From 5c1eabe68501d1e1b1586c7f4c46cc531828c4ab Mon Sep 17 00:00:00 2001 From: Mikulas Patocka Date: Mon, 22 Oct 2012 19:37:47 -0400 Subject: [PATCH] percpu-rw-semaphores: use light/heavy barriers This patch introduces new barrier pair light_mb() and heavy_mb() for percpu rw semaphores. This patch fixes a bug in percpu-rw-semaphores where a barrier was missing in percpu_up_write. This patch improves performance on the read path of percpu-rw-semaphores: on non-x86 cpus, there was a smp_mb() in percpu_up_read. This patch changes it to a compiler barrier and removes the "#if defined(X86) ..." condition. From: Lai Jiangshan Signed-off-by: Mikulas Patocka Signed-off-by: Linus Torvalds --- include/linux/percpu-rwsem.h | 20 +++++++------------- 1 file changed, 7 insertions(+), 13 deletions(-) diff --git a/include/linux/percpu-rwsem.h b/include/linux/percpu-rwsem.h index cf80f7e5277f..18f35b54286c 100644 --- a/include/linux/percpu-rwsem.h +++ b/include/linux/percpu-rwsem.h @@ -12,6 +12,9 @@ struct percpu_rw_semaphore { struct mutex mtx; }; +#define light_mb() barrier() +#define heavy_mb() synchronize_sched() + static inline void percpu_down_read(struct percpu_rw_semaphore *p) { rcu_read_lock(); @@ -24,22 +27,12 @@ static inline void percpu_down_read(struct percpu_rw_semaphore *p) } this_cpu_inc(*p->counters); rcu_read_unlock(); + light_mb(); /* A, between read of p->locked and read of data, paired with D */ } static inline void percpu_up_read(struct percpu_rw_semaphore *p) { - /* - * On X86, write operation in this_cpu_dec serves as a memory unlock - * barrier (i.e. memory accesses may be moved before the write, but - * no memory accesses are moved past the write). - * On other architectures this may not be the case, so we need smp_mb() - * there. - */ -#if defined(CONFIG_X86) && (!defined(CONFIG_X86_PPRO_FENCE) && !defined(CONFIG_X86_OOSTORE)) - barrier(); -#else - smp_mb(); -#endif + light_mb(); /* B, between read of the data and write to p->counter, paired with C */ this_cpu_dec(*p->counters); } @@ -61,11 +54,12 @@ static inline void percpu_down_write(struct percpu_rw_semaphore *p) synchronize_rcu(); while (__percpu_count(p->counters)) msleep(1); - smp_rmb(); /* paired with smp_mb() in percpu_sem_up_read() */ + heavy_mb(); /* C, between read of p->counter and write to data, paired with B */ } static inline void percpu_up_write(struct percpu_rw_semaphore *p) { + heavy_mb(); /* D, between write to data and write to p->locked, paired with A */ p->locked = false; mutex_unlock(&p->mtx); }