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Subject[PATCH][RESEND] scalable rw_mutex
This was posted sometime last year I think and it never got merged. Can this get 
a go around in -mm, it would help in converting the semaphore's in reiser4 to
mutexes.

diff --git a/include/linux/rwmutex.h b/include/linux/rwmutex.h
new file mode 100644
index 0000000..39ec857
--- /dev/null
+++ b/include/linux/rwmutex.h
@@ -0,0 +1,83 @@
+/*
+ * Scalable reader/writer lock.
+ *
+ * Copyright (C) 2007 Red Hat, Inc., Peter Zijlstra <pzijlstr@redhat.com>
+ *
+ * This file contains the public data structure and API definitions.
+ */
+#ifndef _LINUX_RWMUTEX_H
+#define _LINUX_RWMUTEX_H
+
+#include <linux/preempt.h>
+#include <linux/wait.h>
+#include <linux/percpu_counter.h>
+#include <linux/lockdep.h>
+#include <linux/mutex.h>
+#include <asm/atomic.h>
+
+struct rw_mutex {
+ /* Read mostly global */
+ struct percpu_counter readers;
+ unsigned int status;
+
+ /* The following variables are only for the slowpath */
+ struct mutex read_mutex; /* r -> w waiting */
+ struct mutex write_mutex; /* w -> w waiting */
+ struct task_struct *waiter; /* w -> r waiting */
+ atomic_t read_waiters;
+
+#ifdef CONFIG_DEBUG_LOCK_ALLOC
+ struct lockdep_map dep_map;
+#endif
+};
+
+void __rw_mutex_init(struct rw_mutex *rw_mutex, const char * name,
+ struct lock_class_key *key);
+void rw_mutex_destroy(struct rw_mutex *rw_mutex);
+
+#define rw_mutex_init(rw_mutex) \
+ do { \
+ static struct lock_class_key __key; \
+ __rw_mutex_init((rw_mutex), #rw_mutex, &__key); \
+ } while (0)
+
+void rw_mutex_read_lock_slow(struct rw_mutex *rw_mutex);
+
+void rw_mutex_write_lock_nested(struct rw_mutex *rw_mutex, int subclass);
+void rw_mutex_write_unlock(struct rw_mutex *rw_mutex);
+
+int __rw_mutex_read_trylock(struct rw_mutex *rw_mutex);
+
+static inline int rw_mutex_read_trylock(struct rw_mutex *rw_mutex)
+{
+ int ret = __rw_mutex_read_trylock(rw_mutex);
+ if (ret)
+ rwsem_acquire_read(&rw_mutex->dep_map, 0, 1, _RET_IP_);
+ return ret;
+}
+
+static inline void rw_mutex_read_lock(struct rw_mutex *rw_mutex)
+{
+ int ret;
+
+ might_sleep();
+ rwsem_acquire_read(&rw_mutex->dep_map, 0, 0, _RET_IP_);
+
+ ret = __rw_mutex_read_trylock(rw_mutex);
+ if (!ret)
+ rw_mutex_read_lock_slow(rw_mutex);
+}
+
+void rw_mutex_read_unlock(struct rw_mutex *rw_mutex);
+
+static inline int rw_mutex_is_locked(struct rw_mutex *rw_mutex)
+{
+ return mutex_is_locked(&rw_mutex->write_mutex);
+}
+
+static inline void rw_mutex_write_lock(struct rw_mutex *rw_mutex)
+{
+ rw_mutex_write_lock_nested(rw_mutex, 0);
+}
+
+#endif /* _LINUX_RWMUTEX_H */
diff --git a/kernel/Makefile b/kernel/Makefile
index dd58bdc..8277ef5 100644
--- a/kernel/Makefile
+++ b/kernel/Makefile
@@ -9,7 +9,7 @@ obj-y = sched.o fork.o exec_domain.o panic.o printk.o \
rcupdate.o extable.o params.o posix-timers.o \
kthread.o wait.o kfifo.o sys_ni.o posix-cpu-timers.o mutex.o \
hrtimer.o rwsem.o nsproxy.o srcu.o semaphore.o \
- notifier.o ksysfs.o pm_qos_params.o sched_clock.o
+ notifier.o ksysfs.o pm_qos_params.o sched_clock.o rwmutex.o

CFLAGS_REMOVE_sched.o = -mno-spe

diff --git a/kernel/rwmutex.c b/kernel/rwmutex.c
new file mode 100644
index 0000000..2b82d11
--- /dev/null
+++ b/kernel/rwmutex.c
@@ -0,0 +1,256 @@
+/*
+ * Scalable reader/writer lock.
