Inc.
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU Lesser General Public License as published by the
+ * Free Software Foundation; either version 2.1 of the License, or (at
+ * your option) any later version.
+ */
+
+#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
+#include <linux/atomic.h>
+#include <linux/kernel.h>
+#include <linux/sched.h>
+#include <linux/wait.h>
+#include "active.h"
+
+/*
+ * Bias values track states of "active references". They're all negative. If an
+ * object is active, its active-ref-counter is >=0 and tracks all active
+ * references. Once an object is deactivated, we subtract ACTIVE_BIAS. This
+ * means, the counter is now negative but still counts the active references.
+ * Once it drops to exactly ACTIVE_BIAS, we know all active references were
+ * dropped. Exactly one thread will change it to ACTIVE_RELEASE now, perform
+ * cleanup and then put it into ACTIVE_DONE. Once released, all other threads
+ * that tried deactivating the node will now be woken up (thus, they wait until
+ * the object is fully done).
+ * The initial state during object setup is ACTIVE_NEW. If an object is
+ * directly deactivated without having ever been active, it is put into
+ * ACTIVE_RELEASE_DIRECT instead of ACTIVE_BIAS. This tracks this one-bit state
+ * across deactivation. The task putting it into ACTIVE_RELEASE now knows
+ * whether the object was active before or not.
+ *
+ * We support lockdep annotations for 'active references'. We treat active
+ * references as a read-trylock, and deactivation as a write-lock.
+ *
+ * Some archs implement atomic_sub(v) with atomic_add(-v), so reserve INT_MIN
+ * to avoid overflows if multiplied by -1.
+ */
+#define BUS1_ACTIVE_RELEASE_DIRECT (BUS1_ACTIVE_BIAS - 1)
+#define BUS1_ACTIVE_RELEASE (BUS1_ACTIVE_BIAS - 2)
+#define BUS1_ACTIVE_DONE (BUS1_ACTIVE_BIAS - 3)
+#define BUS1_ACTIVE_NEW (BUS1_ACTIVE_BIAS - 4)
+#define _BUS1_ACTIVE_RESERVED (BUS1_ACTIVE_BIAS - 5)
+
+/**
+ * bus1_active_init_private() - initialize object
+ * @active: object to initialize
+ *
+ * This initializes an active-object. The initial state is NEW, and as such no
+ * active reference can be acquired. The object must be activated first.
+ *
+ * This is an internal helper. Always use the public bus1_active_init() macro
+ * which does proper lockdep initialization for private key classes.
+ */
+void bus1_active_init_private(struct bus1_active *active)
+{
+ atomic_set(&active->count, BUS1_ACTIVE_NEW);
+}
+
+/**
+ * bus1_active_deinit() - destroy object
+ * @active: object to destroy
+ *
+ * Destroy an active-object. The object must have been initialized via
+ * bus1_active_init(), deactivated via bus1_active_deactivate(), drained via
+ * bus1_active_drain() and cleaned via bus1_active_cleanup(), before you can
+ * destroy it. Alternatively, it can also be destroyed if still in state NEW.
+ *
+ * This function only does sanity checks, it does not modify the object itself.
+ * There is no allocated memory, so there is nothing to do.
+ */
+void bus1_active_deinit(struct bus1_active *active)
+{
+ int v;
+
+ v = atomic_read(&active->count);
+ WARN_ON(v != BUS1_ACTIVE_NEW && v != BUS1_ACTIVE_DONE);
+}
+
+/**
+ * bus1_active_is_new() - check whether object is new
+ * @active: object to check
+ *
+ * This checks whether the object is new, that is, it was never activated nor
+ * deactivated.
+ *
+ * Return: True if new, false if not.
+ */
+bool bus1_active_is_new(struct bus1_active *active)
+{
+ return atomic_read(&active->count) == BUS1_ACTIVE_NEW;
+}
+
+/**
+ * bus1_active_is_active() - check whether object is active
+ * @active: object to check
+ *
+ * This checks whether the given active-object is active. That is, the object
+ * was already activated, but not deactivated, yet.
+ *
+ * Note that this function does not give any guarantee that the object is still
+ * active/inactive at the time this call returns. It only serves as a barrier.
+ *
+ * Return: True if active, false if not.
+ */
+bool bus1_active_is_active(struct bus1_active *active)
+{
+ return atomic_read(&active->count) >= 0;
+}
+
+/**
+ * bus1_active_is_deactivated() - check whether object was deactivated
+ * @active: object to check
+ *
+ * This checks whether the given active-object was already deactivated. That
+ * is, the object was actively deactivated (state NEW does *not* count as
+ * deactivated) via bus1_active_deactivate().
