linux-kernel - [tip: locking/core] locking/rwsem: Pass the current atomic count to rwsem_down_read

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Date:   Wed, 09 Dec 2020 18:38:48 -0000
From:   "tip-bot2 for Waiman Long" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     Waiman Long <longman@...hat.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Davidlohr Bueso <dbueso@...e.de>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: [tip: locking/core] locking/rwsem: Pass the current atomic count to
 rwsem_down_read_slowpath()

The following commit has been merged into the locking/core branch of tip:

Commit-ID:     c8fe8b0564388f41147326f31e4587171aacccd4
Gitweb:        https://git.kernel.org/tip/c8fe8b0564388f41147326f31e4587171aacccd4
Author:        Waiman Long <longman@...hat.com>
AuthorDate:    Fri, 20 Nov 2020 23:14:12 -05:00
Committer:     Peter Zijlstra <peterz@...radead.org>
CommitterDate: Wed, 09 Dec 2020 17:08:47 +01:00

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>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Reviewed-by: Davidlohr Bueso <dbueso@...e.de>
Link: https://lkml.kernel.org/r/20201121041416.12285-2-longman@redhat.com
---
 kernel/locking/rwsem.c | 18 ++++++++++--------
 1 file changed, 10 insertions(+), 8 deletions(-)

diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c
index 67ae366..5768b90 100644
--- a/kernel/locking/rwsem.c
+++ b/kernel/locking/rwsem.c
@@ -270,14 +270,14 @@ 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 bool rwsem_read_trylock(struct rw_semaphore *sem, long *cntp)
 {
-	long cnt = atomic_long_add_return_acquire(RWSEM_READER_BIAS, &sem->count);
+	*cntp = atomic_long_add_return_acquire(RWSEM_READER_BIAS, &sem->count);
 
-	if (WARN_ON_ONCE(cnt < 0))
+	if (WARN_ON_ONCE(*cntp < 0))
 		rwsem_set_nonspinnable(sem);
 
-	if (!(cnt & RWSEM_READ_FAILED_MASK)) {
+	if (!(*cntp & RWSEM_READ_FAILED_MASK)) {
 		rwsem_set_reader_owned(sem);
 		return true;
 	}
@@ -1008,9 +1008,9 @@ 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, long count, int state)
 {
-	long count, adjustment = -RWSEM_READER_BIAS;
+	long adjustment = -RWSEM_READER_BIAS;
 	struct rwsem_waiter waiter;
 	DEFINE_WAKE_Q(wake_q);
 	bool wake = false;
@@ -1356,8 +1356,10 @@ static struct rw_semaphore *rwsem_downgrade_wake(struct rw_semaphore *sem)
  */
 static inline int __down_read_common(struct rw_semaphore *sem, int state)
 {
-	if (!rwsem_read_trylock(sem)) {
-		if (IS_ERR(rwsem_down_read_slowpath(sem, state)))
+	long count;
+
+	if (!rwsem_read_trylock(sem, &count)) {
+		if (IS_ERR(rwsem_down_read_slowpath(sem, count, state)))
 			return -EINTR;
 		DEBUG_RWSEMS_WARN_ON(!is_rwsem_reader_owned(sem), sem);
 	}