+ *
+ * Copyright (C) 2007 Red Hat, Inc., Peter Zijlstra <pzijlstr@redhat.com>
+ *
+ * Its scalable in that the read count is a percpu counter and the reader fast
+ * path does not write to a shared cache-line.
+ *
+ * Its not FIFO fair, but starvation proof by alternating readers and writers.
+ */
+#include <linux/sched.h>
+#include <linux/rwmutex.h>
+#include <linux/debug_locks.h>
+#include <linux/module.h>
+
+/*
+ * rw mutex - oxymoron when we take mutex to stand for 'MUTual EXlusion'
+ *
+ * However in this context we take mutex to mean a sleeping lock, with the
+ * property that it must be released by the same context that acquired it.
+ *
+ * design goals:
+ *
+ * A sleeping reader writer lock with a scalable read side, to avoid bouncing
+ * cache-lines.
+ *
+ * dynamics:
+ *
+ * The reader fast path is modification of a percpu_counter and a read of a
+ * shared cache-line.
+ *
+ * The write side is quite heavy; it takes two mutexes, a writer mutex and a
+ * readers mutex. The writer mutex is for w <-> w interaction, the read mutex
+ * for r -> w. The read side is forced into the slow path by setting the
+ * status bit. Then it waits for all current readers to disappear.
+ *
+ * The read lock slow path; taken when the status bit is set; takes the read
+ * mutex. Because the write side also takes this mutex, the new readers are
+ * blocked. The read unlock slow path tickles the writer every time a read
+ * lock is released.
+ *
+ * Write unlock clears the status bit, and drops the read mutex; allowing new
+ * readers. It then waits for at least one waiting reader to get a lock (if
+ * there were any readers waiting) before releasing the write mutex which will
+ * allow possible other writers to come in an stop new readers, thus avoiding
+ * starvation by alternating between readers and writers
+ *
+ * considerations:
+ *
+ * The lock's space footprint is quite large (on x86_64):
+ *
+ * 96 bytes [struct rw_mutex]
+ * 8 bytes per cpu NR_CPUS [void *]
+ * 32 bytes per cpu (NR_CPUS ?= cpu_possible_map ?= nr_cpu_ids)
+ * [smallest slab]
+ *
+ * 1376 bytes for x86_64 defconfig (NR_CPUS = 32)
+ */
+
+#define RW_MUTEX_READER_FAST 0
+#define RW_MUTEX_READER_SLOW 1
+
+void __rw_mutex_init(struct rw_mutex *rw_mutex, const char *name,
+ struct lock_class_key *key)
+{
+#ifdef CONFIG_DEBUG_LOCK_ALLOC
+ debug_check_no_locks_freed((void *)rw_mutex, sizeof(*rw_mutex));
+ lockdep_init_map(&rw_mutex->dep_map, name, key, 0);
+#endif
+
+ percpu_counter_init(&rw_mutex->readers, 0);
+ rw_mutex->status = RW_MUTEX_READER_FAST;
+ mutex_init(&rw_mutex->read_mutex);
+ mutex_init(&rw_mutex->write_mutex);
+ rw_mutex->waiter = NULL;
+#ifdef CONFIG_DEBUG_LOCK_ALLOC
+ printk("rw_mutex size: %u\n", sizeof(struct rw_mutex));
+#endif
+}
+EXPORT_SYMBOL(__rw_mutex_init);
+
+void rw_mutex_destroy(struct rw_mutex *rw_mutex)
+{
+ percpu_counter_destroy(&rw_mutex->readers);
+ mutex_destroy(&rw_mutex->read_mutex);
+ mutex_destroy(&rw_mutex->write_mutex);
+}
+EXPORT_SYMBOL(rw_mutex_destroy);
+
+static inline void rw_mutex_readers_inc(struct rw_mutex *rw_mutex)
+{
+ percpu_counter_inc(&rw_mutex->readers);
+ smp_wmb();
+}
+
+static inline void rw_mutex_readers_dec(struct rw_mutex *rw_mutex)
+{
+ percpu_counter_dec(&rw_mutex->readers);
+ smp_wmb();
+}
+
+static inline long rw_mutex_readers(struct rw_mutex *rw_mutex)
+{
+ smp_rmb();
+ return percpu_counter_sum(&rw_mutex->readers);
+}
+
+#define rw_mutex_writer_wait(rw_mutex, condition) \
+do { \
+ struct task_struct *tsk = current; \
+ \
+ BUG_ON((rw_mutex)->waiter); \
+ set_task_state(tsk, TASK_UNINTERRUPTIBLE); \
+ get_task_struct(tsk); \
+ (rw_mutex)->waiter = tsk; \
+ smp_wmb(); \
+ while (!(condition)) { \
+ schedule(); \
+ set_task_state(tsk, TASK_UNINTERRUPTIBLE); \
+ } \
+ tsk->state = TASK_RUNNING; \
+ (rw_mutex)->waiter = NULL; \
+ put_task_struct(tsk); \
+} while (0)
+
+static inline void rw_mutex_writer_wake(struct rw_mutex *rw_mutex)
+{
+ struct task_struct *tsk;
+
+ smp_rmb();
+ tsk = rw_mutex->waiter;
+ if (tsk)
+ wake_up_process(tsk);
+}
+
+void rw_mutex_read_lock_slow(struct rw_mutex *rw_mutex)
+{
+ /*
+ * read lock slow path;
+ * count the number of readers waiting on the read_mutex
+ */
+ atomic_inc(&rw_mutex->read_waiters);
+ mutex_lock(&rw_mutex->read_mutex);
+
+ /*
+ * rw_mutex->state is only set while the read_mutex is held
+ * so by serialising on this lock, we're sure its free.
