More include's and more comments, no changes in code.
To remind, once/if I am sure you agree with this patch I'll send 2 additional
and simple patches:
1. lockdep annotations
2. CONFIG_PERCPU_RWSEM
It seems that we can do much more improvements to a) speedup the writers and
b) make percpu_rw_semaphore more useful, but not right now.
Signed-off-by: Oleg Nesterov <o...@redhat.com>
---
lib/percpu-rwsem.c | 35 +++++++++++++++++++++++++++++++++--
1 files changed, 33 insertions(+), 2 deletions(-)
diff --git a/lib/percpu-rwsem.c b/lib/percpu-rwsem.c
index 0e3bc0f..02bd157 100644
--- a/lib/percpu-rwsem.c
+++ b/lib/percpu-rwsem.c
@@ -1,6 +1,11 @@
+#include <linux/mutex.h>
+#include <linux/rwsem.h>
+#include <linux/percpu.h>
+#include <linux/wait.h>
#include <linux/percpu-rwsem.h>
#include <linux/rcupdate.h>
#include <linux/sched.h>
+#include <linux/errno.h>
int percpu_init_rwsem(struct percpu_rw_semaphore *brw)
{
@@ -21,6 +26,29 @@ void percpu_free_rwsem(struct percpu_rw_semaphore *brw)
brw->fast_read_ctr = NULL; /* catch use after free bugs */
}
+/*
+ * 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
+ * fast per-cpu counter. The writer uses synchronize_sched() to serialize
+ * with the preempt-disabled section below.
+ *
+ * The nontrivial part is that we should guarantee acquire/release semantics
+ * in case when
+ *
+ * R_W: down_write() comes after up_read(), the writer should see all
+ * changes done by the reader
+ * or
+ * W_R: down_read() comes after up_write(), the reader should see all
+ * changes done by the writer
+ *
+ * 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
+ * __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.
+ */
static bool update_fast_ctr(struct percpu_rw_semaphore *brw, unsigned int val)
{
bool success = false;
@@ -98,6 +126,7 @@ void percpu_down_write(struct percpu_rw_semaphore *brw)
*
* 3. Ensures that if any reader has exited its critical section via
* fast-path, it executes a full memory barrier before we return.
+ * See R_W case in the comment above update_fast_ctr().
*/
synchronize_sched();
@@ -116,8 +145,10 @@ void percpu_up_write(struct percpu_rw_semaphore *brw)
/* allow the new readers, but only the slow-path */
up_write(&brw->rw_sem);
- /* insert the barrier before the next fast-path in down_read */
+ /*
+ * Insert the barrier before the next fast-path in down_read,
+ * see W_R case in the comment above update_fast_ctr().
+ */
synchronize_sched();
-
mutex_unlock(&brw->writer_mutex);
}
--
1.5.5.1
--
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