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rwsem: add rwsem_is_contended

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Btrfs needs a simple way to know if it needs to let go of it's read lock on a
rwsem.  Introduce rwsem_is_contended to check to see if there are any waiters on
this rwsem currently.  This is just a hueristic, it is meant to be light and not
100% accurate and called by somebody already holding on to the rwsem in either
read or write.  Thanks,

Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <clm@fb.com>
Acked-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Josef Bacik 2013-08-30 10:05:22 -04:00 • committed by Chris Mason
commit 4a444b1f06
1 changed files with 11 additions and 0 deletions
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11
include/linux/rwsem.h
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@ -74,6 +74,17 @@ do { \
__init_rwsem((sem), #sem, &__key); \ __init_rwsem((sem), #sem, &__key); \
} while (0) } while (0)
/*
* This is the same regardless of which rwsem implementation that is being used.
* It is just a heuristic meant to be called by somebody alreadying holding the
* rwsem to see if somebody from an incompatible type is wanting access to the
* lock.
*/
static inline int rwsem_is_contended(struct rw_semaphore *sem)
{
return !list_empty(&sem->wait_list);
}
/* /*
* lock for reading * lock for reading
*/ */