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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>
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parent
7721fea3d0
commit
345af7bf33
1 changed files with 36 additions and 29 deletions
65
lib/rwsem.c
65
lib/rwsem.c
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@ -41,7 +41,7 @@ struct rwsem_waiter {
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* - if we come here from up_xxxx(), then:
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* - the 'active part' of count (&0x0000ffff) reached 0 (but may have changed)
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* - the 'waiting part' of count (&0xffff0000) is -ve (and will still be so)
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* - there must be someone on the queue
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* - there must be someone on the queue
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* - the spinlock must be held by the caller
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* - woken process blocks are discarded from the list after having task zeroed
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* - writers are only woken if downgrading is false
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@ -54,26 +54,23 @@ __rwsem_do_wake(struct rw_semaphore *sem, int downgrading)
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struct list_head *next;
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signed long oldcount, woken, loop;
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if (downgrading)
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goto dont_wake_writers;
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waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
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if (!(waiter->flags & RWSEM_WAITING_FOR_WRITE))
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goto readers_only;
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/* if we came through an up_xxxx() call, we only only wake someone up
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* if we can transition the active part of the count from 0 -> 1
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if (downgrading)
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goto out;
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/* There's a writer at the front of the queue - try to grant it the
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* write lock. However, we only wake this writer if we can transition
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* the active part of the count from 0 -> 1
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*/
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try_again:
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try_again_write:
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oldcount = rwsem_atomic_update(RWSEM_ACTIVE_BIAS, sem)
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- RWSEM_ACTIVE_BIAS;
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if (oldcount & RWSEM_ACTIVE_MASK)
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goto undo;
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waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
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/* try to grant a single write lock if there's a writer at the front
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* of the queue - note we leave the 'active part' of the count
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* incremented by 1 and the waiting part incremented by 0x00010000
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*/
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if (!(waiter->flags & RWSEM_WAITING_FOR_WRITE))
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goto readers_only;
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/* Someone grabbed the sem already */
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goto undo_write;
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/* We must be careful not to touch 'waiter' after we set ->task = NULL.
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* It is an allocated on the waiter's stack and may become invalid at
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@ -87,18 +84,24 @@ __rwsem_do_wake(struct rw_semaphore *sem, int downgrading)
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put_task_struct(tsk);
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goto out;
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/* don't want to wake any writers */
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dont_wake_writers:
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waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
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if (waiter->flags & RWSEM_WAITING_FOR_WRITE)
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goto out;
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/* grant an infinite number of read locks to the readers at the front
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* of the queue
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* - note we increment the 'active part' of the count by the number of
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* readers before waking any processes up
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*/
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readers_only:
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if (downgrading)
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goto wake_readers;
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/* if we came through an up_xxxx() call, we only only wake someone up
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* if we can transition the active part of the count from 0 -> 1 */
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try_again_read:
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oldcount = rwsem_atomic_update(RWSEM_ACTIVE_BIAS, sem)
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- RWSEM_ACTIVE_BIAS;
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if (oldcount & RWSEM_ACTIVE_MASK)
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/* Someone grabbed the sem already */
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goto undo_read;
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wake_readers:
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/* Grant an infinite number of read locks to the readers at the front
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* of the queue. Note we increment the 'active part' of the count by
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* the number of readers before waking any processes up.
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*/
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woken = 0;
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do {
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woken++;
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@ -138,10 +141,14 @@ __rwsem_do_wake(struct rw_semaphore *sem, int downgrading)
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/* undo the change to the active count, but check for a transition
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* 1->0 */
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undo:
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undo_write:
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if (rwsem_atomic_update(-RWSEM_ACTIVE_BIAS, sem) & RWSEM_ACTIVE_MASK)
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goto out;
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goto try_again;
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goto try_again_write;
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undo_read:
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if (rwsem_atomic_update(-RWSEM_ACTIVE_BIAS, sem) & RWSEM_ACTIVE_MASK)
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goto out;
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goto try_again_read;
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}
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/*
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