From 70af2f8a4f48d6cebdf92d533d3aef37853ce6de Mon Sep 17 00:00:00 2001 From: Waiman Long Date: Mon, 3 Feb 2014 13:18:49 +0100 Subject: locking/rwlocks: Introduce 'qrwlocks' - fair, queued rwlocks 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 [peterz: near complete rewrite] Signed-off-by: Peter Zijlstra Cc: Arnd Bergmann Cc: Linus Torvalds Cc: "Paul E.McKenney" Cc: linux-arch@vger.kernel.org Cc: linux-kernel@vger.kernel.org Link: http://lkml.kernel.org/n/tip-gac1nnl3wvs2ij87zv2xkdzq@git.kernel.org Signed-off-by: Ingo Molnar --- include/asm-generic/qrwlock.h | 166 ++++++++++++++++++++++++++++++++++++ include/asm-generic/qrwlock_types.h | 21 +++++ 2 files changed, 187 insertions(+) create mode 100644 include/asm-generic/qrwlock.h create mode 100644 include/asm-generic/qrwlock_types.h (limited to 'include/asm-generic') diff --git a/include/asm-generic/qrwlock.h b/include/asm-generic/qrwlock.h new file mode 100644 index 0000000..6383d54 --- /dev/null +++ b/include/asm-generic/qrwlock.h @@ -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 + */ +#ifndef __ASM_GENERIC_QRWLOCK_H +#define __ASM_GENERIC_QRWLOCK_H + +#include +#include +#include + +#include + +/* + * 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 */ diff --git a/include/asm-generic/qrwlock_types.h b/include/asm-generic/qrwlock_types.h new file mode 100644 index 0000000..4d76f24 --- /dev/null +++ b/include/asm-generic/qrwlock_types.h @@ -0,0 +1,21 @@ +#ifndef __ASM_GENERIC_QRWLOCK_TYPES_H +#define __ASM_GENERIC_QRWLOCK_TYPES_H + +#include +#include + +/* + * 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 */ -- cgit v1.1