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rwsem: fully separate code paths to wake writers vs readers

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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>
This commit is contained in:
Michel Lespinasse 2010-08-09 17:21:15 -07:00 committed by Linus Torvalds
parent 7721fea3d0
commit 345af7bf33
1 changed files with 36 additions and 29 deletions
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65
lib/rwsem.c
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@ -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;
}
/*