153 lines
4.0 KiB
C
153 lines
4.0 KiB
C
/*
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* Queued read/write locks
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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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#include <linux/smp.h>
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#include <linux/bug.h>
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#include <linux/cpumask.h>
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#include <linux/percpu.h>
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#include <linux/hardirq.h>
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#include <asm/qrwlock.h>
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/*
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* This internal data structure is used for optimizing access to some of
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* the subfields within the atomic_t cnts.
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*/
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struct __qrwlock {
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union {
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atomic_t cnts;
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struct {
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#ifdef __LITTLE_ENDIAN
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u8 wmode; /* Writer mode */
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u8 rcnts[3]; /* Reader counts */
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#else
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u8 rcnts[3]; /* Reader counts */
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u8 wmode; /* Writer mode */
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#endif
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};
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};
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arch_spinlock_t lock;
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};
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/**
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* rspin_until_writer_unlock - inc reader count & spin until writer is gone
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* @lock : Pointer to queue rwlock structure
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* @writer: Current queue rwlock writer status byte
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*
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* In interrupt context or at the head of the queue, the reader will just
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* increment the reader count & wait until the writer releases the lock.
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*/
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static __always_inline void
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rspin_until_writer_unlock(struct qrwlock *lock, u32 cnts)
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{
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while ((cnts & _QW_WMASK) == _QW_LOCKED) {
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cpu_relax_lowlatency();
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cnts = smp_load_acquire((u32 *)&lock->cnts);
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}
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}
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/**
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* queue_read_lock_slowpath - 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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void queue_read_lock_slowpath(struct qrwlock *lock)
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{
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u32 cnts;
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/*
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* Readers come here when they cannot get the lock without waiting
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*/
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if (unlikely(in_interrupt())) {
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/*
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* Readers in interrupt context will spin until the lock is
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* available without waiting in the queue.
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*/
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cnts = smp_load_acquire((u32 *)&lock->cnts);
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rspin_until_writer_unlock(lock, cnts);
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return;
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}
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atomic_sub(_QR_BIAS, &lock->cnts);
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/*
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* Put the reader into the wait queue
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*/
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arch_spin_lock(&lock->lock);
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/*
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* At the head of the wait queue now, wait until the writer state
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* goes to 0 and then try to increment the reader count and get
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* the lock. It is possible that an incoming writer may steal the
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* lock in the interim, so it is necessary to check the writer byte
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* to make sure that the write lock isn't taken.
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*/
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while (atomic_read(&lock->cnts) & _QW_WMASK)
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cpu_relax_lowlatency();
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cnts = atomic_add_return(_QR_BIAS, &lock->cnts) - _QR_BIAS;
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rspin_until_writer_unlock(lock, cnts);
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/*
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* Signal the next one in queue to become queue head
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*/
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arch_spin_unlock(&lock->lock);
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}
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EXPORT_SYMBOL(queue_read_lock_slowpath);
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/**
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* queue_write_lock_slowpath - 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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void queue_write_lock_slowpath(struct qrwlock *lock)
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{
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u32 cnts;
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/* Put the writer into the wait queue */
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arch_spin_lock(&lock->lock);
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/* Try to acquire the lock directly if no reader is present */
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if (!atomic_read(&lock->cnts) &&
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(atomic_cmpxchg(&lock->cnts, 0, _QW_LOCKED) == 0))
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goto unlock;
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/*
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* Set the waiting flag to notify readers that a writer is pending,
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* or wait for a previous writer to go away.
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*/
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for (;;) {
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struct __qrwlock *l = (struct __qrwlock *)lock;
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if (!READ_ONCE(l->wmode) &&
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(cmpxchg(&l->wmode, 0, _QW_WAITING) == 0))
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break;
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cpu_relax_lowlatency();
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}
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/* When no more readers, set the locked flag */
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for (;;) {
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cnts = atomic_read(&lock->cnts);
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if ((cnts == _QW_WAITING) &&
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(atomic_cmpxchg(&lock->cnts, _QW_WAITING,
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_QW_LOCKED) == _QW_WAITING))
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break;
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cpu_relax_lowlatency();
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}
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unlock:
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arch_spin_unlock(&lock->lock);
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}
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EXPORT_SYMBOL(queue_write_lock_slowpath);
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