rwsem: fully separate code paths to wake writers vs readers

This is in preparation for later changes in the series.

In __rwsem_do_wake(), the first queued waiter is checked first in order to
determine whether it's a writer or a reader.  The code paths diverge at
this point.  The code that checks and increments the rwsem active count is
duplicated on both sides - the point is that later changes in the series
will be able to independently modify both sides.

Signed-off-by: Michel Lespinasse <walken@google.com>
Acked-by: David Howells <dhowells@redhat.com>
Cc: Mike Waychison <mikew@google.com>
Cc: Suleiman Souhlal <suleiman@google.com>
Cc: Ying Han <yinghan@google.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

lib/rwsem.c

@@ -41,7 +41,7 @@ struct rwsem_waiter {
  * - if we come here from up_xxxx(), then:
  *   - the 'active part' of count (&0x0000ffff) reached 0 (but may have changed)
  *   - the 'waiting part' of count (&0xffff0000) is -ve (and will still be so)
- *   - there must be someone on the queue
+ * - there must be someone on the queue
  * - the spinlock must be held by the caller
  * - woken process blocks are discarded from the list after having task zeroed
  * - writers are only woken if downgrading is false
@@ -54,26 +54,23 @@ __rwsem_do_wake(struct rw_semaphore *sem, int downgrading)
 	struct list_head *next;
 	signed long oldcount, woken, loop;
 
-	if (downgrading)
-		goto dont_wake_writers;
+	waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
+	if (!(waiter->flags & RWSEM_WAITING_FOR_WRITE))
+		goto readers_only;
 
-	/* if we came through an up_xxxx() call, we only only wake someone up
-	 * if we can transition the active part of the count from 0 -> 1
+	if (downgrading)
+		goto out;
+
+	/* There's a writer at the front of the queue - try to grant it the
+	 * write lock.  However, we only wake this writer if we can transition
+	 * the active part of the count from 0 -> 1
 	 */
- try_again:
+ try_again_write:
 	oldcount = rwsem_atomic_update(RWSEM_ACTIVE_BIAS, sem)
 						- RWSEM_ACTIVE_BIAS;
 	if (oldcount & RWSEM_ACTIVE_MASK)
-		goto undo;
-
-	waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
-
-	/* try to grant a single write lock if there's a writer at the front
-	 * of the queue - note we leave the 'active part' of the count
-	 * incremented by 1 and the waiting part incremented by 0x00010000
-	 */
-	if (!(waiter->flags & RWSEM_WAITING_FOR_WRITE))
-		goto readers_only;
+		/* Someone grabbed the sem already */
+		goto undo_write;
 
 	/* We must be careful not to touch 'waiter' after we set ->task = NULL.
 	 * It is an allocated on the waiter's stack and may become invalid at
@@ -87,18 +84,24 @@ __rwsem_do_wake(struct rw_semaphore *sem, int downgrading)
 	put_task_struct(tsk);
 	goto out;
 
-	/* don't want to wake any writers */
- dont_wake_writers:
-	waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
-	if (waiter->flags & RWSEM_WAITING_FOR_WRITE)
-		goto out;
-
-	/* grant an infinite number of read locks to the readers at the front
-	 * of the queue
-	 * - note we increment the 'active part' of the count by the number of
-	 *   readers before waking any processes up
-	 */
  readers_only:
+	if (downgrading)
+		goto wake_readers;
+
+	/* if we came through an up_xxxx() call, we only only wake someone up
+	 * if we can transition the active part of the count from 0 -> 1 */
+ try_again_read:
+	oldcount = rwsem_atomic_update(RWSEM_ACTIVE_BIAS, sem)
+						- RWSEM_ACTIVE_BIAS;
+	if (oldcount & RWSEM_ACTIVE_MASK)
+		/* Someone grabbed the sem already */
+		goto undo_read;
+
+ wake_readers:
+	/* Grant an infinite number of read locks to the readers at the front
+	 * of the queue.  Note we increment the 'active part' of the count by
+	 * the number of readers before waking any processes up.
+	 */
 	woken = 0;
 	do {
 		woken++;
@@ -138,10 +141,14 @@ __rwsem_do_wake(struct rw_semaphore *sem, int downgrading)
 
 	/* undo the change to the active count, but check for a transition
 	 * 1->0 */
- undo:
+ undo_write:
 	if (rwsem_atomic_update(-RWSEM_ACTIVE_BIAS, sem) & RWSEM_ACTIVE_MASK)
 		goto out;
-	goto try_again;
+	goto try_again_write;
+ undo_read:
+	if (rwsem_atomic_update(-RWSEM_ACTIVE_BIAS, sem) & RWSEM_ACTIVE_MASK)
+		goto out;
+	goto try_again_read;
 }
 
 /*