[PATCH v7 1/4] qrwlock: A queue read/write lock implementation
From: Waiman Long
Date: Fri Nov 22 2013 - 14:05:56 EST
This patch introduces a new read/write lock implementation that put
waiting readers and writers into a queue instead of actively contending
the lock like the current read/write lock implementation. This will
improve performance in highly contended situation by reducing the
cache line bouncing effect.
The queue read/write lock (qrwlock) is a fair lock even though there
is still a slight chance of lock stealing if a reader or writer comes
at the right moment. Other than that, lock granting is done in a
FIFO manner. As a result, it is possible to determine a maximum time
period after which the waiting is over and the lock can be acquired.
Internally, however, there is a second type of readers which try to
steal lock aggressively. They simply increments the reader count and
wait until the writer releases the lock. The transition to aggressive
reader happens in the read lock slowpath when
1. In an interrupt context.
2. When a reader comes to the head of the wait queue and sees
the release of a write lock.
The queue read lock is safe to use in an interrupt context (softirq
or hardirq) as it will switch to become an aggressive reader in such
environment allowing recursive read lock.
The only downside of queue rwlock is the size increase in the lock
structure by 4 bytes for 32-bit systems and by 12 bytes for 64-bit
systems.
In term of single-thread performance (no contention), a 256K
lock/unlock loop was run on a 2.4GHz and 2.93Ghz Westmere x86-64
CPUs. The following table shows the average time (in ns) for a single
lock/unlock sequence (including the looping and timing overhead):
Lock Type 2.4GHz 2.93GHz
--------- ------ -------
Ticket spinlock 14.9 12.3
Read lock 17.0 13.5
Write lock 17.0 13.5
Queue read lock 16.0 13.4
Queue write lock 9.2 7.8
The queue read lock is slightly slower than the spinlock, but is
slightly faster than the read lock. The queue write lock, however,
is the fastest of all. It is almost twice as fast as the write lock
and about 1.5X of the spinlock.
With lock contention, the speed of each individual lock/unlock function
is less important than the amount of contention-induced delays.
To investigate the performance characteristics of the queue rwlock
compared with the regular rwlock, Ingo's anon_vmas patch that converts
rwsem to rwlock was applied to a 3.12 kernel. This kernel was then
tested under the following 3 conditions:
1) Plain 3.12
2) Ingo's patch
3) Ingo's patch + qrwlock
Each of the 3 kernels were booted up twice with and without the
"idle=poll" kernel parameter which keeps the CPUs in C0 state while
idling instead of a more energy-saving sleep state. The jobs per
minutes (JPM) results of the AIM7's high_systime workload at 1500
users on a 8-socket 80-core DL980 (HT off) were:
Kernel JPMs %Change from (1)
------ ---- ----------------
1 145704/227295 -
2 229750/236066 +58%/+3.8%
4 240062/248606 +65%/+9.4%
The first JPM number is without the "idle=poll" kernel parameter,
the second number is with that parameter. It can be seen that most
of the performance benefit of converting rwsem to rwlock actually
come from the latency improvement of not needing to wake up a CPU
from deep sleep state when work is available.
The use of non-sleeping locks did improve performance by eliminating
the context switching cost. Using queue rwlock gave almost tripling
of performance gain. The performance gain was reduced somewhat with
a fair lock which was to be expected.
Looking at the perf profiles (with idle=poll) below, we can clearly see
that other bottlenecks were constraining the performance improvement.
