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
3 changed files with 270 additions and 1 deletions
94
include/linux/rcu_sync.h
Normal file
94
include/linux/rcu_sync.h
Normal file
|
|
@ -0,0 +1,94 @@
|
|||
/*
|
||||
* 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_ */
|
||||
|
|
@ -1,4 +1,4 @@
|
|||
obj-y += update.o
|
||||
obj-y += update.o sync.o
|
||||
obj-$(CONFIG_SRCU) += srcu.o
|
||||
obj-$(CONFIG_RCU_TORTURE_TEST) += rcutorture.o
|
||||
obj-$(CONFIG_TREE_RCU) += tree.o
|
||||
|
|
|
|||
175
kernel/rcu/sync.c
Normal file
175
kernel/rcu/sync.c
Normal file
|
|
@ -0,0 +1,175 @@
|
|||
/*
|
||||
* 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>
|
||||
*/
|
||||
|
||||
#include <linux/rcu_sync.h>
|
||||
#include <linux/sched.h>
|
||||
|
||||
enum { GP_IDLE = 0, GP_PENDING, GP_PASSED };
|
||||
enum { CB_IDLE = 0, CB_PENDING, CB_REPLAY };
|
||||
|
||||
#define rss_lock gp_wait.lock
|
||||
|
||||
/**
|
||||
* rcu_sync_init() - Initialize an rcu_sync structure
|
||||
* @rsp: Pointer to rcu_sync structure to be initialized
|
||||
* @type: Flavor of RCU with which to synchronize rcu_sync structure
|
||||
*/
|
||||
void rcu_sync_init(struct rcu_sync *rsp, enum rcu_sync_type type)
|
||||
{
|
||||
memset(rsp, 0, sizeof(*rsp));
|
||||
init_waitqueue_head(&rsp->gp_wait);
|
||||
|
||||
switch (type) {
|
||||
case RCU_SYNC:
|
||||
rsp->sync = synchronize_rcu;
|
||||
rsp->call = call_rcu;
|
||||
break;
|
||||
|
||||
case RCU_SCHED_SYNC:
|
||||
rsp->sync = synchronize_sched;
|
||||
rsp->call = call_rcu_sched;
|
||||
break;
|
||||
|
||||
case RCU_BH_SYNC:
|
||||
rsp->sync = synchronize_rcu_bh;
|
||||
rsp->call = call_rcu_bh;
|
||||
break;
|
||||
}
|
||||
}
|
||||
|
||||
/**
|
||||
* rcu_sync_enter() - Force readers onto slowpath
|
||||
* @rsp: Pointer to rcu_sync structure to use for synchronization
|
||||
*
|
||||
* This function is used by updaters who need readers to make use of
|
||||
* a slowpath during the update. After this function returns, all
|
||||
* subsequent calls to rcu_sync_is_idle() will return false, which
|
||||
* tells readers to stay off their fastpaths. A later call to
|
||||
* rcu_sync_exit() re-enables reader slowpaths.
|
||||
*
|
||||
* When called in isolation, rcu_sync_enter() must wait for a grace
|
||||
* period, however, closely spaced calls to rcu_sync_enter() can
|
||||
* optimize away the grace-period wait via a state machine implemented
|
||||
* by rcu_sync_enter(), rcu_sync_exit(), and rcu_sync_func().
|
||||
*/
|
||||
void rcu_sync_enter(struct rcu_sync *rsp)
|
||||
{
|
||||
bool need_wait, need_sync;
|
||||
|
||||
spin_lock_irq(&rsp->rss_lock);
|
||||
need_wait = rsp->gp_count++;
|
||||
need_sync = rsp->gp_state == GP_IDLE;
|
||||
if (need_sync)
|
||||
rsp->gp_state = GP_PENDING;
|
||||
spin_unlock_irq(&rsp->rss_lock);
|
||||
|
||||
BUG_ON(need_wait && need_sync);
|
||||
|
||||
if (need_sync) {
|
||||
rsp->sync();
|
||||
rsp->gp_state = GP_PASSED;
|
||||
wake_up_all(&rsp->gp_wait);
|
||||
} else if (need_wait) {
|
||||
wait_event(rsp->gp_wait, rsp->gp_state == GP_PASSED);
|
||||
} else {
|
||||
/*
|
||||
* Possible when there's a pending CB from a rcu_sync_exit().