+ *
+ * Once this function returns true, it cannot change again on this object.
+ *
+ * Return: True if already deactivated, false if not.
+ */
+bool bus1_active_is_deactivated(struct bus1_active *active)
+{
+ int v = atomic_read(&active->count);
+
+ return v > BUS1_ACTIVE_NEW && v < 0;
+}
+
+/**
+ * bus1_active_is_drained() - check whether object is drained
+ * @active: object to check
+ *
+ * This checks whether the given object was already deactivated and is fully
+ * drained. That is, no active references to the object exist, nor can they be
+ * acquired, anymore.
+ *
+ * Return: True if drained, false if not.
+ */
+bool bus1_active_is_drained(struct bus1_active *active)
+{
+ int v = atomic_read(&active->count);
+
+ return v > BUS1_ACTIVE_NEW && v <= BUS1_ACTIVE_BIAS;
+}
+
+/**
+ * bus1_active_activate() - activate object
+ * @active: object to activate
+ *
+ * This activates the given object, if it is still in state NEW. Otherwise, it
+ * is a no-op (and the object might already be deactivated).
+ *
+ * Once this returns successfully, active references can be acquired.
+ *
+ * Return: True if this call activated it, false if it was already activated,
+ * or deactivated.
+ */
+bool bus1_active_activate(struct bus1_active *active)
+{
+ return atomic_cmpxchg(&active->count,
+ BUS1_ACTIVE_NEW, 0) == BUS1_ACTIVE_NEW;
+}
+
+/**
+ * bus1_active_deactivate() - deactivate object
+ * @active: object to deactivate
+ *
+ * This deactivates the given object, if not already done by someone else. Once
+ * this returns, no new active references can be acquired.
+ *
+ * Return: True if this call deactivated the object, false if it was already
+ * deactivated by someone else.
+ */
+bool bus1_active_deactivate(struct bus1_active *active)
+{
+ int v, v1;
+
+ v = atomic_cmpxchg(&active->count,
+ BUS1_ACTIVE_NEW, BUS1_ACTIVE_RELEASE_DIRECT);
+ if (unlikely(v == BUS1_ACTIVE_NEW))
+ return true;
+
+ /*
+ * This adds BUS1_ACTIVE_BIAS to the counter, unless its negative:
+ * atomic_add_unless_negative(&active->count, BUS1_ACTIVE_BIAS)
+ * No such global helper exists, so it is inline here.
+ */
+ for (v = atomic_read(&active->count); v >= 0; v = v1) {
+ v1 = atomic_cmpxchg(&active->count, v, v + BUS1_ACTIVE_BIAS);
+ if (likely(v1 == v))
+ return true;
+ }
+
+ return false;
+}
+
+/**
+ * bus1_active_drain() - drain active references
+ * @active: object to drain
+ * @waitq: wait-queue linked to @active
+ *
+ * This waits for all active-references on @active to be dropped. It uses the
+ * passed wait-queue to sleep. It must be the same wait-queue that is used when
+ * calling bus1_active_release().
+ *
+ * The caller must guarantee that bus1_active_deactivate() was called before.
+ *
+ * This function can be safely called in parallel on multiple CPUs.
+ *
+ * Semantically (and also enforced by lockdep), this call behaves like a
+ * down_write(), followed by an up_write(), on this active object.
+ */
+void bus1_active_drain(struct bus1_active *active, wait_queue_head_t *waitq)
+{
+ if (WARN_ON(!bus1_active_is_deactivated(active)))
+ return;
+
+#ifdef CONFIG_DEBUG_LOCK_ALLOC
+ /*
+ * We pretend this is a down_write_interruptible() and all but
+ * the release-context get interrupted. This is required, as we
+ * cannot call lock_acquired() on multiple threads without
+ * synchronization. Hence, only the release-context will do
+ * this, all others just release the lock.
+ */
+ lock_acquire_exclusive(&active->dep_map, /* lock */
+ 0, /* subclass */
+ 0, /* try-lock */
+ NULL, /* nest underneath */
+ _RET_IP_); /* IP */
+ if (atomic_read(&active->count) > BUS1_ACTIVE_BIAS)
+ lock_contended(&active->dep_map, _RET_IP_);
+#endif
+
+ /* wait until all active references were dropped */
+ wait_event(*waitq, atomic_read(&active->count) <= BUS1_ACTIVE_BIAS);
+
+#ifdef CONFIG_DEBUG_LOCK_ALLOC
+ /*
+ * Pretend that no-one got the lock, but everyone got interrupted
+ * instead. That is, they released the lock without ever actually
+ * getting it locked.
