linux-kernel - [PATCH 1/5] locking: Add rwsem_is_write

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Message-Id: <20230907174705.2976191-2-willy@infradead.org>
Date:   Thu,  7 Sep 2023 18:47:01 +0100
From:   "Matthew Wilcox (Oracle)" <willy@...radead.org>
To:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Waiman Long <longman@...hat.com>
Cc:     "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Chandan Babu R <chandan.babu@...cle.com>,
        "Darrick J . Wong" <djwong@...nel.org>, linux-xfs@...r.kernel.org
Subject: [PATCH 1/5] locking: Add rwsem_is_write_locked()

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@...radead.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