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locking/rwlocks: Introduce 'qrwlocks' - fair, queued rwlocks
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This rwlock uses the arch_spin_lock_t as a waitqueue, and assuming the
arch_spin_lock_t is a fair lock (ticket,mcs etc..) the resulting
rwlock is a fair lock.

It fits in the same 8 bytes as the regular rwlock_t by folding the
reader and writer count into a single integer, using the remaining 4
bytes for the arch_spinlock_t.

Architectures that can single-copy adress bytes can optimize
queue_write_unlock() with a 0 write to the LSB (the write count).

Performance as measured by Davidlohr Bueso (rwlock_t -> qrwlock_t):

 +--------------+-------------+---------------+
 |   Workload   |   #users    |     delta     |
 +--------------+-------------+---------------+
 | alltests     | > 1400      | -4.83%        |
 | custom       | 0-100,> 100 | +1.43%,-1.57% |
 | high_systime | > 1000      | -2.61         |
 | shared       | all         | +0.32         |
 +--------------+-------------+---------------+

http://www.stgolabs.net/qrwlock-stuff/aim7-results-vs-rwsem_optsin/

Signed-off-by: Waiman Long <[email protected]>
[peterz: near complete rewrite]
Signed-off-by: Peter Zijlstra <[email protected]>
Cc: Arnd Bergmann <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: "Paul E.McKenney" <[email protected]>
Cc: [email protected]
Cc: [email protected]
Link: http://lkml.kernel.org/n/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>
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longman88 authored and Ingo Molnar committed Jun 6, 2014
1 parent 9161f54 commit 70af2f8
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166 changes: 166 additions & 0 deletions include/asm-generic/qrwlock.h
Original file line number Diff line number Diff line change
@@ -0,0 +1,166 @@
/*
* Queue read/write lock
*
* 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.
*
* (C) Copyright 2013-2014 Hewlett-Packard Development Company, L.P.
*
* Authors: Waiman Long <[email protected]>
*/
#ifndef __ASM_GENERIC_QRWLOCK_H
#define __ASM_GENERIC_QRWLOCK_H

#include <linux/atomic.h>
#include <asm/barrier.h>
#include <asm/processor.h>

#include <asm-generic/qrwlock_types.h>

/*
* Writer states & reader shift and bias
*/
#define _QW_WAITING 1 /* A writer is waiting */
#define _QW_LOCKED 0xff /* A writer holds the lock */
#define _QW_WMASK 0xff /* Writer mask */
#define _QR_SHIFT 8 /* Reader count shift */
#define _QR_BIAS (1U << _QR_SHIFT)

/*
* External function declarations
*/
extern void queue_read_lock_slowpath(struct qrwlock *lock);
extern void queue_write_lock_slowpath(struct qrwlock *lock);

/**
* queue_read_can_lock- would read_trylock() succeed?
* @lock: Pointer to queue rwlock structure
*/
static inline int queue_read_can_lock(struct qrwlock *lock)
{
return !(atomic_read(&lock->cnts) & _QW_WMASK);
}

/**
* queue_write_can_lock- would write_trylock() succeed?
* @lock: Pointer to queue rwlock structure
*/
static inline int queue_write_can_lock(struct qrwlock *lock)
{
return !atomic_read(&lock->cnts);
}

/**
* queue_read_trylock - try to acquire read lock of a queue rwlock
* @lock : Pointer to queue rwlock structure
* Return: 1 if lock acquired, 0 if failed
*/
static inline int queue_read_trylock(struct qrwlock *lock)
{
u32 cnts;

cnts = atomic_read(&lock->cnts);
if (likely(!(cnts & _QW_WMASK))) {
cnts = (u32)atomic_add_return(_QR_BIAS, &lock->cnts);
if (likely(!(cnts & _QW_WMASK)))
return 1;
atomic_sub(_QR_BIAS, &lock->cnts);
}
return 0;
}