+ */
+ lock_release(&active->dep_map, /* lock */
+ 1, /* nested (no-op) */
+ _RET_IP_); /* instruction pointer */
+#endif
+}
+
+/**
+ * bus1_active_cleanup() - cleanup drained object
+ * @active: object to release
+ * @waitq: wait-queue linked to @active, or NULL
+ * @cleanup: cleanup callback, or NULL
+ * @userdata: userdata for callback
+ *
+ * This performs the final object cleanup. The caller must guarantee that the
+ * object is drained, by calling bus1_active_drain().
+ *
+ * This function invokes the passed cleanup callback on the object. However, it
+ * guarantees that this is done exactly once. If there're multiple parallel
+ * callers, this will pick one randomly and make all others wait until it is
+ * done. If you call this after it was already cleaned up, this is a no-op
+ * and only serves as barrier.
+ *
+ * If @waitq is NULL, the wait is skipped and the call returns immediately. In
+ * this case, another thread has entered before, but there is no guarantee that
+ * they finished executing the cleanup callback, yet.
+ *
+ * If @waitq is non-NULL, this call behaves like a down_write(), followed by an
+ * up_write(), just like bus1_active_drain(). If @waitq is NULL, this rather
+ * behaves like a down_write_trylock(), optionally followed by an up_write().
+ *
+ * Return: True if this is the thread that released it, false otherwise.
+ */
+bool bus1_active_cleanup(struct bus1_active *active,
+ wait_queue_head_t *waitq,
+ void (*cleanup)(struct bus1_active *, void *),
+ void *userdata)
+{
+ int v;
+
+ if (WARN_ON(!bus1_active_is_drained(active)))
+ return false;
+
+#ifdef CONFIG_DEBUG_LOCK_ALLOC
+ /*
+ * We pretend this is a down_write_interruptible() and all but
+ * the release-context get interrupted. This is required, as we
+ * cannot call lock_acquired() on multiple threads without
+ * synchronization. Hence, only the release-context will do
+ * this, all others just release the lock.
+ */
+ lock_acquire_exclusive(&active->dep_map,/* lock */
+ 0, /* subclass */
+ !waitq, /* try-lock */
+ NULL, /* nest underneath */
+ _RET_IP_); /* IP */
+#endif
+
+ /* mark object as RELEASE */
+ v = atomic_cmpxchg(&active->count,
+ BUS1_ACTIVE_RELEASE_DIRECT, BUS1_ACTIVE_RELEASE);
+ if (v != BUS1_ACTIVE_RELEASE_DIRECT)
+ v = atomic_cmpxchg(&active->count,
+ BUS1_ACTIVE_BIAS, BUS1_ACTIVE_RELEASE);
+
+ /*
+ * If this is the thread that marked the object as RELEASE, we
+ * perform the actual release. Otherwise, we wait until the
+ * release is done and the node is marked as DRAINED.
+ */
+ if (v == BUS1_ACTIVE_BIAS || v == BUS1_ACTIVE_RELEASE_DIRECT) {
+
+#ifdef CONFIG_DEBUG_LOCK_ALLOC
+ /* we're the release-context and acquired the lock */
+ lock_acquired(&active->dep_map, _RET_IP_);
+#endif
+
+ if (cleanup)
+ cleanup(active, userdata);
+
+ /* mark as DONE */
+ atomic_set(&active->count, BUS1_ACTIVE_DONE);
+ if (waitq)
+ wake_up_all(waitq);
+ } else if (waitq) {
+
+#ifdef CONFIG_DEBUG_LOCK_ALLOC
+ /* we're contended against the release context */
+ lock_contended(&active->dep_map, _RET_IP_);
+#endif
+
+ /* wait until object is DONE */
+ wait_event(*waitq,
+ atomic_read(&active->count) == BUS1_ACTIVE_DONE);
+ }
+
+#ifdef CONFIG_DEBUG_LOCK_ALLOC
+ /*
+ * No-one but the release-context acquired the lock. However,
+ * that does not matter as we simply treat this as
+ * 'interrupted'. Everyone releases the lock, but only one
+ * caller really got it.
