d4d74af4b8
Currently the percpu-rwsem switches to (global) atomic ops while a writer is waiting; which could be quite a while and slows down releasing the readers. This patch cures this problem by ordering the reader-state vs reader-count (see the comments in __percpu_down_read() and percpu_down_write()). This changes a global atomic op into a full memory barrier, which doesn't have the global cacheline contention. This also enables using the percpu-rwsem with rcu_sync disabled in order to bias the implementation differently, reducing the writer latency by adding some cost to readers. Mailing-list-URL: https://lkml.org/lkml/2016/8/9/181 Cc: Paul McKenney <paulmck@linux.vnet.ibm.com> Reviewed-by: Oleg Nesterov <oleg@redhat.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> [jstultz: Backported to 4.4] Change-Id: I8ea04b4dca2ec36f1c2469eccafde1423490572f Signed-off-by: John Stultz <john.stultz@linaro.org>
121 lines
3.2 KiB
C
121 lines
3.2 KiB
C
#ifndef _LINUX_PERCPU_RWSEM_H
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#define _LINUX_PERCPU_RWSEM_H
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#include <linux/atomic.h>
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#include <linux/rwsem.h>
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#include <linux/percpu.h>
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#include <linux/wait.h>
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#include <linux/rcu_sync.h>
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#include <linux/lockdep.h>
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struct percpu_rw_semaphore {
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struct rcu_sync rss;
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unsigned int __percpu *read_count;
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struct rw_semaphore rw_sem;
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wait_queue_head_t writer;
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int readers_block;
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};
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extern int __percpu_down_read(struct percpu_rw_semaphore *, int);
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extern void __percpu_up_read(struct percpu_rw_semaphore *);
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static inline void percpu_down_read(struct percpu_rw_semaphore *sem)
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{
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might_sleep();
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rwsem_acquire_read(&sem->rw_sem.dep_map, 0, 0, _RET_IP_);
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preempt_disable();
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/*
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* We are in an RCU-sched read-side critical section, so the writer
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* cannot both change sem->state from readers_fast and start checking
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* counters while we are here. So if we see !sem->state, we know that
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* the writer won't be checking until we're past the preempt_enable()
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* and that one the synchronize_sched() is done, the writer will see
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* anything we did within this RCU-sched read-size critical section.
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*/
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__this_cpu_inc(*sem->read_count);
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if (unlikely(!rcu_sync_is_idle(&sem->rss)))
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__percpu_down_read(sem, false); /* Unconditional memory barrier */
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preempt_enable();
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/*
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* The barrier() from preempt_enable() prevents the compiler from
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* bleeding the critical section out.
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*/
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}
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static inline int percpu_down_read_trylock(struct percpu_rw_semaphore *sem)
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{
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int ret = 1;
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preempt_disable();
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/*
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* Same as in percpu_down_read().
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*/
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__this_cpu_inc(*sem->read_count);
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if (unlikely(!rcu_sync_is_idle(&sem->rss)))
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ret = __percpu_down_read(sem, true); /* Unconditional memory barrier */
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preempt_enable();
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/*
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* The barrier() from preempt_enable() prevents the compiler from
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* bleeding the critical section out.
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*/
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if (ret)
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rwsem_acquire_read(&sem->rw_sem.dep_map, 0, 1, _RET_IP_);
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return ret;
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}
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static inline void percpu_up_read(struct percpu_rw_semaphore *sem)
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{
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/*
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* The barrier() in preempt_disable() prevents the compiler from
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* bleeding the critical section out.
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*/
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preempt_disable();
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/*
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* Same as in percpu_down_read().
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*/
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if (likely(rcu_sync_is_idle(&sem->rss)))
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__this_cpu_dec(*sem->read_count);
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else
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__percpu_up_read(sem); /* Unconditional memory barrier */
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preempt_enable();
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rwsem_release(&sem->rw_sem.dep_map, 1, _RET_IP_);
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}
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extern void percpu_down_write(struct percpu_rw_semaphore *);
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extern void percpu_up_write(struct percpu_rw_semaphore *);
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extern int __percpu_init_rwsem(struct percpu_rw_semaphore *,
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const char *, struct lock_class_key *);
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extern void percpu_free_rwsem(struct percpu_rw_semaphore *);
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#define percpu_init_rwsem(sem) \
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({ \
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static struct lock_class_key rwsem_key; \
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__percpu_init_rwsem(sem, #sem, &rwsem_key); \
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})
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#define percpu_rwsem_is_held(sem) lockdep_is_held(&(sem)->rw_sem)
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static inline void percpu_rwsem_release(struct percpu_rw_semaphore *sem,
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bool read, unsigned long ip)
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{
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lock_release(&sem->rw_sem.dep_map, 1, ip);
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#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
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if (!read)
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sem->rw_sem.owner = NULL;
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#endif
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}
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static inline void percpu_rwsem_acquire(struct percpu_rw_semaphore *sem,
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bool read, unsigned long ip)
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{
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lock_acquire(&sem->rw_sem.dep_map, 0, 1, read, 1, NULL, ip);
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}
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#endif
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