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Date: Wed, 11 Oct 2017 14:01:54 -0400
From: Waiman Long <longman@...hat.com>
To: Peter Zijlstra <peterz@...radead.org>,
Ingo Molnar <mingo@...hat.com>
Cc: linux-kernel@...r.kernel.org, x86@...nel.org,
linux-alpha@...r.kernel.org, linux-ia64@...r.kernel.org,
linux-s390@...r.kernel.org, linux-arch@...r.kernel.org,
Davidlohr Bueso <dave@...olabs.net>,
Dave Chinner <david@...morbit.com>,
Waiman Long <longman@...hat.com>
Subject: [PATCH v6 03/11] locking/rwsem: Move owner setting code from rwsem.c to rwsem-xadd.h
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@...hat.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
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