+ */
+ lock_release(&active->dep_map, /* lock */
+ 1, /* nested (no-op) */
+ _RET_IP_); /* instruction pointer */
+#endif
+
+ /* true if we released it */
+ return v == BUS1_ACTIVE_BIAS || v == BUS1_ACTIVE_RELEASE_DIRECT;
+}
+
+/**
+ * bus1_active_lockdep_acquired() - acquire lockdep reader
+ * @active: object to acquire lockdep reader of, or NULL
+ *
+ * Whenever you acquire an active reference via bus1_active_acquire(), this
+ * function is implicitly called afterwards. It enables lockdep annotations and
+ * tells lockdep that you acquired the active reference.
+ *
+ * However, lockdep cannot support arbitrary depths, hence, we allow
+ * temporarily dropping the lockdep-annotation via
+ * bus1_active_lockdep_release(), and acquiring them later again via
+ * bus1_active_lockdep_acquire().
+ *
+ * Example: If you need to pin a large number of objects, you would acquire each
+ * of them individually via bus1_active_acquire(). Then you would
+ * perform state tracking, etc. on that object. Before you continue
+ * with the next, you call bus1_active_lockdep_released(), to pretend
+ * you released the lock (but you still retain your active reference).
+ * Now you continue with pinning the next object, etc. until you
+ * pinned all objects you need.
+ *
+ * If you now need to access one of your pinned objects (or want to
+ * release them eventually), you call bus1_active_lockdep_acquired()
+ * before accessing the object. This enables the lockdep annotations
+ * again. This cannot fail, ever. You still own the active reference
+ * at all times.
+ * Once you're done with the single object, you either release your
+ * entire active reference via bus1_active_release(), or you
+ * temporarily disable lockdep via bus1_active_lockdep_released()
+ * again, in case you need the pinned object again later.
+ *
+ * Note that you can acquired multiple active references just fine. The only
+ * reason those lockdep helpers are provided, is if you need to acquire a
+ * *large* number at the same time. Lockdep is usually limited to a depths of 64
+ * so you cannot hold more locks at the same time.
+ */
+#ifdef CONFIG_DEBUG_LOCK_ALLOC
+void bus1_active_lockdep_acquired(struct bus1_active *active)
+{
+ if (active)
+ lock_acquire_shared(&active->dep_map, /* lock */
+ 0, /* subclass */
+ 1, /* try-lock */
+ NULL, /* nest underneath */
+ _RET_IP_); /* IP */
+}
+#endif
+
+/**
+ * bus1_active_lockdep_released() - release lockdep reader
+ * @active: object to release lockdep reader of, or NULL
+ *
+ * This is the counterpart of bus1_active_lockdep_acquired(). See its
+ * documentation for details.
+ */
+#ifdef CONFIG_DEBUG_LOCK_ALLOC
+void bus1_active_lockdep_released(struct bus1_active *active)
+{
+ if (active)
+ lock_release(&active->dep_map, /* lock */
+ 1, /* nested (no-op) */
+ _RET_IP_); /* instruction pointer */
+}
+#endif
diff --git a/ipc/bus1/util/active.h b/ipc/bus1/util/active.h
new file mode 100644
index 0000000..462e7cf
--- /dev/null
+++ b/ipc/bus1/util/active.h
@@ -0,0 +1,154 @@
+#ifndef __BUS1_ACTIVE_H
+#define __BUS1_ACTIVE_H
+
+/*
+ * Copyright (C) 2013-2016 Red Hat, Inc.
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU Lesser General Public License as published by the
+ * Free Software Foundation; either version 2.1 of the License, or (at
+ * your option) any later version.
+ */
+
+/**
+ * DOC: Active References
+ *
+ * The bus1_active object implements active references. They work similarly to
+ * plain object reference counters, but allow disabling any new references from
+ * being taken.
+ *
+ * Each bus1_active object goes through a set of states:
+ * NEW: Initial state, no active references can be acquired
+ * ACTIVE: Live state, active references can be acquired
+ * DRAINING: Deactivated but lingering, no active references can be acquired
+ * DRAINED: Deactivated and all active references were dropped
+ * RELEASED: Fully drained and synchronously released
+ *
+ * Initially, all bus1_active objects are in state NEW. As soon as they're
+ * activated, they enter ACTIVE and active references can be acquired. This is
+ * the normal, live state. Once the object is deactivated, it enters state
+ * DRAINING. No new active references can be acquired, but some threads might
+ * still own active references. Once all those are dropped, the object enters
+ * state DRAINED. Now the object can be released a *single* time, before it
+ * enters state RELEASED and is finished. It cannot be re-used anymore.