/**
* queue_write_trylock - try to acquire write lock of a queue rwlock
* @lock : Pointer to queue rwlock structure
* Return: 1 if lock acquired, 0 if failed
*/
static inline int queue_write_trylock(struct qrwlock *lock)
{
u32 cnts;

cnts = atomic_read(&lock->cnts);
if (unlikely(cnts))
return 0;

return likely(atomic_cmpxchg(&lock->cnts,
cnts, cnts | _QW_LOCKED) == cnts);
}
/**
* queue_read_lock - acquire read lock of a queue rwlock
* @lock: Pointer to queue rwlock structure
*/
static inline void queue_read_lock(struct qrwlock *lock)
{
u32 cnts;

cnts = atomic_add_return(_QR_BIAS, &lock->cnts);
if (likely(!(cnts & _QW_WMASK)))
return;

/* The slowpath will decrement the reader count, if necessary. */
queue_read_lock_slowpath(lock);
}

/**
* queue_write_lock - acquire write lock of a queue rwlock
* @lock : Pointer to queue rwlock structure
*/
static inline void queue_write_lock(struct qrwlock *lock)
{
/* Optimize for the unfair lock case where the fair flag is 0. */
if (atomic_cmpxchg(&lock->cnts, 0, _QW_LOCKED) == 0)
return;

queue_write_lock_slowpath(lock);
}

/**
* queue_read_unlock - release read lock of a queue rwlock
* @lock : Pointer to queue rwlock structure
*/
static inline void queue_read_unlock(struct qrwlock *lock)
{
/*
* Atomically decrement the reader count
*/
smp_mb__before_atomic();
atomic_sub(_QR_BIAS, &lock->cnts);
}

#ifndef queue_write_unlock
/**
* queue_write_unlock - release write lock of a queue rwlock
* @lock : Pointer to queue rwlock structure
*/
static inline void queue_write_unlock(struct qrwlock *lock)
{
/*
* If the writer field is atomic, it can be cleared directly.
* Otherwise, an atomic subtraction will be used to clear it.
*/
smp_mb__before_atomic();
atomic_sub(_QW_LOCKED, &lock->cnts);
}
#endif

/*
* Remapping rwlock architecture specific functions to the corresponding
* queue rwlock functions.
*/
#define arch_read_can_lock(l) queue_read_can_lock(l)
#define arch_write_can_lock(l) queue_write_can_lock(l)
#define arch_read_lock(l) queue_read_lock(l)
#define arch_write_lock(l) queue_write_lock(l)
#define arch_read_trylock(l) queue_read_trylock(l)
#define arch_write_trylock(l) queue_write_trylock(l)
#define arch_read_unlock(l) queue_read_unlock(l)
#define arch_write_unlock(l) queue_write_unlock(l)

#endif /* __ASM_GENERIC_QRWLOCK_H */
21 changes: 21 additions & 0 deletions include/asm-generic/qrwlock_types.h
Original file line number Diff line number Diff line change
@@ -0,0 +1,21 @@
#ifndef __ASM_GENERIC_QRWLOCK_TYPES_H
#define __ASM_GENERIC_QRWLOCK_TYPES_H

#include <linux/types.h>
#include <asm/spinlock_types.h>

/*
* The queue read/write lock data structure
*/

typedef struct qrwlock {
atomic_t cnts;
arch_spinlock_t lock;
} arch_rwlock_t;

#define __ARCH_RW_LOCK_UNLOCKED { \
.cnts = ATOMIC_INIT(0), \
.lock = __ARCH_SPIN_LOCK_UNLOCKED, \
}

#endif /* __ASM_GENERIC_QRWLOCK_TYPES_H */
7 changes: 7 additions & 0 deletions kernel/Kconfig.locks
Original file line number Diff line number Diff line change
Expand Up @@ -223,3 +223,10 @@ endif
config MUTEX_SPIN_ON_OWNER
def_bool y
depends on SMP && !DEBUG_MUTEXES

