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rcu: Create rcu_sync infrastructure

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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>
This commit is contained in:
Oleg Nesterov
2015-08-21 19:42:44 +02:00
committed by Paul E. McKenney
parent 3836f5337f
commit cc44ca848f
3 changed files with 270 additions and 1 deletions
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94
include/linux/rcu_sync.h Normal file
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/*
* RCU-based infrastructure for lightweight reader-writer locking
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation; either version 2 of the License, or
* (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program; if not, you can access it online at
* http://www.gnu.org/licenses/gpl-2.0.html.
*
* Copyright (c) 2015, Red Hat, Inc.
*
* Author: Oleg Nesterov <oleg@redhat.com>
*/
#ifndef _LINUX_RCU_SYNC_H_
#define _LINUX_RCU_SYNC_H_
#include <linux/wait.h>
#include <linux/rcupdate.h>
/* Structure to mediate between updaters and fastpath-using readers. */
struct rcu_sync {
int gp_state;
int gp_count;
wait_queue_head_t gp_wait;
int cb_state;
struct rcu_head cb_head;
void (*sync)(void);
void (*call)(struct rcu_head *, void (*)(struct rcu_head *));
};
#define ___RCU_SYNC_INIT(name) \
.gp_state = 0, \
.gp_count = 0, \
.gp_wait = __WAIT_QUEUE_HEAD_INITIALIZER(name.gp_wait), \
.cb_state = 0
#define __RCU_SCHED_SYNC_INIT(name) { \
___RCU_SYNC_INIT(name), \
.sync = synchronize_sched, \
.call = call_rcu_sched, \
}
#define __RCU_BH_SYNC_INIT(name) { \
___RCU_SYNC_INIT(name), \
.sync = synchronize_rcu_bh, \
.call = call_rcu_bh, \
}
#define __RCU_SYNC_INIT(name) { \
___RCU_SYNC_INIT(name), \
.sync = synchronize_rcu, \
.call = call_rcu, \
}
#define DEFINE_RCU_SCHED_SYNC(name) \
struct rcu_sync name = __RCU_SCHED_SYNC_INIT(name)
#define DEFINE_RCU_BH_SYNC(name) \
struct rcu_sync name = __RCU_BH_SYNC_INIT(name)
#define DEFINE_RCU_SYNC(name) \
struct rcu_sync name = __RCU_SYNC_INIT(name)
/**
* rcu_sync_is_idle() - Are readers permitted to use their fastpaths?
* @rsp: Pointer to rcu_sync structure to use for synchronization
*
* Returns true if readers are permitted to use their fastpaths.
* Must be invoked within an RCU read-side critical section whose
* flavor matches that of the rcu_sync struture.
*/
static inline bool rcu_sync_is_idle(struct rcu_sync *rsp)
{
return !rsp->gp_state; /* GP_IDLE */
}
enum rcu_sync_type { RCU_SYNC, RCU_SCHED_SYNC, RCU_BH_SYNC };
extern void rcu_sync_init(struct rcu_sync *, enum rcu_sync_type);
extern void rcu_sync_enter(struct rcu_sync *);
extern void rcu_sync_exit(struct rcu_sync *);
#endif /* _LINUX_RCU_SYNC_H_ */