On 09/19/2013 11:48 AM, Josef Bacik wrote:
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 <jba...@fusionio.com>
---
V1->V2: took everybodys suggestions and simplified it to just one function in
rwsem.h so it works for both the spinlock case and non-spinlock case.
include/linux/rwsem.h | 13 +++++++++++++
1 file changed, 13 insertions(+)
diff --git a/include/linux/rwsem.h b/include/linux/rwsem.h
index 0616ffe..c340493 100644
--- a/include/linux/rwsem.h
+++ b/include/linux/rwsem.h
@@ -75,6 +75,19 @@ do {
\
} 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 the opposite type is wanting access to the
^^^^^^^^^^^^^
Readers can infer that at least one writer is waiting if the wait_list is
!empty; however, writers cannot infer anything other than some other
thread is waiting -- it could be a reader or a writer or multiples of either.
+ * lock.
+ */
+static inline int rwsem_is_contended(struct rw_semaphore *sem)
+{
+ if (!list_empty(&sem->wait_list))
+ return 1;
+ return 0;
How about
return !list_empty(&sem->wait_list);
?
+}
+
+/*
* lock for reading
*/
extern void down_read(struct rw_semaphore *sem);
--
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