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authorWaiman Long <Waiman.Long@hp.com>2014-02-03 16:18:49 +0400
committerIngo Molnar <mingo@kernel.org>2014-06-06 09:58:28 +0400
commit70af2f8a4f48d6cebdf92d533d3aef37853ce6de (patch)
tree2a8d7b54053ef9fc7d45794ab28addbd00870819 /include/asm-generic/qrwlock.h
parent9161f5409798d52aa8598ff12575fde2327bed84 (diff)
downloadlinux-70af2f8a4f48d6cebdf92d533d3aef37853ce6de.tar.xz
locking/rwlocks: Introduce 'qrwlocks' - fair, queued rwlocks
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>
Diffstat (limited to 'include/asm-generic/qrwlock.h')
-rw-r--r--include/asm-generic/qrwlock.h166
1 files changed, 166 insertions, 0 deletions
diff --git a/include/asm-generic/qrwlock.h b/include/asm-generic/qrwlock.h
new file mode 100644
index 000000000000..6383d54bf983
--- /dev/null
+++ b/include/asm-generic/qrwlock.h
@@ -0,0 +1,166 @@
+/*
+ * 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>
+ */
+#ifndef __ASM_GENERIC_QRWLOCK_H
+#define __ASM_GENERIC_QRWLOCK_H
+
+#include <linux/atomic.h>
+#include <asm/barrier.h>
+#include <asm/processor.h>
+
+#include <asm-generic/qrwlock_types.h>
+
+/*
+ * Writer states & reader shift and bias
+ */
+#define _QW_WAITING 1 /* A writer is waiting */
+#define _QW_LOCKED 0xff /* A writer holds the lock */
+#define _QW_WMASK 0xff /* Writer mask */
+#define _QR_SHIFT 8 /* Reader count shift */
+#define _QR_BIAS (1U << _QR_SHIFT)
+
+/*
+ * External function declarations
+ */
+extern void queue_read_lock_slowpath(struct qrwlock *lock);
+extern void queue_write_lock_slowpath(struct qrwlock *lock);
+
+/**
+ * queue_read_can_lock- would read_trylock() succeed?
+ * @lock: Pointer to queue rwlock structure
+ */
+static inline int queue_read_can_lock(struct qrwlock *lock)
+{
+ return !(atomic_read(&lock->cnts) & _QW_WMASK);
+}
+
+/**
+ * queue_write_can_lock- would write_trylock() succeed?
+ * @lock: Pointer to queue rwlock structure
+ */
+static inline int queue_write_can_lock(struct qrwlock *lock)
+{
+ return !atomic_read(&lock->cnts);
+}
+
+/**
+ * queue_read_trylock - try to acquire read lock of a queue rwlock
+ * @lock : Pointer to queue rwlock structure
+ * Return: 1 if lock acquired, 0 if failed
+ */
+static inline int queue_read_trylock(struct qrwlock *lock)
+{
+ u32 cnts;
+
+ cnts = atomic_read(&lock->cnts);
+ if (likely(!(cnts & _QW_WMASK))) {
+ cnts = (u32)atomic_add_return(_QR_BIAS, &lock->cnts);
+ if (likely(!(cnts & _QW_WMASK)))
+ return 1;
+ atomic_sub(_QR_BIAS, &lock->cnts);
+ }
+ return 0;
+}
+
+/**
+ * queue_write_trylock - try to acquire write lock of a queue rwlock
+ * @lock : Pointer to queue rwlock structure
+ * Return: 1 if lock acquired, 0 if failed
+ */
+static inline int queue_write_trylock(struct qrwlock *lock)
+{
+ u32 cnts;
+
+ cnts = atomic_read(&lock->cnts);
+ if (unlikely(cnts))
+ return 0;
+
+ return likely(atomic_cmpxchg(&lock->cnts,
+ cnts, cnts | _QW_LOCKED) == cnts);
+}
+/**
+ * queue_read_lock - acquire read lock of a queue rwlock
+ * @lock: Pointer to queue rwlock structure
+ */
+static inline void queue_read_lock(struct qrwlock *lock)
+{
+ u32 cnts;
+
+ cnts = atomic_add_return(_QR_BIAS, &lock->cnts);
+ if (likely(!(cnts & _QW_WMASK)))
+ return;
+
+ /* The slowpath will decrement the reader count, if necessary. */
+ queue_read_lock_slowpath(lock);
+}
+
+/**
+ * queue_write_lock - acquire write lock of a queue rwlock
+ * @lock : Pointer to queue rwlock structure
+ */
+static inline void queue_write_lock(struct qrwlock *lock)
+{
+ /* Optimize for the unfair lock case where the fair flag is 0. */
+ if (atomic_cmpxchg(&lock->cnts, 0, _QW_LOCKED) == 0)
+ return;
+
+ queue_write_lock_slowpath(lock);
+}
+
+/**
+ * queue_read_unlock - release read lock of a queue rwlock
+ * @lock : Pointer to queue rwlock structure
+ */
+static inline void queue_read_unlock(struct qrwlock *lock)
+{
+ /*
+ * Atomically decrement the reader count
+ */
+ smp_mb__before_atomic();
+ atomic_sub(_QR_BIAS, &lock->cnts);
+}
+
+#ifndef queue_write_unlock
+/**
+ * queue_write_unlock - release write lock of a queue rwlock
+ * @lock : Pointer to queue rwlock structure
+ */
+static inline void queue_write_unlock(struct qrwlock *lock)
+{
+ /*
+ * If the writer field is atomic, it can be cleared directly.
+ * Otherwise, an atomic subtraction will be used to clear it.
+ */
+ smp_mb__before_atomic();
+ atomic_sub(_QW_LOCKED, &lock->cnts);
+}
+#endif
+
+/*
+ * Remapping rwlock architecture specific functions to the corresponding
+ * queue rwlock functions.
+ */
+#define arch_read_can_lock(l) queue_read_can_lock(l)
+#define arch_write_can_lock(l) queue_write_can_lock(l)
+#define arch_read_lock(l) queue_read_lock(l)
+#define arch_write_lock(l) queue_write_lock(l)
+#define arch_read_trylock(l) queue_read_trylock(l)
+#define arch_write_trylock(l) queue_write_trylock(l)
+#define arch_read_unlock(l) queue_read_unlock(l)
+#define arch_write_unlock(l) queue_write_unlock(l)
+
+#endif /* __ASM_GENERIC_QRWLOCK_H */