qrwlock.h
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/*
* 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 <waiman.long@hp.com>
*/
#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.
*
* | +0 | +1 | +2 | +3 |
* ----+----+----+----+----+
* LE | 78 | 56 | 34 | 12 | 0x12345678
* ----+----+----+----+----+
* | wr | rd |
* +----+----+----+----+
*
* ----+----+----+----+----+
* BE | 12 | 34 | 56 | 78 | 0x12345678
* ----+----+----+----+----+
* | rd | wr |
* +----+----+----+----+
*/
#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 queued_read_lock_slowpath(struct qrwlock *lock, u32 cnts);
extern void queued_write_lock_slowpath(struct qrwlock *lock);
/**
* queued_read_can_lock- would read_trylock() succeed?
* @lock: Pointer to queue rwlock structure
*/
static inline int queued_read_can_lock(struct qrwlock *lock)
{
return !(atomic_read(&lock->cnts) & _QW_WMASK);
}
/**
* queued_write_can_lock- would write_trylock() succeed?
* @lock: Pointer to queue rwlock structure
*/
static inline int queued_write_can_lock(struct qrwlock *lock)
{
return !atomic_read(&lock->cnts);
}
/**
* queued_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 queued_read_trylock(struct qrwlock *lock)
{
u32 cnts;
cnts = atomic_read(&lock->cnts);
if (likely(!(cnts & _QW_WMASK))) {
cnts = (u32)atomic_add_return_acquire(_QR_BIAS, &lock->cnts);
if (likely(!(cnts & _QW_WMASK)))
return 1;
atomic_sub(_QR_BIAS, &lock->cnts);
}
return 0;
}
/**
* queued_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 queued_write_trylock(struct qrwlock *lock)
{
u32 cnts;
cnts = atomic_read(&lock->cnts);
if (unlikely(cnts))
return 0;
return likely(atomic_cmpxchg_acquire(&lock->cnts,
cnts, cnts | _QW_LOCKED) == cnts);
}
/**
* queued_read_lock - acquire read lock of a queue rwlock
* @lock: Pointer to queue rwlock structure
*/
static inline void queued_read_lock(struct qrwlock *lock)
{
u32 cnts;
cnts = atomic_add_return_acquire(_QR_BIAS, &lock->cnts);
if (likely(!(cnts & _QW_WMASK)))
return;
/* The slowpath will decrement the reader count, if necessary. */
queued_read_lock_slowpath(lock, cnts);
}
/**
* queued_write_lock - acquire write lock of a queue rwlock
* @lock : Pointer to queue rwlock structure
*/
static inline void queued_write_lock(struct qrwlock *lock)
{
/* Optimize for the unfair lock case where the fair flag is 0. */
if (atomic_cmpxchg_acquire(&lock->cnts, 0, _QW_LOCKED) == 0)
return;
queued_write_lock_slowpath(lock);
}
/**
* queued_read_unlock - release read lock of a queue rwlock
* @lock : Pointer to queue rwlock structure
*/
static inline void queued_read_unlock(struct qrwlock *lock)
{
/*
* Atomically decrement the reader count
*/
(void)atomic_sub_return_release(_QR_BIAS, &lock->cnts);
}
/**
* __qrwlock_write_byte - retrieve the write byte address of a queue rwlock
* @lock : Pointer to queue rwlock structure
* Return: the write byte address of a queue rwlock
*/
static inline u8 *__qrwlock_write_byte(struct qrwlock *lock)
{
return (u8 *)lock + 3 * IS_BUILTIN(CONFIG_CPU_BIG_ENDIAN);
}
/**
* queued_write_unlock - release write lock of a queue rwlock
* @lock : Pointer to queue rwlock structure
*/
static inline void queued_write_unlock(struct qrwlock *lock)
{
smp_store_release(__qrwlock_write_byte(lock), 0);
}
/*
* Remapping rwlock architecture specific functions to the corresponding
* queue rwlock functions.
*/
#define arch_read_can_lock(l) queued_read_can_lock(l)
#define arch_write_can_lock(l) queued_write_can_lock(l)
#define arch_read_lock(l) queued_read_lock(l)
#define arch_write_lock(l) queued_write_lock(l)
#define arch_read_trylock(l) queued_read_trylock(l)
#define arch_write_trylock(l) queued_write_trylock(l)
#define arch_read_unlock(l) queued_read_unlock(l)
#define arch_write_unlock(l) queued_write_unlock(l)
#endif /* __ASM_GENERIC_QRWLOCK_H */