linux-kernel - [PATCH v2 1/5] locking/rwsem: Pass the current atomic count to rwsem_down_read

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Date:   Fri, 20 Nov 2020 23:14:12 -0500
From:   Waiman Long <longman@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>
Cc:     linux-kernel@...r.kernel.org, Davidlohr Bueso <dave@...olabs.net>,
        Phil Auld <pauld@...hat.com>, Waiman Long <longman@...hat.com>
Subject: [PATCH v2 1/5] locking/rwsem: Pass the current atomic count to rwsem_down_read_slowpath()

The atomic count value right after reader count increment can be useful
to determine the rwsem state at trylock time. So the count value is
passed down to rwsem_down_read_slowpath() to be used when appropriate.

Signed-off-by: Waiman Long <longman@...hat.com>
---
 kernel/locking/rwsem.c | 20 ++++++++++++--------
 1 file changed, 12 insertions(+), 8 deletions(-)

diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c
index f11b9bd3431d..12761e02ab9b 100644
--- a/kernel/locking/rwsem.c
+++ b/kernel/locking/rwsem.c
@@ -270,12 +270,12 @@ static inline void rwsem_set_nonspinnable(struct rw_semaphore *sem)
 					  owner | RWSEM_NONSPINNABLE));
 }
 
-static inline bool rwsem_read_trylock(struct rw_semaphore *sem)
+static inline long rwsem_read_trylock(struct rw_semaphore *sem)
 {
 	long cnt = atomic_long_add_return_acquire(RWSEM_READER_BIAS, &sem->count);
 	if (WARN_ON_ONCE(cnt < 0))
 		rwsem_set_nonspinnable(sem);
-	return !(cnt & RWSEM_READ_FAILED_MASK);
+	return cnt;
 }
 
 /*
@@ -989,9 +989,9 @@ rwsem_spin_on_owner(struct rw_semaphore *sem, unsigned long nonspinnable)
  * Wait for the read lock to be granted
  */
 static struct rw_semaphore __sched *
-rwsem_down_read_slowpath(struct rw_semaphore *sem, int state)
+rwsem_down_read_slowpath(struct rw_semaphore *sem, int state, long count)
 {
-	long count, adjustment = -RWSEM_READER_BIAS;
+	long adjustment = -RWSEM_READER_BIAS;
 	struct rwsem_waiter waiter;
 	DEFINE_WAKE_Q(wake_q);
 	bool wake = false;
@@ -1337,8 +1337,10 @@ static struct rw_semaphore *rwsem_downgrade_wake(struct rw_semaphore *sem)
  */
 static inline void __down_read(struct rw_semaphore *sem)
 {
-	if (!rwsem_read_trylock(sem)) {
-		rwsem_down_read_slowpath(sem, TASK_UNINTERRUPTIBLE);
+	long count = rwsem_read_trylock(sem);
+
+	if (count & RWSEM_READ_FAILED_MASK) {
+		rwsem_down_read_slowpath(sem, TASK_UNINTERRUPTIBLE, count);
 		DEBUG_RWSEMS_WARN_ON(!is_rwsem_reader_owned(sem), sem);
 	} else {
 		rwsem_set_reader_owned(sem);
@@ -1347,8 +1349,10 @@ static inline void __down_read(struct rw_semaphore *sem)
 
 static inline int __down_read_killable(struct rw_semaphore *sem)
 {
-	if (!rwsem_read_trylock(sem)) {
-		if (IS_ERR(rwsem_down_read_slowpath(sem, TASK_KILLABLE)))
+	long count = rwsem_read_trylock(sem);
+
+	if (count & RWSEM_READ_FAILED_MASK) {
+		if (IS_ERR(rwsem_down_read_slowpath(sem, TASK_KILLABLE, count)))
 			return -EINTR;
 		DEBUG_RWSEMS_WARN_ON(!is_rwsem_reader_owned(sem), sem);
 	} else {
-- 
2.18.1