config ARCH_USE_QUEUE_RWLOCK
bool

config QUEUE_RWLOCK
def_bool y if ARCH_USE_QUEUE_RWLOCK
depends on SMP
1 change: 1 addition & 0 deletions kernel/locking/Makefile
Original file line number Diff line number Diff line change
Expand Up @@ -24,4 +24,5 @@ obj-$(CONFIG_DEBUG_SPINLOCK) += spinlock_debug.o
obj-$(CONFIG_RWSEM_GENERIC_SPINLOCK) += rwsem-spinlock.o
obj-$(CONFIG_RWSEM_XCHGADD_ALGORITHM) += rwsem-xadd.o
obj-$(CONFIG_PERCPU_RWSEM) += percpu-rwsem.o
obj-$(CONFIG_QUEUE_RWLOCK) += qrwlock.o
obj-$(CONFIG_LOCK_TORTURE_TEST) += locktorture.o
133 changes: 133 additions & 0 deletions kernel/locking/qrwlock.c
Original file line number Diff line number Diff line change
@@ -0,0 +1,133 @@
/*
* Queue read/write lock
*
* 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.
*
* (C) Copyright 2013-2014 Hewlett-Packard Development Company, L.P.
*
* Authors: Waiman Long <[email protected]>
*/
#include <linux/smp.h>
#include <linux/bug.h>
#include <linux/cpumask.h>
#include <linux/percpu.h>
#include <linux/hardirq.h>
#include <linux/mutex.h>
#include <asm/qrwlock.h>

/**
* rspin_until_writer_unlock - inc reader count & spin until writer is gone
* @lock : Pointer to queue rwlock structure
* @writer: Current queue rwlock writer status byte
*
* In interrupt context or at the head of the queue, the reader will just
* increment the reader count & wait until the writer releases the lock.
*/
static __always_inline void
rspin_until_writer_unlock(struct qrwlock *lock, u32 cnts)
{
while ((cnts & _QW_WMASK) == _QW_LOCKED) {
arch_mutex_cpu_relax();
cnts = smp_load_acquire((u32 *)&lock->cnts);
}
}

/**
* queue_read_lock_slowpath - acquire read lock of a queue rwlock
* @lock: Pointer to queue rwlock structure
*/
void queue_read_lock_slowpath(struct qrwlock *lock)
{
u32 cnts;

/*
* Readers come here when they cannot get the lock without waiting
*/
if (unlikely(in_interrupt())) {
/*
* Readers in interrupt context will spin until the lock is
* available without waiting in the queue.
*/
cnts = smp_load_acquire((u32 *)&lock->cnts);
rspin_until_writer_unlock(lock, cnts);
return;
}
atomic_sub(_QR_BIAS, &lock->cnts);

/*
* Put the reader into the wait queue
*/
arch_spin_lock(&lock->lock);

/*
* At the head of the wait queue now, wait until the writer state
* goes to 0 and then try to increment the reader count and get
* the lock. It is possible that an incoming writer may steal the
* lock in the interim, so it is necessary to check the writer byte
* to make sure that the write lock isn't taken.
*/
while (atomic_read(&lock->cnts) & _QW_WMASK)
arch_mutex_cpu_relax();

cnts = atomic_add_return(_QR_BIAS, &lock->cnts) - _QR_BIAS;
rspin_until_writer_unlock(lock, cnts);

/*
* Signal the next one in queue to become queue head
*/
arch_spin_unlock(&lock->lock);
}
EXPORT_SYMBOL(queue_read_lock_slowpath);

/**
* queue_write_lock_slowpath - acquire write lock of a queue rwlock
* @lock : Pointer to queue rwlock structure
*/
void queue_write_lock_slowpath(struct qrwlock *lock)
{
u32 cnts;

/* Put the writer into the wait queue */
arch_spin_lock(&lock->lock);

/* Try to acquire the lock directly if no reader is present */
if (!atomic_read(&lock->cnts) &&
(atomic_cmpxchg(&lock->cnts, 0, _QW_LOCKED) == 0))
goto unlock;

/*
* Set the waiting flag to notify readers that a writer is pending,
* or wait for a previous writer to go away.
*/
for (;;) {
cnts = atomic_read(&lock->cnts);
if (!(cnts & _QW_WMASK) &&
(atomic_cmpxchg(&lock->cnts, cnts,
cnts | _QW_WAITING) == cnts))
break;

arch_mutex_cpu_relax();
}

/* When no more readers, set the locked flag */
for (;;) {
cnts = atomic_read(&lock->cnts);
if ((cnts == _QW_WAITING) &&
(atomic_cmpxchg(&lock->cnts, _QW_WAITING,
_QW_LOCKED) == _QW_WAITING))
break;

arch_mutex_cpu_relax();
}
unlock:
arch_spin_unlock(&lock->lock);
}
EXPORT_SYMBOL(queue_write_lock_slowpath);

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