a1fd3e24d8
Currently the writer does msleep() plus synchronize_sched() 3 times to acquire/release the semaphore, and during this time the readers are blocked completely. Even if the "write" section was not actually started or if it was already finished. With this patch down_write/up_write does synchronize_sched() twice and down_read/up_read are still possible during this time, just they use the slow path. percpu_down_write() first forces the readers to use rw_semaphore and increment the "slow" counter to take the lock for reading, then it takes that rw_semaphore for writing and blocks the readers. Also. With this patch the code relies on the documented behaviour of synchronize_sched(), it doesn't try to pair synchronize_sched() with barrier. Signed-off-by: Oleg Nesterov <oleg@redhat.com> Reviewed-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Mikulas Patocka <mpatocka@redhat.com> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Ingo Molnar <mingo@elte.hu> Cc: Srikar Dronamraju <srikar@linux.vnet.ibm.com> Cc: Ananth N Mavinakayanahalli <ananth@in.ibm.com> Cc: Anton Arapov <anton@redhat.com> Cc: Jens Axboe <axboe@kernel.dk> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
27 lines
719 B
C
27 lines
719 B
C
#ifndef _LINUX_PERCPU_RWSEM_H
|
|
#define _LINUX_PERCPU_RWSEM_H
|
|
|
|
#include <linux/mutex.h>
|
|
#include <linux/rwsem.h>
|
|
#include <linux/percpu.h>
|
|
#include <linux/wait.h>
|
|
|
|
struct percpu_rw_semaphore {
|
|
unsigned int __percpu *fast_read_ctr;
|
|
struct mutex writer_mutex;
|
|
struct rw_semaphore rw_sem;
|
|
atomic_t slow_read_ctr;
|
|
wait_queue_head_t write_waitq;
|
|
};
|
|
|
|
extern void percpu_down_read(struct percpu_rw_semaphore *);
|
|
extern void percpu_up_read(struct percpu_rw_semaphore *);
|
|
|
|
extern void percpu_down_write(struct percpu_rw_semaphore *);
|
|
extern void percpu_up_write(struct percpu_rw_semaphore *);
|
|
|
|
extern int percpu_init_rwsem(struct percpu_rw_semaphore *);
|
|
extern void percpu_free_rwsem(struct percpu_rw_semaphore *);
|
|
|
|
#endif
|