Perf profile of kernel (2):
18.65% reaim [kernel.kallsyms] [k] __write_lock_failed
9.00% reaim [kernel.kallsyms] [k] _raw_spin_lock_irqsave
5.21% swapper [kernel.kallsyms] [k] cpu_idle_loop
3.08% reaim [kernel.kallsyms] [k] mspin_lock
2.50% reaim [kernel.kallsyms] [k] anon_vma_interval_tree_insert
2.00% ls [kernel.kallsyms] [k] _raw_spin_lock_irqsave
1.29% reaim [kernel.kallsyms] [k] update_cfs_rq_blocked_load
1.21% reaim [kernel.kallsyms] [k] __read_lock_failed
1.12% reaim [kernel.kallsyms] [k] _raw_spin_lock
1.10% reaim [kernel.kallsyms] [k] perf_event_aux
1.09% true [kernel.kallsyms] [k] _raw_spin_lock_irqsave
Perf profile of kernel (3):
20.14% swapper [kernel.kallsyms] [k] cpu_idle_loop
7.94% reaim [kernel.kallsyms] [k] _raw_spin_lock_irqsave
5.41% reaim [kernel.kallsyms] [k] queue_write_lock_slowpath
5.01% reaim [kernel.kallsyms] [k] mspin_lock
2.12% reaim [kernel.kallsyms] [k] anon_vma_interval_tree_insert
2.07% ls [kernel.kallsyms] [k] _raw_spin_lock_irqsave
1.58% reaim [kernel.kallsyms] [k] update_cfs_rq_blocked_load
1.25% reaim [kernel.kallsyms] [k] queue_write_3step_lock
1.18% reaim [kernel.kallsyms] [k] queue_read_lock_slowpath
1.14% true [kernel.kallsyms] [k] _raw_spin_lock_irqsave
0.95% reaim [kernel.kallsyms] [k] mutex_spin_on_owner
The spinlock bottlenecks were shown below.
7.94% reaim [kernel.kallsyms] [k] _raw_spin_lock_irqsave
|--59.72%-- release_pages
|--37.41%-- pagevec_lru_move_fn
|--0.82%-- get_page_from_freelist
|--0.73%-- __page_cache_release
--1.32%-- [...]
For both release_pages() & pagevec_lru_move_fn() function, the
spinlock contention was on zone->lru_lock. With the queue spinlock
patch, however, the contention went away with a lot more idle time
available and the JPM number went up to 265532 which was an additional
performance improvement.
28.40% swapper [kernel.kallsyms] [k] cpu_idle_loop
6.89% reaim [kernel.kallsyms] [k] mspin_lock
4.17% reaim [kernel.kallsyms] [k] queue_write_lock_slowpath
2.10% reaim [kernel.kallsyms] [k] anon_vma_interval_tree_insert
1.82% reaim [kernel.kallsyms] [k] update_cfs_rq_blocked_load
1.34% reaim [kernel.kallsyms] [k] entity_tick
1.17% reaim [kernel.kallsyms] [k] queue_write_3step_lock
1.06% reaim [kernel.kallsyms] [k] mutex_spin_on_owner
0.86% reaim [kernel.kallsyms] [k] perf_event_aux
0.83% ls [kernel.kallsyms] [k] mspin_lock
:
0.53% reaim [kernel.kallsyms] [k] _raw_spin_lock
0.14% reaim [kernel.kallsyms] [k] _raw_spin_lock_irqsave
Tim Chen also tested the qrwlock with Ingo's patch on a 4-socket
machine. It was found the performance improvement of 11% was the
same with regular rwlock or queue rwlock.
Signed-off-by: Waiman Long <Waiman.Long@xxxxxx>
---
include/asm-generic/qrwlock.h | 205 +++++++++++++++++++++++++++++++
kernel/Kconfig.locks | 7 +
kernel/locking/Makefile | 1 +
kernel/locking/qrwlock.c | 265 +++++++++++++++++++++++++++++++++++++++++
4 files changed, 478 insertions(+), 0 deletions(-)
create mode 100644 include/asm-generic/qrwlock.h
create mode 100644 kernel/locking/qrwlock.c
diff --git a/include/asm-generic/qrwlock.h b/include/asm-generic/qrwlock.h
new file mode 100644
index 0000000..9d085cb
--- /dev/null
+++ b/include/asm-generic/qrwlock.h
@@ -0,0 +1,205 @@
+/*
+ * 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 Hewlett-Packard Development Company, L.P.
+ *
+ * Authors: Waiman Long <waiman.long@xxxxxx>
+ */
+#ifndef __ASM_GENERIC_QRWLOCK_H
+#define __ASM_GENERIC_QRWLOCK_H
+
+#include <linux/types.h>
+#include <asm/bitops.h>
+#include <asm/cmpxchg.h>
+#include <asm/barrier.h>
+#include <asm/processor.h>
+#include <asm/byteorder.h>
+
+#if !defined(__LITTLE_ENDIAN) && !defined(__BIG_ENDIAN)
+#error "Missing either LITTLE_ENDIAN or BIG_ENDIAN definition."