+ *
+ * Active-references are very useful to track threads that call methods on an
+ * object. As long as a method is running, an active reference is held, and as
+ * such the object is usually protected from being destroyed. The destructor of
+ * the object needs to deactivate *and* drain the object, before releasing
+ * resources.
+ *
+ * Note that active-references cannot be used to manage their own backing
+ * memory. That is, they do not replace normal reference counts.
+ */
+
+#include <linux/atomic.h>
+#include <linux/lockdep.h>
+#include <linux/sched.h>
+#include <linux/wait.h>
+
+/* base value for counter-bias, see BUS1_ACTIVE_* constants for details */
+#define BUS1_ACTIVE_BIAS (INT_MIN + 5)
+
+/**
+ * struct bus1_active - active references
+ * @count: active reference counter
+ * @dep_map: lockdep annotations
+ *
+ * This object should be treated like a simple atomic_t. It will only contain
+ * more fields in the case of lockdep-enabled compilations.
+ *
+ * Users must embed this object into their parent structures and create/destroy
+ * it via bus1_active_init() and bus1_active_deinit().
+ */
+struct bus1_active {
+ atomic_t count;
+
+#ifdef CONFIG_DEBUG_LOCK_ALLOC
+ struct lockdep_map dep_map;
+#endif
+};
+
+void bus1_active_init_private(struct bus1_active *active);
+void bus1_active_deinit(struct bus1_active *active);
+bool bus1_active_is_new(struct bus1_active *active);
+bool bus1_active_is_active(struct bus1_active *active);
+bool bus1_active_is_deactivated(struct bus1_active *active);
+bool bus1_active_is_drained(struct bus1_active *active);
+bool bus1_active_activate(struct bus1_active *active);
+bool bus1_active_deactivate(struct bus1_active *active);
+void bus1_active_drain(struct bus1_active *active, wait_queue_head_t *waitq);
+bool bus1_active_cleanup(struct bus1_active *active,
+ wait_queue_head_t *waitq,
+ void (*cleanup) (struct bus1_active *, void *),
+ void *userdata);
+
+#ifdef CONFIG_DEBUG_LOCK_ALLOC
+# define bus1_active_init(_active) \
+ ({ \
+ static struct lock_class_key bus1_active_lock_key; \
+ lockdep_init_map(&(_active)->dep_map, "bus1.active", \
+ &bus1_active_lock_key, 0); \
+ bus1_active_init_private(_active); \
+ })
+void bus1_active_lockdep_acquired(struct bus1_active *active);
+void bus1_active_lockdep_released(struct bus1_active *active);
+#else
+# define bus1_active_init(_active) bus1_active_init_private(_active)
+static inline void bus1_active_lockdep_acquired(struct bus1_active *active) {}
+static inline void bus1_active_lockdep_released(struct bus1_active *active) {}
+#endif
+
+/**
+ * bus1_active_acquire() - acquire active reference
+ * @active: object to acquire active reference to, or NULL
+ *
+ * This acquires an active reference to the passed object. If the object was
+ * not activated, yet, or if it was already deactivated, this will fail and
+ * return NULL. If a reference was successfully acquired, this will return
+ * @active.
+ *
+ * If NULL is passed, this is a no-op and always returns NULL.
+ *
+ * This behaves as a down_read_trylock(). Use bus1_active_release() to release
+ * the reference again and get the matching up_read().
+ *
+ * Return: @active if reference was acquired, NULL if not.
+ */
+static inline struct bus1_active *
+bus1_active_acquire(struct bus1_active *active)
+{
+ if (active && atomic_inc_unless_negative(&active->count))
+ bus1_active_lockdep_acquired(active);
+ else
+ active = NULL;
+ return active;
+}
+
+/**
+ * bus1_active_release() - release active reference
+ * @active: object to release active reference of, or NULL
+ * @waitq: wait-queue linked to @active, or NULL
+ *
+ * This releases an active reference that was previously acquired via
+ * bus1_active_acquire().
+ *
+ * This is a no-op if NULL is passed.
+ *
+ * This behaves like an up_read().
+ *
+ * Return: NULL is returned.
+ */
+static inline struct bus1_active *
+bus1_active_release(struct bus1_active *active, wait_queue_head_t *waitq)
+{
+ if (active) {
+ bus1_active_lockdep_released(active);
+ if (atomic_dec_return(&active->count) == BUS1_ACTIVE_BIAS)
+ if (waitq)
+ wake_up(waitq);
+ }
+ return NULL;
+}
+
+#endif /* __BUS1_ACTIVE_H */
--
2.10.1
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