This patch enables readers to optimistically spin on a
rwsem when it is owned by a writer instead of going to sleep
directly. The rwsem_can_spin_on_owner() function is extracted
out of rwsem_optimistic_spin() and is called directly by
rwsem_down_read_failed() and rwsem_down_write_failed().
This patch may actually reduce performance under certain circumstances
as the readers may not be grouped together in the wait queue anymore.
So we may have a number of small reader groups among writers instead
of a large reader group. However, this change is needed for some of
the subsequent patches.
Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
---
kernel/locking/rwsem-xadd.c | 68 +++++++++++++++++++++++++++++++++++++++------
kernel/locking/rwsem-xadd.h | 1 +
2 files changed, 60 insertions(+), 9 deletions(-)
diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
index 65717d8..15cdb28 100644
--- a/kernel/locking/rwsem-xadd.c
+++ b/kernel/locking/rwsem-xadd.c
@@ -224,6 +224,30 @@ static void __rwsem_mark_wake(struct rw_semaphore *sem,
#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
/*
+ * Try to acquire read lock before the reader is put on wait queue.
+ * Lock acquisition isn't allowed if the rwsem is locked or a writer handoff
+ * is ongoing.
+ */
+static inline bool rwsem_try_read_lock_unqueued(struct rw_semaphore *sem)
+{
+ int count = atomic_read(&sem->count);
+
+ if (RWSEM_COUNT_WLOCKED(count) || RWSEM_COUNT_HANDOFF_WRITER(count))
+ return false;
+
+ count = atomic_fetch_add_acquire(RWSEM_READER_BIAS, &sem->count);
+ if (!RWSEM_COUNT_WLOCKED(count) && !RWSEM_COUNT_HANDOFF_WRITER(count)) {
+ if (!(count >> RWSEM_READER_SHIFT))
+ rwsem_set_reader_owned(sem);
+ return true;
+ }
+
+ /* Back out the change */
+ atomic_add(-RWSEM_READER_BIAS, &sem->count);
+ return false;
+}
+
+/*
* Try to acquire write lock before the writer has been put on wait queue.
*/
static inline bool rwsem_try_write_lock_unqueued(struct rw_semaphore *sem)
@@ -314,16 +338,14 @@ static noinline bool rwsem_spin_on_owner(struct rw_semaphore *sem)
return !rwsem_owner_is_reader(READ_ONCE(sem->owner));
}
-static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
+static bool rwsem_optimistic_spin(struct rw_semaphore *sem,
+ enum rwsem_waiter_type type)
{
bool taken = false;
preempt_disable();
/* sem->wait_lock should not be held when doing optimistic spinning */
- if (!rwsem_can_spin_on_owner(sem))
- goto done;
-
if (!osq_lock(&sem->osq))
goto done;
@@ -338,10 +360,12 @@ static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
/*
* Try to acquire the lock
*/
- if (rwsem_try_write_lock_unqueued(sem)) {
- taken = true;
+ taken = (type == RWSEM_WAITING_FOR_WRITE)
+ ? rwsem_try_write_lock_unqueued(sem)
+ : rwsem_try_read_lock_unqueued(sem);
+
+ if (taken)
break;
- }
/*
* When there's no owner, we might have preempted between the
@@ -375,7 +399,13 @@ static inline bool rwsem_has_spinner(struct rw_semaphore *sem)
}
#else
-static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
+static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem)
+{
+ return false;
+}
+
+static inline bool
+rwsem_optimistic_spin(struct rw_semaphore *sem, enum rwsem_waiter_type type)
{
return false;
}
@@ -402,10 +432,29 @@ static inline bool rwsem_has_spinner(struct rw_semaphore *sem)
static inline struct rw_semaphore __sched *
__rwsem_down_read_failed_common(struct rw_semaphore *sem, int state)
{
+ bool can_spin;
int count, adjustment = -RWSEM_READER_BIAS;
struct rwsem_waiter waiter;
DEFINE_WAKE_Q(wake_q);
+ /*
+ * Undo read bias from down_read operation to stop active locking if:
+ * 1) Optimistic spinners are present; or
+ * 2) optimistic spinning is allowed.
+ */
+ can_spin = rwsem_can_spin_on_owner(sem);
+ if (can_spin || rwsem_has_spinner(sem)) {
+ atomic_add(-RWSEM_READER_BIAS, &sem->count);
+ adjustment = 0;
+
+ /*
+ * Do optimistic spinning and steal lock if possible.
+ */
+ if (can_spin &&
+ rwsem_optimistic_spin(sem, RWSEM_WAITING_FOR_READ))
+ return sem;
+ }
+
waiter.task = current;
waiter.type = RWSEM_WAITING_FOR_READ;
waiter.timeout = jiffies + RWSEM_WAIT_TIMEOUT;
@@ -487,7 +536,8 @@ static inline bool rwsem_has_spinner(struct rw_semaphore *sem)
DEFINE_WAKE_Q(wake_q);
/* do optimistic spinning and steal lock if possible */
- if (rwsem_optimistic_spin(sem))
+ if (rwsem_can_spin_on_owner(sem) &&
+ rwsem_optimistic_spin(sem, RWSEM_WAITING_FOR_WRITE))
return sem;
/*
diff --git a/kernel/locking/rwsem-xadd.h b/kernel/locking/rwsem-xadd.h
index a340ea4..db9726b 100644
--- a/kernel/locking/rwsem-xadd.h
+++ b/kernel/locking/rwsem-xadd.h
@@ -98,6 +98,7 @@ static inline void rwsem_set_reader_owned(struct rw_semaphore *sem)
RWSEM_FLAG_HANDOFF)
#define RWSEM_COUNT_LOCKED(c) ((c) & RWSEM_LOCK_MASK)
+#define RWSEM_COUNT_WLOCKED(c) ((c) & RWSEM_WRITER_LOCKED)
#define RWSEM_COUNT_HANDOFF(c) ((c) & RWSEM_FLAG_HANDOFF)
#define RWSEM_COUNT_HANDOFF_WRITER(c) \
(((c) & RWSEM_FLAG_HANDOFFS) == RWSEM_FLAG_WHANDOFF)
--
1.8.3.1
--
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