+#endif
+
+/*
+ * The queue read/write lock data structure
+ *
+ * The layout of the structure is endian-sensitive to make sure that adding
+ * _QR_BIAS to the rw field to increment the reader count won't disturb
+ * the writer field. The least significant 8 bits is the writer field
+ * whereas the remaining 24 bits is the reader count.
+ */
+struct qrwnode {
+ struct qrwnode *next;
+ bool wait; /* Waiting flag */
+};
+
+typedef struct qrwlock {
+ union qrwcnts {
+ struct {
+#ifdef __LITTLE_ENDIAN
+ u8 writer; /* Writer state */
+#else
+ u16 r16; /* Reader count - msb */
+ u8 r8; /* Reader count - lsb */
+ u8 writer; /* Writer state */
+#endif
+ };
+ u32 rw; /* Reader/writer number pair */
+ } cnts;
+ struct qrwnode *waitq; /* Tail of waiting queue */
+} arch_rwlock_t;
+
+/*
+ * Writer states & reader shift and bias
+ */
+#define _QW_WAITING 1 /* A writer is waiting */
+#define _QW_LOCKED 0xff /* A writer holds the lock */
+#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 !ACCESS_ONCE(lock->cnts.writer);
+}
+
+/**
+ * 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 !ACCESS_ONCE(lock->cnts.rw);
+}
+
+/**
+ * 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)
+{
+ union qrwcnts cnts;
+
+ cnts.rw = ACCESS_ONCE(lock->cnts.rw);
+ if (likely(!cnts.writer)) {
+ cnts.rw = xadd(&lock->cnts.rw, _QR_BIAS);
+ if (likely(!cnts.writer))
+ return 1;
+ add_smp(&lock->cnts.rw, -_QR_BIAS);
+ }
+ 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)
+{
+ union qrwcnts old, new;
+
+ old.rw = ACCESS_ONCE(lock->cnts.rw);
+ if (likely(!old.rw)) {
+ new.rw = old.rw;
+ new.writer = _QW_LOCKED;
+ if (likely(cmpxchg(&lock->cnts.rw, old.rw, new.rw) == old.rw))
+ return 1;
+ }
+ return 0;
+}
+/**
+ * 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)
+{
+ union qrwcnts cnts;
+
+ cnts.rw = xadd(&lock->cnts.rw, _QR_BIAS);
+ if (likely(!cnts.writer))
+ return;
+ /*
+ * 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 (cmpxchg(&lock->cnts.rw, 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
+ */
+ add_smp(&lock->cnts.rw, -_QR_BIAS);
+}
+
+/**
+ * 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)
+{
+ /*
+ * Make sure that none of the critical section will be leaked out.
+ */
+ smp_mb__before_clear_bit();
+ ACCESS_ONCE(lock->cnts.writer) = 0;
+ smp_mb__after_clear_bit();
+}
+
+/*
+ * Initializier
+ */
+#define __ARCH_RW_LOCK_UNLOCKED { .cnts = { .rw = 0 }, .waitq = NULL }
+
+/*
+ * 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 */
diff --git a/kernel/Kconfig.locks b/kernel/Kconfig.locks
index d2b32ac..b665478 100644
--- a/kernel/Kconfig.locks
+++ b/kernel/Kconfig.locks
@@ -223,3 +223,10 @@ endif
config MUTEX_SPIN_ON_OWNER
def_bool y
depends on SMP && !DEBUG_MUTEXES
+
+config ARCH_QUEUE_RWLOCK
+ bool
+
+config QUEUE_RWLOCK
+ def_bool y if ARCH_QUEUE_RWLOCK
+ depends on SMP
diff --git a/kernel/locking/Makefile b/kernel/locking/Makefile
index baab8e5..3e7bab1 100644
--- a/kernel/locking/Makefile
+++ b/kernel/locking/Makefile
@@ -23,3 +23,4 @@ obj-$(CONFIG_DEBUG_SPINLOCK) += spinlock_debug.o
obj-$(CONFIG_RWSEM_GENERIC_SPINLOCK) += rwsem-spinlock.o
obj-$(CONFIG_RWSEM_XCHGADD_ALGORITHM) += rwsem-xadd.o
obj-$(CONFIG_PERCPU_RWSEM) += percpu-rwsem.o
+obj-$(CONFIG_QUEUE_RWLOCK) += qrwlock.o
diff --git a/kernel/locking/qrwlock.c b/kernel/locking/qrwlock.c
new file mode 100644
index 0000000..ea5553d
--- /dev/null
+++ b/kernel/locking/qrwlock.c
@@ -0,0 +1,265 @@
+/*
+ * 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 Hewlett-Packard Development Company, L.P.