+ */
+ BUG_ON(rw_mutex->status);
+
+ rw_mutex_readers_inc(rw_mutex);
+
+ /*
+ * wake up a possible write unlock; waiting for at least a single
+ * reader to pass before letting a new writer through.
+ */
+ atomic_dec(&rw_mutex->read_waiters);
+ rw_mutex_writer_wake(rw_mutex);
+ mutex_unlock(&rw_mutex->read_mutex);
+}
+EXPORT_SYMBOL(rw_mutex_read_lock_slow);
+
+static inline
+void rw_mutex_status_set(struct rw_mutex *rw_mutex, unsigned int status)
+{
+ rw_mutex->status = status;
+ /*
+ * allow new readers to see this change in status
+ */
+ smp_wmb();
+}
+
+static inline unsigned int rw_mutex_reader_slow(struct rw_mutex *rw_mutex)
+{
+ /*
+ * match rw_mutex_status_set()
+ */
+ smp_rmb();
+ return rw_mutex->status;
+}
+
+int __rw_mutex_read_trylock(struct rw_mutex *rw_mutex)
+{
+ rw_mutex_readers_inc(rw_mutex);
+ if (unlikely(rw_mutex_reader_slow(rw_mutex))) {
+ rw_mutex_readers_dec(rw_mutex);
+ /*
+ * possibly wake up a writer waiting for this reference to
+ * disappear
+ */
+ rw_mutex_writer_wake(rw_mutex);
+ return 0;
+ }
+ return 1;
+}
+EXPORT_SYMBOL(__rw_mutex_read_trylock);
+
+void rw_mutex_read_unlock(struct rw_mutex *rw_mutex)
+{
+ rwsem_release(&rw_mutex->dep_map, 1, _RET_IP_);
+
+ rw_mutex_readers_dec(rw_mutex);
+ /*
+ * on the slow path;
+ * nudge the writer waiting for the last reader to go away
+ */
+ if (unlikely(rw_mutex_reader_slow(rw_mutex)))
+ rw_mutex_writer_wake(rw_mutex);
+}
+EXPORT_SYMBOL(rw_mutex_read_unlock);
+
+void rw_mutex_write_lock_nested(struct rw_mutex *rw_mutex, int subclass)
+{
+ might_sleep();
+ rwsem_acquire(&rw_mutex->dep_map, subclass, 0, _RET_IP_);
+
+ mutex_lock_nested(&rw_mutex->write_mutex, subclass);
+
+ /*
+ * block new readers
+ */
+ mutex_lock_nested(&rw_mutex->read_mutex, subclass);
+ rw_mutex_status_set(rw_mutex, RW_MUTEX_READER_SLOW);
+ /*
+ * and wait for all current readers to go away
+ */
+ rw_mutex_writer_wait(rw_mutex, (rw_mutex_readers(rw_mutex) == 0));
+}
+EXPORT_SYMBOL(rw_mutex_write_lock_nested);
+
+void rw_mutex_write_unlock(struct rw_mutex *rw_mutex)
+{
+ int waiters;
+
+ might_sleep();
+ rwsem_release(&rw_mutex->dep_map, 1, _RET_IP_);
+
+ /*
+ * let the readers rip
+ */
+ rw_mutex_status_set(rw_mutex, RW_MUTEX_READER_FAST);
+ waiters = atomic_read(&rw_mutex->read_waiters);
+ mutex_unlock(&rw_mutex->read_mutex);
+ /*
+ * wait for at least 1 reader to get through
+ */
+ if (waiters) {
+ rw_mutex_writer_wait(rw_mutex,
+ (atomic_read(&rw_mutex->read_waiters) < waiters));
+ }
+ /*
+ * before we let the writers rip
+ */
+ mutex_unlock(&rw_mutex->write_mutex);
+}
+EXPORT_SYMBOL(rw_mutex_write_unlock);

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