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From: Michel Lespinasse on 14 May 2010 08:50
This is in preparation to 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 incremetns 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(a)google.com>
---
lib/rwsem.c | 54 +++++++++++++++++++++++++++++++-----------------------
1 files changed, 31 insertions(+), 23 deletions(-)

diff --git a/lib/rwsem.c b/lib/rwsem.c
index 8d6a13e..e3e133f 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
@@ -54,27 +54,24 @@ __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;
+ goto out;

- /* 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
+ /* 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:
oldcount = rwsem_atomic_update(RWSEM_ACTIVE_BIAS, sem)
- RWSEM_ACTIVE_BIAS;
if (oldcount & RWSEM_ACTIVE_MASK)
+ /* Someone grabbed the sem already */
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;
-
/* 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
* any time after that point (due to a wakeup from another source).
@@ -87,18 +84,25 @@ __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;
+ readers_only:
+ if (!downgrading) {
+
+ /* 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;
+ }

- /* 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
+ /* 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:
woken = 0;
do {
woken++;
@@ -141,6 +145,10 @@ __rwsem_do_wake(struct rw_semaphore *sem, int downgrading)
if (rwsem_atomic_update(-RWSEM_ACTIVE_BIAS, sem) & RWSEM_ACTIVE_MASK)
goto out;
goto try_again;
+ undo_read:
+ if (rwsem_atomic_update(-RWSEM_ACTIVE_BIAS, sem) & RWSEM_ACTIVE_MASK)
+ goto out;
+ goto try_again_read;
}

/*
--
1.7.0.1

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Prev: [PATCH 10/10] Use down_read_unfair() for /sys/<pid>/exe and /sys/<pid>/maps files
Next: [PATCH 01/10] x86 rwsem: minor cleanups