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Date: Fri, 8 Aug 2008 10:04:05 -0400
From: "Ryan Hope" <rmh3093@...il.com>
To: "Peter Zijlstra" <peterz@...radead.org>
Cc: LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH][RESEND] scalable rw_mutex
Sorry for not CCing you, I had intended to CC you and others but I
clicked send too quick. One of the reiser4 todo's was remove all
semaphore's, I didnt realize rw_semaphores were not real semaphores
On Fri, Aug 8, 2008 at 6:21 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Thu, 2008-08-07 at 18:56 -0400, Ryan Hope wrote:
>> 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.
>
> Thanks for CC'ing me :-/
>
> I dropped it because its only more scalable up to around 4 cpus.
>
> Also, how would it help reiser4? using rwsems is perfectly fine - as
> they aren't actual semaphores.
>
>> 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@...hat.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@...hat.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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>
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