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-rw-r--r--lib/percpu-rwsem.c34
1 files changed, 16 insertions, 18 deletions
diff --git a/lib/percpu-rwsem.c b/lib/percpu-rwsem.c
index 2e03bcf..ce92ab5 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);
}
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