[PATCH 1/5] locking: Add rwsem_is_write_locked()

["Matthew Wilcox (Oracle)" <willy@infradead.org>]

Several places want to know whether the lock is held by a writer, instead
of just whether it's held.  We can implement this for both normal and
rt rwsems.  RWSEM_WRITER_LOCKED is declared in rwsem.c and exposing
it outside that file might tempt other people to use it, so just use
a comment to note that's what the 1 means, and help anybody find it if
they're looking to change the implementation.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
---
 include/linux/rwbase_rt.h |  5 +++++
 include/linux/rwsem.h     | 10 ++++++++++
 2 files changed, 15 insertions(+)

diff --git a/include/linux/rwbase_rt.h b/include/linux/rwbase_rt.h
index 1d264dd08625..3c25b14edc05 100644
--- a/include/linux/rwbase_rt.h
+++ b/include/linux/rwbase_rt.h
@@ -31,6 +31,11 @@ static __always_inline bool rw_base_is_locked(struct rwbase_rt *rwb)
 	return atomic_read(&rwb->readers) != READER_BIAS;
 }
 
+static __always_inline bool rw_base_is_write_locked(struct rwbase_rt *rwb)
+{
+	return atomic_read(&rwb->readers) == WRITER_BIAS;
+}
+
 static __always_inline bool rw_base_is_contended(struct rwbase_rt *rwb)
 {
 	return atomic_read(&rwb->readers) > 0;
diff --git a/include/linux/rwsem.h b/include/linux/rwsem.h
index 1dd530ce8b45..0f78b8d2e653 100644
--- a/include/linux/rwsem.h
+++ b/include/linux/rwsem.h
@@ -72,6 +72,11 @@ static inline int rwsem_is_locked(struct rw_semaphore *sem)
 	return atomic_long_read(&sem->count) != 0;
 }
 
+static inline int rwsem_is_write_locked(struct rw_semaphore *sem)
+{
+	return atomic_long_read(&sem->count) & 1 /* RWSEM_WRITER_LOCKED */;
+}
+
 #define RWSEM_UNLOCKED_VALUE		0L
 #define __RWSEM_COUNT_INIT(name)	.count = ATOMIC_LONG_INIT(RWSEM_UNLOCKED_VALUE)
 
@@ -157,6 +162,11 @@ static __always_inline int rwsem_is_locked(struct rw_semaphore *sem)
 	return rw_base_is_locked(&sem->rwbase);
 }
 
+static __always_inline int rwsem_is_write_locked(struct rw_semaphore *sem)
+{
+	return rw_base_is_write_locked(&sem->rwbase);
+}
+
 static __always_inline int rwsem_is_contended(struct rw_semaphore *sem)
 {
 	return rw_base_is_contended(&sem->rwbase);
-- 
2.40.1