linux-kernel - [PATCH] Locking: Let PREEMPT_RT compile again with new rwsem asserts.

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Date: Tue, 19 Mar 2024 08:05:50 +0100
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: linux-kernel@...r.kernel.org
Cc: "Matthew Wilcox (Oracle)" <willy@...radead.org>,
	Boqun Feng <boqun.feng@...il.com>, Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Waiman Long <longman@...hat.com>, Will Deacon <will@...nel.org>,
	Clark Williams <williams@...hat.com>
Subject: [PATCH] Locking: Let PREEMPT_RT compile again with new rwsem asserts.

The commit cited below broke the build for PREEMPT_RT because
rwsem_assert_held_write_nolockdep() passes a struct rw_semaphore but
rw_base_assert_held_write() expects struct rwbase_rt. Fixing the type
alone leads to the problem that WARN_ON() is not found because bug.h is
missing.

In order to resolve this:
- Keep the assert (WARN_ON()) in rwsem.h (not rwbase_rt.h)
- Add the rwsem_held_write() which returns true if the lock is already
  write-locked and no reader are around.
- Make rw_base_is_write_locked() do the implementation specific
  (rw_base) writer check.
- Replace the "inline" with __always_inline which was used before.

Fixes: f70405afc99b1 ("locking: Add rwsem_assert_held() and rwsem_assert_held_write()")
Reported-by: Clark Williams <williams@...hat.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
---
 include/linux/rwbase_rt.h |  4 ++--
 include/linux/rwsem.h     | 11 ++++++++---
 2 files changed, 10 insertions(+), 5 deletions(-)

diff --git a/include/linux/rwbase_rt.h b/include/linux/rwbase_rt.h
index 29c4e4f243e47..f2394a409c9d5 100644
--- a/include/linux/rwbase_rt.h
+++ b/include/linux/rwbase_rt.h
@@ -31,9 +31,9 @@ static __always_inline bool rw_base_is_locked(const struct rwbase_rt *rwb)
 	return atomic_read(&rwb->readers) != READER_BIAS;
 }
 
-static inline void rw_base_assert_held_write(const struct rwbase_rt *rwb)
+static __always_inline bool rw_base_is_write_locked(const struct rwbase_rt *rwb)
 {
-	WARN_ON(atomic_read(&rwb->readers) != WRITER_BIAS);
+	return atomic_read(&rwb->readers) == WRITER_BIAS;
 }
 
 static __always_inline bool rw_base_is_contended(const struct rwbase_rt *rwb)
diff --git a/include/linux/rwsem.h b/include/linux/rwsem.h
index 4f1c18992f768..c81630d81018d 100644
--- a/include/linux/rwsem.h
+++ b/include/linux/rwsem.h
@@ -167,14 +167,19 @@ static __always_inline int rwsem_is_locked(const struct rw_semaphore *sem)
 	return rw_base_is_locked(&sem->rwbase);
 }
 
-static inline void rwsem_assert_held_nolockdep(const struct rw_semaphore *sem)
+static __always_inline void rwsem_assert_held_nolockdep(const struct rw_semaphore *sem)
 {
 	WARN_ON(!rwsem_is_locked(sem));
 }
 
-static inline void rwsem_assert_held_write_nolockdep(const struct rw_semaphore *sem)
+static __always_inline bool rwsem_held_write(const struct rw_semaphore *sem)
 {
-	rw_base_assert_held_write(sem);
+	return rw_base_is_write_locked(&sem->rwbase);
+}
+
+static __always_inline void rwsem_assert_held_write_nolockdep(const struct rw_semaphore *sem)
+{
+	WARN_ON(!rwsem_held_write(sem));
 }
 
 static __always_inline int rwsem_is_contended(struct rw_semaphore *sem)
-- 
2.43.0