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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>
95 lines
2.7 KiB
C
95 lines
2.7 KiB
C
/*
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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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