[PATCH -mm]percpu_rw_semaphore-reimplement-to-not-block-the-readers-unnecessarily.fix

[Oleg Nesterov]

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 <oleg@xxxxxxxxxx>
---
 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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