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authorAndré Fabian Silva Delgado <emulatorman@parabola.nu>2015-08-05 17:04:01 -0300
committerAndré Fabian Silva Delgado <emulatorman@parabola.nu>2015-08-05 17:04:01 -0300
commit57f0f512b273f60d52568b8c6b77e17f5636edc0 (patch)
tree5e910f0e82173f4ef4f51111366a3f1299037a7b /kernel/locking/qrwlock.c
Initial import
Diffstat (limited to 'kernel/locking/qrwlock.c')
-rw-r--r--kernel/locking/qrwlock.c132
1 files changed, 132 insertions, 0 deletions
diff --git a/kernel/locking/qrwlock.c b/kernel/locking/qrwlock.c
new file mode 100644
index 000000000..f956ede7f
--- /dev/null
+++ b/kernel/locking/qrwlock.c
@@ -0,0 +1,132 @@
+/*
+ * 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 <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>
+
+/**
+ * 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 = smp_load_acquire((u32 *)&lock->cnts);
+ }
+}
+
+/**
+ * queue_read_lock_slowpath - acquire read lock of a queue rwlock
+ * @lock: Pointer to queue rwlock structure
+ */
+void queue_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 spin until the lock is
+ * available without waiting in the queue.
+ */
+ cnts = smp_load_acquire((u32 *)&lock->cnts);
+ rspin_until_writer_unlock(lock, cnts);
+ return;
+ }
+ atomic_sub(_QR_BIAS, &lock->cnts);
+
+ /*
+ * Put the reader into the wait queue
+ */
+ arch_spin_lock(&lock->lock);
+
+ /*
+ * At the head of the wait queue now, wait until the writer state
+ * goes to 0 and then try to increment the reader count and get
+ * the lock. It is possible that an incoming writer may steal the
+ * lock in the interim, so it is necessary to check the writer byte
+ * to make sure that the write lock isn't taken.
+ */
+ while (atomic_read(&lock->cnts) & _QW_WMASK)
+ cpu_relax_lowlatency();
+
+ cnts = atomic_add_return(_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->lock);
+}
+EXPORT_SYMBOL(queue_read_lock_slowpath);
+
+/**
+ * queue_write_lock_slowpath - acquire write lock of a queue rwlock
+ * @lock : Pointer to queue rwlock structure
+ */
+void queue_write_lock_slowpath(struct qrwlock *lock)
+{
+ u32 cnts;
+
+ /* Put the writer into the wait queue */
+ arch_spin_lock(&lock->lock);
+
+ /* Try to acquire the lock directly if no reader is present */
+ if (!atomic_read(&lock->cnts) &&
+ (atomic_cmpxchg(&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 (;;) {
+ cnts = atomic_read(&lock->cnts);
+ if (!(cnts & _QW_WMASK) &&
+ (atomic_cmpxchg(&lock->cnts, cnts,
+ cnts | _QW_WAITING) == cnts))
+ break;
+
+ cpu_relax_lowlatency();
+ }
+
+ /* When no more readers, set the locked flag */
+ for (;;) {
+ cnts = atomic_read(&lock->cnts);
+ if ((cnts == _QW_WAITING) &&
+ (atomic_cmpxchg(&lock->cnts, _QW_WAITING,
+ _QW_LOCKED) == _QW_WAITING))
+ break;
+
+ cpu_relax_lowlatency();
+ }
+unlock:
+ arch_spin_unlock(&lock->lock);
+}
+EXPORT_SYMBOL(queue_write_lock_slowpath);