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- /*
- * Queued read/write locks
- *
- * 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>
- */
- #include <linux/smp.h>
- #include <linux/bug.h>
- #include <linux/cpumask.h>
- #include <linux/percpu.h>
- #include <linux/hardirq.h>
- #include <asm/qrwlock.h>
- /*
- * This internal data structure is used for optimizing access to some of
- * the subfields within the atomic_t cnts.
- */
- struct __qrwlock {
- union {
- atomic_t cnts;
- struct {
- #ifdef __LITTLE_ENDIAN
- u8 wmode; /* Writer mode */
- u8 rcnts[3]; /* Reader counts */
- #else
- u8 rcnts[3]; /* Reader counts */
- u8 wmode; /* Writer mode */
- #endif
- };
- };
- arch_spinlock_t lock;
- };
- /**
- * rspin_until_writer_unlock - inc reader count & spin until writer is gone
- * @lock : Pointer to queue rwlock structure
- * @writer: Current queue rwlock writer status byte
- *
- * In interrupt context or at the head of the queue, the reader will just
- * increment the reader count & wait until the writer releases the lock.
- */
- static __always_inline void
- rspin_until_writer_unlock(struct qrwlock *lock, u32 cnts)
- {
- while ((cnts & _QW_WMASK) == _QW_LOCKED) {
- cpu_relax_lowlatency();
- cnts = atomic_read_acquire(&lock->cnts);
- }
- }
- /**
- * queued_read_lock_slowpath - acquire read lock of a queue rwlock
- * @lock: Pointer to queue rwlock structure
- * @cnts: Current qrwlock lock value
- */
- void queued_read_lock_slowpath(struct qrwlock *lock, u32 cnts)
- {
- /*
- * Readers come here when they cannot get the lock without waiting
- */
- if (unlikely(in_interrupt())) {
- /*
- * Readers in interrupt context will get the lock immediately
- * if the writer is just waiting (not holding the lock yet).
- * The rspin_until_writer_unlock() function returns immediately
- * in this case. Otherwise, they will spin (with ACQUIRE
- * semantics) until the lock is available without waiting in
- * the queue.
- */
- rspin_until_writer_unlock(lock, cnts);
- return;
- }
- atomic_sub(_QR_BIAS, &lock->cnts);
- /*
- * Put the reader into the wait queue
- */
- arch_spin_lock(&lock->wait_lock);
- /*
- * The ACQUIRE semantics of the following spinning code ensure
- * that accesses can't leak upwards out of our subsequent critical
- * section in the case that the lock is currently held for write.
- */
- cnts = atomic_add_return_acquire(_QR_BIAS, &lock->cnts) - _QR_BIAS;
- rspin_until_writer_unlock(lock, cnts);
- /*
- * Signal the next one in queue to become queue head
- */
- arch_spin_unlock(&lock->wait_lock);
- }
- EXPORT_SYMBOL(queued_read_lock_slowpath);
- /**
- * queued_write_lock_slowpath - acquire write lock of a queue rwlock
- * @lock : Pointer to queue rwlock structure
- */
- void queued_write_lock_slowpath(struct qrwlock *lock)
- {
- u32 cnts;
- /* Put the writer into the wait queue */
- arch_spin_lock(&lock->wait_lock);
- /* Try to acquire the lock directly if no reader is present */
- if (!atomic_read(&lock->cnts) &&
- (atomic_cmpxchg_acquire(&lock->cnts, 0, _QW_LOCKED) == 0))
- goto unlock;
- /*
- * Set the waiting flag to notify readers that a writer is pending,
- * or wait for a previous writer to go away.
- */
- for (;;) {
- struct __qrwlock *l = (struct __qrwlock *)lock;
- if (!READ_ONCE(l->wmode) &&
- (cmpxchg_relaxed(&l->wmode, 0, _QW_WAITING) == 0))
- break;
- cpu_relax_lowlatency();
- }
- /* When no more readers, set the locked flag */
- for (;;) {
- cnts = atomic_read(&lock->cnts);
- if ((cnts == _QW_WAITING) &&
- (atomic_cmpxchg_acquire(&lock->cnts, _QW_WAITING,
- _QW_LOCKED) == _QW_WAITING))
- break;
- cpu_relax_lowlatency();
- }
- unlock:
- arch_spin_unlock(&lock->wait_lock);
- }
- EXPORT_SYMBOL(queued_write_lock_slowpath);
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