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Subject[PATCH 02/12] rwsem: use single atomic update for sem count when waking up readers.
Date
When waking up queued readers, __rwsem_do_wake was using a pair of atomic
operations (one before looking at the first queued thread and one after
having looped through all readers). Restructure code to use a single
atomic operation instead.

Signed-off-by: Michel Lespinasse <walken@google.com>
---
lib/rwsem.c | 52 ++++++++++++++++++++++++++--------------------------
1 files changed, 26 insertions(+), 26 deletions(-)
diff --git a/lib/rwsem.c b/lib/rwsem.c
index 8d6a13e..caee9b7 100644
--- a/lib/rwsem.c
+++ b/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 +54,26 @@ __rwsem_do_wake(struct rw_semaphore *sem, int downgrading)
struct list_head *next;
signed long oldcount, woken, loop;

+ waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
+ if (!(waiter->flags & RWSEM_WAITING_FOR_WRITE))
+ goto readers_only;
+
if (downgrading)
- goto dont_wake_writers;
+ /* Caller's lock is still active, so we can't possibly
+ * succeed waking writers.
+ */
+ goto out;

- /* if we came through an up_xxxx() call, we only only wake someone up
+ /* There's a writer at the front of the queue - try to grant it the
+ * write lock. However, we only only wake someone up
* if we can transition the active part of the count from 0 -> 1
*/
- try_again:
+ retry_writer:
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_writer;

/* 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
@@ -87,12 +87,6 @@ __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
@@ -113,11 +107,12 @@ __rwsem_do_wake(struct rw_semaphore *sem, int downgrading)

loop = woken;
woken *= RWSEM_ACTIVE_BIAS - RWSEM_WAITING_BIAS;
- if (!downgrading)
- /* we'd already done one increment earlier */
- woken -= RWSEM_ACTIVE_BIAS;

- rwsem_atomic_add(woken, sem);
+ retry_readers:
+ oldcount = rwsem_atomic_update(woken, sem) - woken;
+ if (!downgrading && (oldcount & RWSEM_ACTIVE_MASK))
+ /* Someone grabbed the sem already */
+ goto undo_readers;

next = sem->wait_list.next;
for (; loop > 0; loop--) {
@@ -137,10 +132,15 @@ __rwsem_do_wake(struct rw_semaphore *sem, int downgrading)
return sem;

/* undo the change to count, but check for a transition 1->0 */
- undo:
+ undo_writer:
if (rwsem_atomic_update(-RWSEM_ACTIVE_BIAS, sem) & RWSEM_ACTIVE_MASK)
goto out;
- goto try_again;
+ goto retry_writer;
+
+ undo_readers:
+ if (rwsem_atomic_update(-woken, sem) & RWSEM_ACTIVE_MASK)
+ goto out;
+ goto retry_readers;
}

/*
--
1.7.0.1


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