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From: Waiman Long <longman@redhat.com>
To: Peter Zijlstra <peterz@infradead.org>, Ingo Molnar <mingo@redhat.com>
Cc: linux-kernel@vger.kernel.org, x86@kernel.org,
	linux-alpha@vger.kernel.org, linux-ia64@vger.kernel.org,
	linux-s390@vger.kernel.org, linux-arch@vger.kernel.org,
	Davidlohr Bueso <dave@stgolabs.net>,
	Dave Chinner <david@fromorbit.com>,
	Waiman Long <longman@redhat.com>
Subject: [PATCH v6 03/11] locking/rwsem: Move owner setting code from rwsem.c to rwsem-xadd.h
Date: Wed, 11 Oct 2017 14:01:54 -0400	[thread overview]
Message-ID: <1507744922-15196-4-git-send-email-longman@redhat.com> (raw)
In-Reply-To: <1507744922-15196-1-git-send-email-longman@redhat.com>

The setting of owner field is specific to rwsem-xadd, it is not needed
for rwsem-spinlock. This patch moves all the owner setting code to
the fast paths directly within rwsem-add.h file.

The rwsem_set_reader_owned() is now only called by the first reader.
So there is no need to do a read to check if the owner is properly
set first.

Signed-off-by: Waiman Long <longman@redhat.com>
---
 kernel/locking/rwsem-xadd.c |  7 +++----
 kernel/locking/rwsem-xadd.h | 19 ++++++++++++++++---
 kernel/locking/rwsem.c      | 17 ++---------------
 kernel/locking/rwsem.h      | 11 ++++-------
 4 files changed, 25 insertions(+), 29 deletions(-)

diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
index 39dc5be..30bc163 100644
--- a/kernel/locking/rwsem-xadd.c
+++ b/kernel/locking/rwsem-xadd.c
@@ -132,11 +132,10 @@ static void __rwsem_mark_wake(struct rw_semaphore *sem,
 			return;
 		}
 		/*
-		 * It is not really necessary to set it to reader-owned here,
-		 * but it gives the spinners an early indication that the
-		 * readers now have the lock.
+		 * Set it to reader-owned for first reader.
 		 */
-		rwsem_set_reader_owned(sem);
+		if (!(oldcount >> RWSEM_READER_SHIFT))
+			rwsem_set_reader_owned(sem);
 	}
 
 	/*
diff --git a/kernel/locking/rwsem-xadd.h b/kernel/locking/rwsem-xadd.h
index 3820719..abcb484 100644
--- a/kernel/locking/rwsem-xadd.h
+++ b/kernel/locking/rwsem-xadd.h
@@ -29,9 +29,12 @@
  */
 static inline void __down_read(struct rw_semaphore *sem)
 {
-	if (unlikely(atomic_fetch_add_acquire(RWSEM_READER_BIAS, &sem->count)
-		     & RWSEM_READ_FAILED_MASK))
+	int count = atomic_fetch_add_acquire(RWSEM_READER_BIAS, &sem->count);
+
+	if (unlikely(count & RWSEM_READ_FAILED_MASK))
 		rwsem_down_read_failed(sem);
+	else if ((count >> RWSEM_READER_SHIFT) == 1)
+		rwsem_set_reader_owned(sem);
 }
 
 static inline int __down_read_trylock(struct rw_semaphore *sem)
@@ -41,6 +44,8 @@ static inline int __down_read_trylock(struct rw_semaphore *sem)
 	while (!((tmp = atomic_read(&sem->count)) & RWSEM_READ_FAILED_MASK)) {
 		if (tmp == atomic_cmpxchg_acquire(&sem->count, tmp,
 				   tmp + RWSEM_READER_BIAS)) {
+			if (!(tmp >> RWSEM_READER_SHIFT))
+				rwsem_set_reader_owned(sem);
 			return 1;
 		}
 	}
@@ -55,6 +60,7 @@ static inline void __down_write(struct rw_semaphore *sem)
 	if (unlikely(atomic_cmpxchg_acquire(&sem->count, 0,
 					    RWSEM_WRITER_LOCKED)))
 		rwsem_down_write_failed(sem);
+	rwsem_set_owner(sem);
 }
 
 static inline int __down_write_killable(struct rw_semaphore *sem)
@@ -63,12 +69,17 @@ static inline int __down_write_killable(struct rw_semaphore *sem)
 					    RWSEM_WRITER_LOCKED)))
 		if (IS_ERR(rwsem_down_write_failed_killable(sem)))
 			return -EINTR;
+	rwsem_set_owner(sem);
 	return 0;
 }
 
 static inline int __down_write_trylock(struct rw_semaphore *sem)
 {
-	return !atomic_cmpxchg_acquire(&sem->count, 0, RWSEM_WRITER_LOCKED);
+	bool taken = !atomic_cmpxchg_acquire(&sem->count, 0,
+					     RWSEM_WRITER_LOCKED);
+	if (taken)
+		rwsem_set_owner(sem);
+	return taken;
 }
 
