94 lines
2.8 KiB
C
94 lines
2.8 KiB
C
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/*
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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 <linux/spinlock.h>
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#include <asm/qrwlock.h>
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/**
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* queued_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 queued_read_lock_slowpath(struct qrwlock *lock)
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{
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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 get the lock immediately
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* if the writer is just waiting (not holding the lock yet),
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* so spin with ACQUIRE semantics until the lock is available
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* without waiting in the queue.
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*/
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atomic_cond_read_acquire(&lock->cnts, !(VAL & _QW_LOCKED));
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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->wait_lock);
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atomic_add(_QR_BIAS, &lock->cnts);
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/*
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* The ACQUIRE semantics of the following spinning code ensure
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* that accesses can't leak upwards out of our subsequent critical
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* section in the case that the lock is currently held for write.
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*/
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atomic_cond_read_acquire(&lock->cnts, !(VAL & _QW_LOCKED));
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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->wait_lock);
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}
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EXPORT_SYMBOL(queued_read_lock_slowpath);
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/**
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* queued_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 queued_write_lock_slowpath(struct qrwlock *lock)
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{
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int cnts;
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/* Put the writer into the wait queue */
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arch_spin_lock(&lock->wait_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_acquire(&lock->cnts, 0, _QW_LOCKED) == 0))
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goto unlock;
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/* Set the waiting flag to notify readers that a writer is pending */
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atomic_add(_QW_WAITING, &lock->cnts);
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/* When no more readers or writers, set the locked flag */
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do {
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cnts = atomic_cond_read_relaxed(&lock->cnts, VAL == _QW_WAITING);
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} while (!atomic_try_cmpxchg_acquire(&lock->cnts, &cnts, _QW_LOCKED));
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unlock:
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arch_spin_unlock(&lock->wait_lock);
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}
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EXPORT_SYMBOL(queued_write_lock_slowpath);
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