+ *
+ * Authors: Waiman Long <waiman.long@xxxxxx>
+ */
+#include <linux/smp.h>
+#include <linux/bug.h>
+#include <linux/cpumask.h>
+#include <linux/percpu.h>
+#include <linux/hardirq.h>
+#include <asm-generic/qrwlock.h>
+
+/*
+ * Compared with regular rwlock, the queue rwlock has has the following
+ * advantages:
+ * 1. Even though there is a slight chance of stealing the lock if come at
+ * the right moment, the granting of the lock is mostly in FIFO order.
+ * 2. It is usually faster in high contention situation.
+ *
+ * The only downside is that the lock is 4 bytes larger in 32-bit systems
+ * and 12 bytes larger in 64-bit systems.
+ *
+ * There are two queues for writers. The writer field of the lock is a
+ * one-slot wait queue. The writers that follow will have to wait in the
+ * combined reader/writer queue (waitq).
+ *
+ * Compared with x86 ticket spinlock, the queue rwlock is faster in high
+ * contention situation. The writer lock is also faster in single thread
+ * operations. Therefore, queue rwlock can be considered as a replacement
+ * for those spinlocks that are highly contended as long as an increase
+ * in lock size is not an issue.
+ */
+
+#ifndef arch_mutex_cpu_relax
+# define arch_mutex_cpu_relax() cpu_relax()
+#endif
+
+#ifndef smp_mb__load_acquire
+# ifdef CONFIG_X86
+# define smp_mb__load_acquire() barrier()
+# else
+# define smp_mb__load_acquire() smp_mb()
+# endif
+#endif
+
+#ifndef smp_mb__store_release
+# ifdef CONFIG_X86
+# define smp_mb__store_release() barrier()
+# else
+# define smp_mb__store_release() smp_mb()
+# endif
+#endif
+
+/**
+ * wait_in_queue - Add to queue and wait until it is at the head
+ * @lock: Pointer to queue rwlock structure
+ * @node: Node pointer to be added to the queue
+ */
+static __always_inline void
+wait_in_queue(struct qrwlock *lock, struct qrwnode *node)
+{
+ struct qrwnode *prev;
+
+ node->next = NULL;
+ node->wait = true;
+ prev = xchg(&lock->waitq, node);
+ if (prev) {
+ prev->next = node;
+ /*
+ * Wait until the waiting flag is off
+ */
+ while (ACCESS_ONCE(node->wait))
+ arch_mutex_cpu_relax();
+ smp_mb__load_acquire();
+ }
+}
+
+/**
+ * signal_next - Signal the next one in queue to be at the head
+ * @lock: Pointer to queue rwlock structure
+ * @node: Node pointer to the current head of queue
+ */
+static __always_inline void
+signal_next(struct qrwlock *lock, struct qrwnode *node)
+{
+ struct qrwnode *next;
+
+ /*
+ * Try to notify the next node first without disturbing the cacheline
+ * of the lock. If that fails, check to see if it is the last node
+ * and so should clear the wait queue.