 /*
@@ -89,6 +100,7 @@ static inline void __up_read(struct rw_semaphore *sem)
  */
 static inline void __up_write(struct rw_semaphore *sem)
 {
+	rwsem_clear_owner(sem);
 	if (unlikely(atomic_fetch_add_release(-RWSEM_WRITER_LOCKED,
 			&sem->count) & RWSEM_FLAG_WAITERS))
 		rwsem_wake(sem);
@@ -110,6 +122,7 @@ static inline void __downgrade_write(struct rw_semaphore *sem)
 	 */
 	tmp = atomic_fetch_add_release(-RWSEM_WRITER_LOCKED+RWSEM_READER_BIAS,
 					&sem->count);
+	rwsem_set_reader_owned(sem);
 	if (tmp & RWSEM_FLAG_WAITERS)
 		rwsem_downgrade_wake(sem);
 }
diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c
index 4d48b1c..0a32725 100644
--- a/kernel/locking/rwsem.c
+++ b/kernel/locking/rwsem.c
@@ -23,7 +23,6 @@ void __sched down_read(struct rw_semaphore *sem)
 	rwsem_acquire_read(&sem->dep_map, 0, 0, _RET_IP_);
 
 	LOCK_CONTENDED(sem, __down_read_trylock, __down_read);
-	rwsem_set_reader_owned(sem);
 }
 
 EXPORT_SYMBOL(down_read);
@@ -35,10 +34,8 @@ int down_read_trylock(struct rw_semaphore *sem)
 {
 	int ret = __down_read_trylock(sem);
 
-	if (ret == 1) {
+	if (ret == 1)
 		rwsem_acquire_read(&sem->dep_map, 0, 1, _RET_IP_);
-		rwsem_set_reader_owned(sem);
-	}
 	return ret;
 }
 
@@ -53,7 +50,6 @@ void __sched down_write(struct rw_semaphore *sem)
 	rwsem_acquire(&sem->dep_map, 0, 0, _RET_IP_);
 
 	LOCK_CONTENDED(sem, __down_write_trylock, __down_write);
-	rwsem_set_owner(sem);
 }
 
 EXPORT_SYMBOL(down_write);
@@ -71,7 +67,6 @@ int __sched down_write_killable(struct rw_semaphore *sem)
 		return -EINTR;
 	}
 
-	rwsem_set_owner(sem);
 	return 0;
 }
 
@@ -84,10 +79,8 @@ int down_write_trylock(struct rw_semaphore *sem)
 {
 	int ret = __down_write_trylock(sem);
 
-	if (ret == 1) {
+	if (ret == 1)
 		rwsem_acquire(&sem->dep_map, 0, 1, _RET_IP_);
-		rwsem_set_owner(sem);
-	}
 
 	return ret;
 }
@@ -113,7 +106,6 @@ void up_write(struct rw_semaphore *sem)
 {
 	rwsem_release(&sem->dep_map, 1, _RET_IP_);
 
-	rwsem_clear_owner(sem);
 	__up_write(sem);
 }
 
@@ -126,7 +118,6 @@ void downgrade_write(struct rw_semaphore *sem)
 {
 	lock_downgrade(&sem->dep_map, _RET_IP_);
 
-	rwsem_set_reader_owned(sem);
 	__downgrade_write(sem);
 }
 
@@ -140,7 +131,6 @@ void down_read_nested(struct rw_semaphore *sem, int subclass)
 	rwsem_acquire_read(&sem->dep_map, subclass, 0, _RET_IP_);
 
 	LOCK_CONTENDED(sem, __down_read_trylock, __down_read);
-	rwsem_set_reader_owned(sem);
 }
 
