diff options
| -rw-r--r-- | include/linux/percpu-rwsem.h | 4 | ||||
| -rw-r--r-- | lib/percpu-rwsem.c | 34 |
2 files changed, 18 insertions, 20 deletions
diff --git a/include/linux/percpu-rwsem.h b/include/linux/percpu-rwsem.h index 592f0d610d8..d2146a4f833 100644 --- a/include/linux/percpu-rwsem.h +++ b/include/linux/percpu-rwsem.h @@ -1,14 +1,14 @@ #ifndef _LINUX_PERCPU_RWSEM_H #define _LINUX_PERCPU_RWSEM_H -#include <linux/mutex.h> +#include <linux/atomic.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; + atomic_t write_ctr; struct rw_semaphore rw_sem; atomic_t slow_read_ctr; wait_queue_head_t write_waitq; diff --git a/lib/percpu-rwsem.c b/lib/percpu-rwsem.c index 2e03bcfe48f..ce92ab563a0 100644 --- a/lib/percpu-rwsem.c +++ b/lib/percpu-rwsem.c @@ -1,4 +1,4 @@ -#include <linux/mutex.h> +#include <linux/atomic.h> #include <linux/rwsem.h> #include <linux/percpu.h> #include <linux/wait.h> @@ -13,8 +13,8 @@ int percpu_init_rwsem(struct percpu_rw_semaphore *brw) if (unlikely(!brw->fast_read_ctr)) return -ENOMEM; - mutex_init(&brw->writer_mutex); init_rwsem(&brw->rw_sem); + atomic_set(&brw->write_ctr, 0); atomic_set(&brw->slow_read_ctr, 0); init_waitqueue_head(&brw->write_waitq); return 0; @@ -28,7 +28,7 @@ void percpu_free_rwsem(struct percpu_rw_semaphore *brw) /* * This is the fast-path for down_read/up_read, it only needs to ensure - * there is no pending writer (!mutex_is_locked() check) and inc/dec the + * there is no pending writer (atomic_read(write_ctr) == 0) and inc/dec the * fast per-cpu counter. The writer uses synchronize_sched_expedited() to * serialize with the preempt-disabled section below. * @@ -44,7 +44,7 @@ void percpu_free_rwsem(struct percpu_rw_semaphore *brw) * If this helper fails the callers rely on the normal rw_semaphore and * atomic_dec_and_test(), so in this case we have the necessary barriers. * - * But if it succeeds we do not have any barriers, mutex_is_locked() or + * But if it succeeds we do not have any barriers, atomic_read(write_ctr) or * __this_cpu_add() below can be reordered with any LOAD/STORE done by the * reader inside the critical section. See the comments in down_write and * up_write below. @@ -54,7 +54,7 @@ static bool update_fast_ctr(struct percpu_rw_semaphore *brw, unsigned int val) bool success = false; preempt_disable(); - if (likely(!mutex_is_locked(&brw->writer_mutex))) { + if (likely(!atomic_read(&brw->write_ctr))) { __this_cpu_add(*brw->fast_read_ctr, val); success = true; } @@ -101,9 +101,8 @@ static int clear_fast_ctr(struct percpu_rw_semaphore *brw) } /* - * A writer takes ->writer_mutex to exclude other writers and to force the - * readers to switch to the slow mode, note the mutex_is_locked() check in - * update_fast_ctr(). + * A writer increments ->write_ctr to force the readers to switch to the + * slow mode, note the atomic_read() check in update_fast_ctr(). * * After that the readers can only inc/dec the slow ->slow_read_ctr counter, * ->fast_read_ctr is stable. Once the writer moves its sum into the slow @@ -114,11 +113,10 @@ static int clear_fast_ctr(struct percpu_rw_semaphore *brw) */ void percpu_down_write(struct percpu_rw_semaphore *brw) { - /* also blocks update_fast_ctr() which checks mutex_is_locked() */ - mutex_lock(&brw->writer_mutex); - + /* tell update_fast_ctr() there is a pending writer */ + atomic_inc(&brw->write_ctr); /* - * 1. Ensures mutex_is_locked() is visible to any down_read/up_read + * 1. Ensures that write_ctr != 0 is visible to any down_read/up_read * so that update_fast_ctr() can't succeed. * * 2. Ensures we see the result of every previous this_cpu_add() in @@ -130,25 +128,25 @@ void percpu_down_write(struct percpu_rw_semaphore *brw) */ synchronize_sched_expedited(); + /* exclude other writers, and block the new readers completely */ + down_write(&brw->rw_sem); + /* nobody can use fast_read_ctr, move its sum into slow_read_ctr */ atomic_add(clear_fast_ctr(brw), &brw->slow_read_ctr); - /* block the new readers completely */ - down_write(&brw->rw_sem); - /* wait for all readers to complete their percpu_up_read() */ wait_event(brw->write_waitq, !atomic_read(&brw->slow_read_ctr)); } void percpu_up_write(struct percpu_rw_semaphore *brw) { - /* allow the new readers, but only the slow-path */ + /* release the lock, but the readers can't use the fast-path */ up_write(&brw->rw_sem); - /* * Insert the barrier before the next fast-path in down_read, * see W_R case in the comment above update_fast_ctr(). */ synchronize_sched_expedited(); - mutex_unlock(&brw->writer_mutex); + /* the last writer unblocks update_fast_ctr() */ + atomic_dec(&brw->write_ctr); } |