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![Waiman Long](/assets/img/avatar_default.png)
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 <Waiman.Long@hp.com> [peterz: near complete rewrite] Signed-off-by: Peter Zijlstra <peterz@infradead.org> Cc: Arnd Bergmann <arnd@arndb.de> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: "Paul E.McKenney" <paulmck@linux.vnet.ibm.com> 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 <mingo@kernel.org>
166 lines
4.4 KiB
C
166 lines
4.4 KiB
C
/*
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* Queue read/write lock
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*
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* This program is free software; you can redistribute it and/or modify
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* it under the terms of the GNU General Public License as published by
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* the Free Software Foundation; either version 2 of the License, or
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* (at your option) any later version.
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*
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*
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* (C) Copyright 2013-2014 Hewlett-Packard Development Company, L.P.
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*
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* Authors: Waiman Long <waiman.long@hp.com>
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*/
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#ifndef __ASM_GENERIC_QRWLOCK_H
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#define __ASM_GENERIC_QRWLOCK_H
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#include <linux/atomic.h>
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#include <asm/barrier.h>
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#include <asm/processor.h>
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#include <asm-generic/qrwlock_types.h>
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/*
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* Writer states & reader shift and bias
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*/
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#define _QW_WAITING 1 /* A writer is waiting */
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#define _QW_LOCKED 0xff /* A writer holds the lock */
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#define _QW_WMASK 0xff /* Writer mask */
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#define _QR_SHIFT 8 /* Reader count shift */
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#define _QR_BIAS (1U << _QR_SHIFT)
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/*
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* External function declarations
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*/
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extern void queue_read_lock_slowpath(struct qrwlock *lock);
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extern void queue_write_lock_slowpath(struct qrwlock *lock);
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/**
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* queue_read_can_lock- would read_trylock() succeed?
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* @lock: Pointer to queue rwlock structure
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*/
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static inline int queue_read_can_lock(struct qrwlock *lock)
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{
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return !(atomic_read(&lock->cnts) & _QW_WMASK);
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}
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/**
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* queue_write_can_lock- would write_trylock() succeed?
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* @lock: Pointer to queue rwlock structure
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*/
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static inline int queue_write_can_lock(struct qrwlock *lock)
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{
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return !atomic_read(&lock->cnts);
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}
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/**
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* queue_read_trylock - try to acquire read lock of a queue rwlock
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* @lock : Pointer to queue rwlock structure
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* Return: 1 if lock acquired, 0 if failed
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*/
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static inline int queue_read_trylock(struct qrwlock *lock)
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{
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u32 cnts;
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cnts = atomic_read(&lock->cnts);
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if (likely(!(cnts & _QW_WMASK))) {
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cnts = (u32)atomic_add_return(_QR_BIAS, &lock->cnts);
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if (likely(!(cnts & _QW_WMASK)))
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return 1;
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atomic_sub(_QR_BIAS, &lock->cnts);
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}
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return 0;
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}
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/**
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* queue_write_trylock - try to acquire write lock of a queue rwlock
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* @lock : Pointer to queue rwlock structure
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* Return: 1 if lock acquired, 0 if failed
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*/
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static inline int queue_write_trylock(struct qrwlock *lock)
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{
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u32 cnts;
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cnts = atomic_read(&lock->cnts);
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if (unlikely(cnts))
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return 0;
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return likely(atomic_cmpxchg(&lock->cnts,
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cnts, cnts | _QW_LOCKED) == cnts);
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}
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/**
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* queue_read_lock - acquire read lock of a queue rwlock
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* @lock: Pointer to queue rwlock structure
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*/
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static inline void queue_read_lock(struct qrwlock *lock)
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{
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u32 cnts;
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cnts = atomic_add_return(_QR_BIAS, &lock->cnts);
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if (likely(!(cnts & _QW_WMASK)))
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return;
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/* The slowpath will decrement the reader count, if necessary. */
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queue_read_lock_slowpath(lock);
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}
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/**
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* queue_write_lock - acquire write lock of a queue rwlock
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* @lock : Pointer to queue rwlock structure
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*/
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static inline void queue_write_lock(struct qrwlock *lock)
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{
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/* Optimize for the unfair lock case where the fair flag is 0. */
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if (atomic_cmpxchg(&lock->cnts, 0, _QW_LOCKED) == 0)
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return;
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queue_write_lock_slowpath(lock);
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}
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/**
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* queue_read_unlock - release read lock of a queue rwlock
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* @lock : Pointer to queue rwlock structure
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*/
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static inline void queue_read_unlock(struct qrwlock *lock)
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{
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/*
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* Atomically decrement the reader count
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*/
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smp_mb__before_atomic();
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atomic_sub(_QR_BIAS, &lock->cnts);
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}
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#ifndef queue_write_unlock
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/**
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* queue_write_unlock - release write lock of a queue rwlock
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* @lock : Pointer to queue rwlock structure
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*/
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static inline void queue_write_unlock(struct qrwlock *lock)
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{
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/*
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* If the writer field is atomic, it can be cleared directly.
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* Otherwise, an atomic subtraction will be used to clear it.
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*/
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smp_mb__before_atomic();
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atomic_sub(_QW_LOCKED, &lock->cnts);
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}
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#endif
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/*
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* Remapping rwlock architecture specific functions to the corresponding
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* queue rwlock functions.
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*/
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#define arch_read_can_lock(l) queue_read_can_lock(l)
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#define arch_write_can_lock(l) queue_write_can_lock(l)
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#define arch_read_lock(l) queue_read_lock(l)
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#define arch_write_lock(l) queue_write_lock(l)
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#define arch_read_trylock(l) queue_read_trylock(l)
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#define arch_write_trylock(l) queue_write_trylock(l)
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#define arch_read_unlock(l) queue_read_unlock(l)
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#define arch_write_unlock(l) queue_write_unlock(l)
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#endif /* __ASM_GENERIC_QRWLOCK_H */
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