 EXPORT_SYMBOL(down_read_nested);
@@ -151,7 +141,6 @@ void _down_write_nest_lock(struct rw_semaphore *sem, struct lockdep_map *nest)
 	rwsem_acquire_nest(&sem->dep_map, 0, 0, nest, _RET_IP_);
 
 	LOCK_CONTENDED(sem, __down_write_trylock, __down_write);
-	rwsem_set_owner(sem);
 }
 
 EXPORT_SYMBOL(_down_write_nest_lock);
@@ -171,7 +160,6 @@ void down_write_nested(struct rw_semaphore *sem, int subclass)
 	rwsem_acquire(&sem->dep_map, subclass, 0, _RET_IP_);
 
 	LOCK_CONTENDED(sem, __down_write_trylock, __down_write);
-	rwsem_set_owner(sem);
 }
 
 EXPORT_SYMBOL(down_write_nested);
@@ -186,7 +174,6 @@ int __sched down_write_killable_nested(struct rw_semaphore *sem, int subclass)
 		return -EINTR;
 	}
 
-	rwsem_set_owner(sem);
 	return 0;
 }
 
diff --git a/kernel/locking/rwsem.h b/kernel/locking/rwsem.h
index adcc5af..612109f 100644
--- a/kernel/locking/rwsem.h
+++ b/kernel/locking/rwsem.h
@@ -33,15 +33,12 @@ static inline void rwsem_clear_owner(struct rw_semaphore *sem)
 	WRITE_ONCE(sem->owner, NULL);
 }
 
+/*
+ * This should only be called by the first reader that acquires the read lock.
+ */
 static inline void rwsem_set_reader_owned(struct rw_semaphore *sem)
 {
-	/*
-	 * We check the owner value first to make sure that we will only
-	 * do a write to the rwsem cacheline when it is really necessary
-	 * to minimize cacheline contention.
-	 */
-	if (sem->owner != RWSEM_READER_OWNED)
-		WRITE_ONCE(sem->owner, RWSEM_READER_OWNED);
+	WRITE_ONCE(sem->owner, RWSEM_READER_OWNED);
 }
 
 static inline bool rwsem_owner_is_writer(struct task_struct *owner)
-- 
1.8.3.1

  parent reply	other threads:[~2017-10-11 18:05 UTC|newest]

Thread overview: 21+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-10-11 18:01 [PATCH v6 00/11] locking/rwsem: Rework rwsem-xadd & enable new rwsem features Waiman Long
2017-10-11 18:01 ` [PATCH v6 01/11] locking/rwsem: relocate rwsem_down_read_failed() Waiman Long
2017-10-11 18:01 ` [PATCH v6 02/11] locking/rwsem: Implement a new locking scheme Waiman Long
2017-10-11 18:40   ` Peter Zijlstra
2017-10-11 18:58     ` Waiman Long
2017-10-11 19:05       ` Waiman Long
2017-10-11 19:36       ` Peter Zijlstra
2017-10-11 18:01 ` Waiman Long [this message]
2017-10-11 18:01 ` [PATCH v6 04/11] locking/rwsem: Remove kernel/locking/rwsem.h Waiman Long
2017-10-11 18:01 ` [PATCH v6 05/11] locking/rwsem: Move rwsem internal function declarations to rwsem-xadd.h Waiman Long
2017-10-11 18:01 ` [PATCH v6 06/11] locking/rwsem: Remove arch specific rwsem files Waiman Long
2017-10-11 18:01 ` [PATCH v6 07/11] locking/rwsem: Implement lock handoff to prevent lock starvation Waiman Long
2017-10-11 18:01 ` [PATCH v6 08/11] locking/rwsem: Enable readers spinning on writer Waiman Long
2017-10-11 18:02 ` [PATCH v6 09/11] locking/rwsem: Enable time-based reader lock stealing Waiman Long
2017-10-11 18:02 ` [PATCH v6 10/11] locking/rwsem: Make rwsem_spin_on_owner() return a tri-state value Waiman Long
2017-10-11 18:02 ` [PATCH v6 11/11] locking/rwsem: Enable count-based spinning on reader Waiman Long
2017-10-11 18:48 ` [PATCH v6 00/11] locking/rwsem: Rework rwsem-xadd & enable new rwsem features Peter Zijlstra
2017-10-11 18:50   ` Waiman Long
2017-10-11 20:45   ` Dave Chinner
2017-10-11 20:50 ` Dave Chinner
2017-10-11 20:57   ` Waiman Long

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