+ */
+ next = ACCESS_ONCE(node->next);
+ if (likely(next))
+ goto notify_next;
+
+ /*
+ * Clear the wait queue if it is the last node
+ */
+ if ((ACCESS_ONCE(lock->waitq) == node) &&
+ (cmpxchg(&lock->waitq, node, NULL) == node))
+ return;
+ /*
+ * Wait until the next one in queue set up the next field
+ */
+ while (likely(!(next = ACCESS_ONCE(node->next))))
+ arch_mutex_cpu_relax();
+ /*
+ * The next one in queue is now at the head
+ */
+notify_next:
+ smp_mb__store_release();
+ ACCESS_ONCE(next->wait) = false;
+}
+
+/**
+ * rspin_until_writer_unlock - inc reader count & spin until writer is gone
+ * @lock: Pointer to queue rwlock structure
+ * @cnts: Current queue rwlock counts structure
+ *
+ * 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, union qrwcnts cnts)
+{
+ while (cnts.writer == _QW_LOCKED) {
+ arch_mutex_cpu_relax();
+ cnts.rw = ACCESS_ONCE(lock->cnts.rw);
+ }
+}
+
+/**
+ * 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)
+{
+ struct qrwnode node;
+ union qrwcnts cnts;
+
+ /*
+ * Readers come here when it cannot get the lock without waiting
+ */
+ if (unlikely(irq_count())) {
+ /*
+ * Readers in interrupt context will spin until the lock is
+ * available without waiting in the queue.
+ */
+ cnts.rw = ACCESS_ONCE(lock->cnts.rw);
+ rspin_until_writer_unlock(lock, cnts);
+ return;
+ }
+ add_smp(&lock->cnts.rw, -_QR_BIAS);
+
+ /*
+ * Put the reader into the wait queue
+ */
+ wait_in_queue(lock, &node);
+
+ /*
+ * 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.
+ */
+ while (ACCESS_ONCE(lock->cnts.writer))
+ arch_mutex_cpu_relax();
+ cnts.rw = xadd(&lock->cnts.rw, _QR_BIAS);
+ rspin_until_writer_unlock(lock, cnts);
+ /*
+ * Need to have a barrier with read-acquire semantics
+ */
+ smp_mb__load_acquire();
+ signal_next(lock, &node);
+}
+EXPORT_SYMBOL(queue_read_lock_slowpath);
+
+/**
+ * qwrite_trylock - Try to acquire the write lock
+ * @lock : Pointer to queue rwlock structure
+ * @old : The current queue rwlock count structure
+ * Return: 1 if lock acquired, 0 otherwise
+ */
+static __always_inline int
+qwrite_trylock(struct qrwlock *lock, union qrwcnts old)
+{
+ register union qrwcnts new;
+
+ new.rw = old.rw;
+ new.writer = _QW_LOCKED;
+ if (likely(cmpxchg(&lock->cnts.rw, old.rw, new.rw) == old.rw))
+ return 1;
+ return 0;
+}
+
+/**
+ * queue_write_3step_lock - acquire write lock in 3 steps
+ * @lock : Pointer to queue rwlock structure
+ * Return: 1 if lock acquired, 0 otherwise
+ *
+ * Step 1 - Try to acquire the lock directly if no reader is present
+ * Step 2 - Set the waiting flag to notify readers that a writer is waiting
+ * Step 3 - When the readers field goes to 0, set the locked flag
+ *
+ * In x86, the use of noinline generates a slight better optimized code
+ * with less memory access.
+ */
+static noinline int queue_write_3step_lock(struct qrwlock *lock)
+{
+ register union qrwcnts old;
+
+ old.rw = ACCESS_ONCE(lock->cnts.rw);
+
+ /* Step 1 */
+ if (!old.rw && qwrite_trylock(lock, old))
+ return 1;
+
+ /* Step 2 */
+ if (old.writer || (cmpxchg(&lock->cnts.writer, 0, _QW_WAITING) != 0))
+ return 0;
+
+ /* Step 3 */
+ arch_mutex_cpu_relax();
+ old.rw = ACCESS_ONCE(lock->cnts.rw);
+ while ((old.rw > _QW_WAITING) || !qwrite_trylock(lock, old)) {
+ arch_mutex_cpu_relax();
+ old.rw = ACCESS_ONCE(lock->cnts.rw);
+ }
+ return 1;
+}
+
+/**
+ * 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)
+{
+ struct qrwnode node;
+
+ /*
+ * Put the writer into the wait queue
+ */
+ wait_in_queue(lock, &node);
+
+ /*
+ * At the head of the wait queue now, call queue_write_3step_lock()
+ * to acquire the lock until it is done.
+ */
+ while (!queue_write_3step_lock(lock))
+ arch_mutex_cpu_relax();
+ signal_next(lock, &node);
+}
+EXPORT_SYMBOL(queue_write_lock_slowpath);
--
1.7.1
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