rcu: Create rcu_sync infrastructure
The rcu_sync infrastructure can be thought of as infrastructure to be
used to implement reader-writer primitives having extremely lightweight
readers during times when there are no writers. The first use is in
the percpu_rwsem used by the VFS subsystem.
This infrastructure is functionally equivalent to
struct rcu_sync_struct {
atomic_t counter;
};
/* Check possibility of fast-path read-side operations. */
static inline bool rcu_sync_is_idle(struct rcu_sync_struct *rss)
{
return atomic_read(&rss->counter) == 0;
}
/* Tell readers to use slowpaths. */
static inline void rcu_sync_enter(struct rcu_sync_struct *rss)
{
atomic_inc(&rss->counter);
synchronize_sched();
}
/* Allow readers to once again use fastpaths. */
static inline void rcu_sync_exit(struct rcu_sync_struct *rss)
{
synchronize_sched();
atomic_dec(&rss->counter);
}
The main difference is that it records the state and only calls
synchronize_sched() if required. At least some of the calls to
synchronize_sched() will be optimized away when rcu_sync_enter() and
rcu_sync_exit() are invoked repeatedly in quick succession.
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
This commit is contained in:
Oleg Nesterov
Paul E. McKenney
parent
3836f5337f
commit
cc44ca848f
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/*
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* RCU-based infrastructure for lightweight reader-writer locking
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*
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* This program is free software; you can redistribute it and/or modify
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* it under the terms of the GNU General Public License as published by
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* the Free Software Foundation; either version 2 of the License, or
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* (at your option) any later version.
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*
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License
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* along with this program; if not, you can access it online at
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* http://www.gnu.org/licenses/gpl-2.0.html.
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*
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* Copyright (c) 2015, Red Hat, Inc.
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*
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* Author: Oleg Nesterov <oleg@redhat.com>
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*/
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#ifndef _LINUX_RCU_SYNC_H_
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#define _LINUX_RCU_SYNC_H_
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#include <linux/wait.h>
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#include <linux/rcupdate.h>
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/* Structure to mediate between updaters and fastpath-using readers. */
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struct rcu_sync {
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int gp_state;
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int gp_count;
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wait_queue_head_t gp_wait;
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int cb_state;
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struct rcu_head cb_head;
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void (*sync)(void);
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void (*call)(struct rcu_head *, void (*)(struct rcu_head *));
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};
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#define ___RCU_SYNC_INIT(name) \
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.gp_state = 0, \
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.gp_count = 0, \
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.gp_wait = __WAIT_QUEUE_HEAD_INITIALIZER(name.gp_wait), \
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.cb_state = 0
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#define __RCU_SCHED_SYNC_INIT(name) { \
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___RCU_SYNC_INIT(name), \
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.sync = synchronize_sched, \
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.call = call_rcu_sched, \
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}
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#define __RCU_BH_SYNC_INIT(name) { \
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___RCU_SYNC_INIT(name), \
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.sync = synchronize_rcu_bh, \
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.call = call_rcu_bh, \
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}
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#define __RCU_SYNC_INIT(name) { \
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___RCU_SYNC_INIT(name), \
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.sync = synchronize_rcu, \
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.call = call_rcu, \
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}
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#define DEFINE_RCU_SCHED_SYNC(name) \
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struct rcu_sync name = __RCU_SCHED_SYNC_INIT(name)
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#define DEFINE_RCU_BH_SYNC(name) \
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struct rcu_sync name = __RCU_BH_SYNC_INIT(name)
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#define DEFINE_RCU_SYNC(name) \
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struct rcu_sync name = __RCU_SYNC_INIT(name)
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/**
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* rcu_sync_is_idle() - Are readers permitted to use their fastpaths?
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* @rsp: Pointer to rcu_sync structure to use for synchronization
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*
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* Returns true if readers are permitted to use their fastpaths.
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* Must be invoked within an RCU read-side critical section whose
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* flavor matches that of the rcu_sync struture.
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*/
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static inline bool rcu_sync_is_idle(struct rcu_sync *rsp)
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{
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return !rsp->gp_state; /* GP_IDLE */
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}
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enum rcu_sync_type { RCU_SYNC, RCU_SCHED_SYNC, RCU_BH_SYNC };
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extern void rcu_sync_init(struct rcu_sync *, enum rcu_sync_type);
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extern void rcu_sync_enter(struct rcu_sync *);
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extern void rcu_sync_exit(struct rcu_sync *);
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#endif /* _LINUX_RCU_SYNC_H_ */
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@ -1,4 +1,4 @@
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obj-y += update.o
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obj-y += update.o sync.o
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obj-$(CONFIG_SRCU) += srcu.o
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obj-$(CONFIG_RCU_TORTURE_TEST) += rcutorture.o
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obj-$(CONFIG_TREE_RCU) += tree.o
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@ -0,0 +1,175 @@
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/*
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* RCU-based infrastructure for lightweight reader-writer locking
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*
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* This program is free software; you can redistribute it and/or modify
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* it under the terms of the GNU General Public License as published by
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* the Free Software Foundation; either version 2 of the License, or
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* (at your option) any later version.
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*
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License
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* along with this program; if not, you can access it online at
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* http://www.gnu.org/licenses/gpl-2.0.html.
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*
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* Copyright (c) 2015, Red Hat, Inc.