|
||||
* Nobody has yet been allowed the 'fast' path and thus we can
|
||||
* avoid doing any sync(). The callback will get 'dropped'.
|
||||
*/
|
||||
BUG_ON(rsp->gp_state != GP_PASSED);
|
||||
}
|
||||
}
|
||||
|
||||
/**
|
||||
* rcu_sync_func() - Callback function managing reader access to fastpath
|
||||
* @rsp: Pointer to rcu_sync structure to use for synchronization
|
||||
*
|
||||
* This function is passed to one of the call_rcu() functions by
|
||||
* rcu_sync_exit(), so that it is invoked after a grace period following the
|
||||
* that invocation of rcu_sync_exit(). It takes action based on events that
|
||||
* have taken place in the meantime, so that closely spaced rcu_sync_enter()
|
||||
* and rcu_sync_exit() pairs need not wait for a grace period.
|
||||
*
|
||||
* If another rcu_sync_enter() is invoked before the grace period
|
||||
* ended, reset state to allow the next rcu_sync_exit() to let the
|
||||
* readers back onto their fastpaths (after a grace period). If both
|
||||
* another rcu_sync_enter() and its matching rcu_sync_exit() are invoked
|
||||
* before the grace period ended, re-invoke call_rcu() on behalf of that
|
||||
* rcu_sync_exit(). Otherwise, set all state back to idle so that readers
|
||||
* can again use their fastpaths.
|
||||
*/
|
||||
static void rcu_sync_func(struct rcu_head *rcu)
|
||||
{
|
||||
struct rcu_sync *rsp = container_of(rcu, struct rcu_sync, cb_head);
|
||||
unsigned long flags;
|
||||
|
||||
BUG_ON(rsp->gp_state != GP_PASSED);
|
||||
BUG_ON(rsp->cb_state == CB_IDLE);
|
||||
|
||||
spin_lock_irqsave(&rsp->rss_lock, flags);
|
||||
if (rsp->gp_count) {
|
||||
/*
|
||||
* A new rcu_sync_begin() has happened; drop the callback.
|
||||
*/
|
||||
rsp->cb_state = CB_IDLE;
|
||||
} else if (rsp->cb_state == CB_REPLAY) {
|
||||
/*
|
||||
* A new rcu_sync_exit() has happened; requeue the callback
|
||||
* to catch a later GP.
|
||||
*/
|
||||
rsp->cb_state = CB_PENDING;
|
||||
rsp->call(&rsp->cb_head, rcu_sync_func);
|
||||
} else {
|
||||
/*
|
||||
* We're at least a GP after rcu_sync_exit(); eveybody will now
|
||||
* have observed the write side critical section. Let 'em rip!.
|
||||
*/
|
||||
rsp->cb_state = CB_IDLE;
|
||||
rsp->gp_state = GP_IDLE;
|
||||
}
|
||||
spin_unlock_irqrestore(&rsp->rss_lock, flags);
|
||||
}
|
||||
|
||||
/**
|
||||
* rcu_sync_exit() - Allow readers back onto fast patch after grace period
|
||||
* @rsp: Pointer to rcu_sync structure to use for synchronization
|
||||
*
|
||||
* This function is used by updaters who have completed, and can therefore
|
||||
* now allow readers to make use of their fastpaths after a grace period
|
||||
* has elapsed. After this grace period has completed, all subsequent
|
||||
* calls to rcu_sync_is_idle() will return true, which tells readers that
|
||||
* they can once again use their fastpaths.
|
||||
*/
|
||||
void rcu_sync_exit(struct rcu_sync *rsp)
|
||||
{
|
||||
spin_lock_irq(&rsp->rss_lock);
|
||||
if (!--rsp->gp_count) {
|
||||
if (rsp->cb_state == CB_IDLE) {
|
||||
rsp->cb_state = CB_PENDING;
|
||||
rsp->call(&rsp->cb_head, rcu_sync_func);
|
||||
} else if (rsp->cb_state == CB_PENDING) {
|
||||
rsp->cb_state = CB_REPLAY;
|
||||
}
|
||||
}
|
||||
spin_unlock_irq(&rsp->rss_lock);
|
||||
}
|
||||
Loading…
Reference in a new issue