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*
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* Author: Oleg Nesterov <oleg@redhat.com>
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*/
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#include <linux/rcu_sync.h>
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#include <linux/sched.h>
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enum { GP_IDLE = 0, GP_PENDING, GP_PASSED };
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enum { CB_IDLE = 0, CB_PENDING, CB_REPLAY };
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#define rss_lock gp_wait.lock
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/**
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* rcu_sync_init() - Initialize an rcu_sync structure
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* @rsp: Pointer to rcu_sync structure to be initialized
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* @type: Flavor of RCU with which to synchronize rcu_sync structure
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*/
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void rcu_sync_init(struct rcu_sync *rsp, enum rcu_sync_type type)
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{
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memset(rsp, 0, sizeof(*rsp));
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init_waitqueue_head(&rsp->gp_wait);
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switch (type) {
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case RCU_SYNC:
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rsp->sync = synchronize_rcu;
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rsp->call = call_rcu;
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break;
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case RCU_SCHED_SYNC:
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rsp->sync = synchronize_sched;
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rsp->call = call_rcu_sched;
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break;
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case RCU_BH_SYNC:
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rsp->sync = synchronize_rcu_bh;
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rsp->call = call_rcu_bh;
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break;
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}
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}
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/**
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* rcu_sync_enter() - Force readers onto slowpath
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* @rsp: Pointer to rcu_sync structure to use for synchronization
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*
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* This function is used by updaters who need readers to make use of
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* a slowpath during the update. After this function returns, all
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* subsequent calls to rcu_sync_is_idle() will return false, which
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* tells readers to stay off their fastpaths. A later call to
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* rcu_sync_exit() re-enables reader slowpaths.
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*
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* When called in isolation, rcu_sync_enter() must wait for a grace
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* period, however, closely spaced calls to rcu_sync_enter() can
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* optimize away the grace-period wait via a state machine implemented
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* by rcu_sync_enter(), rcu_sync_exit(), and rcu_sync_func().
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*/
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void rcu_sync_enter(struct rcu_sync *rsp)
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{
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bool need_wait, need_sync;
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spin_lock_irq(&rsp->rss_lock);
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need_wait = rsp->gp_count++;
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need_sync = rsp->gp_state == GP_IDLE;
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if (need_sync)
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rsp->gp_state = GP_PENDING;
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spin_unlock_irq(&rsp->rss_lock);
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BUG_ON(need_wait && need_sync);
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if (need_sync) {
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rsp->sync();
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rsp->gp_state = GP_PASSED;
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wake_up_all(&rsp->gp_wait);
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} else if (need_wait) {
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wait_event(rsp->gp_wait, rsp->gp_state == GP_PASSED);
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} else {
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/*
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* Possible when there's a pending CB from a rcu_sync_exit().
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* Nobody has yet been allowed the 'fast' path and thus we can
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* avoid doing any sync(). The callback will get 'dropped'.
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*/
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BUG_ON(rsp->gp_state != GP_PASSED);
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}
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}
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/**
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* rcu_sync_func() - Callback function managing reader access to fastpath
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* @rsp: Pointer to rcu_sync structure to use for synchronization
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*
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* This function is passed to one of the call_rcu() functions by
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* rcu_sync_exit(), so that it is invoked after a grace period following the
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* that invocation of rcu_sync_exit(). It takes action based on events that
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* have taken place in the meantime, so that closely spaced rcu_sync_enter()
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* and rcu_sync_exit() pairs need not wait for a grace period.
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*
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* If another rcu_sync_enter() is invoked before the grace period
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* ended, reset state to allow the next rcu_sync_exit() to let the
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* readers back onto their fastpaths (after a grace period). If both
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* another rcu_sync_enter() and its matching rcu_sync_exit() are invoked
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* before the grace period ended, re-invoke call_rcu() on behalf of that
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* rcu_sync_exit(). Otherwise, set all state back to idle so that readers
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* can again use their fastpaths.
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*/
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static void rcu_sync_func(struct rcu_head *rcu)
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{
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struct rcu_sync *rsp = container_of(rcu, struct rcu_sync, cb_head);
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unsigned long flags;
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BUG_ON(rsp->gp_state != GP_PASSED);
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BUG_ON(rsp->cb_state == CB_IDLE);
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spin_lock_irqsave(&rsp->rss_lock, flags);
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if (rsp->gp_count) {
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/*
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* A new rcu_sync_begin() has happened; drop the callback.
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*/
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rsp->cb_state = CB_IDLE;
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} else if (rsp->cb_state == CB_REPLAY) {
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/*
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* A new rcu_sync_exit() has happened; requeue the callback
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* to catch a later GP.
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*/
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rsp->cb_state = CB_PENDING;
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rsp->call(&rsp->cb_head, rcu_sync_func);
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} else {
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/*
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* We're at least a GP after rcu_sync_exit(); eveybody will now
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* have observed the write side critical section. Let 'em rip!.
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*/
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rsp->cb_state = CB_IDLE;
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rsp->gp_state = GP_IDLE;
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}
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spin_unlock_irqrestore(&rsp->rss_lock, flags);
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}
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/**
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* rcu_sync_exit() - Allow readers back onto fast patch after grace period
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* @rsp: Pointer to rcu_sync structure to use for synchronization
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*
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* This function is used by updaters who have completed, and can therefore
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* now allow readers to make use of their fastpaths after a grace period
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* has elapsed. After this grace period has completed, all subsequent
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* calls to rcu_sync_is_idle() will return true, which tells readers that
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* they can once again use their fastpaths.
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*/
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void rcu_sync_exit(struct rcu_sync *rsp)
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{
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spin_lock_irq(&rsp->rss_lock);
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if (!--rsp->gp_count) {
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if (rsp->cb_state == CB_IDLE) {
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rsp->cb_state = CB_PENDING;
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rsp->call(&rsp->cb_head, rcu_sync_func);
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} else if (rsp->cb_state == CB_PENDING) {
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rsp->cb_state = CB_REPLAY;
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}
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}
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spin_unlock_irq(&rsp->rss_lock);
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}
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