[PATCH 3/5] locking/osq: Allow early break from OSQ

Waiman Long
2019-09-11T15:06:22Z

The current osq_lock() function will spin until it gets the lock or
when its time slice has been used up. There may be other reasons that
a task may want to back out from the OSQ before getting the lock. This
patch extends the osq_lock() function by adding two new arguments - a
break function pointer and its argument. That break function will be
called, if defined, in each iteration of the loop to see if it should
break out early.

The optimistic_spin_node structure in osq_lock.h isn't needed by callers,
so it is moved into osq_lock.c.

Signed-off-by: Waiman Long <longman@redhat.com>
---
include/linux/osq_lock.h | 13 ++-----------
kernel/locking/mutex.c | 2 +-
kernel/locking/osq_lock.c | 12 +++++++++++-
kernel/locking/rwsem.c | 2 +-
4 files changed, 15 insertions(+), 14 deletions(-)

diff --git a/include/linux/osq_lock.h b/include/linux/osq_lock.h
index 5581dbd3bd34..161eb6b26d6d 100644
--- a/include/linux/osq_lock.h
+++ b/include/linux/osq_lock.h
@@ -2,16 +2,6 @@
#ifndef __LINUX_OSQ_LOCK_H
#define __LINUX_OSQ_LOCK_H

-/*
- * An MCS like lock especially tailored for optimistic spinning for sleeping
- * lock implementations (mutex, rwsem, etc).
- */
-struct optimistic_spin_node {
- struct optimistic_spin_node *next, *prev;
- int locked; /* 1 if lock acquired */
- int cpu; /* encoded CPU # + 1 value */
-};
-
struct optimistic_spin_queue {
/*
* Stores an encoded value of the CPU # of the tail node in the queue.
@@ -30,7 +20,8 @@ static inline void osq_lock_init(struct optimistic_spin_queue *lock)
atomic_set(&lock->tail, OSQ_UNLOCKED_VAL);
}

-extern bool osq_lock(struct optimistic_spin_queue *lock);
+extern bool osq_lock(struct optimistic_spin_queue *lock,
+ bool (*break_fn)(void *), void *break_arg);
extern void osq_unlock(struct optimistic_spin_queue *lock);

static inline bool osq_is_locked(struct optimistic_spin_queue *lock)
diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
index 468a9b8422e3..8a1df82fd71a 100644
--- a/kernel/locking/mutex.c
+++ b/kernel/locking/mutex.c
@@ -654,7 +654,7 @@ mutex_optimistic_spin(struct mutex *lock, struct ww_acquire_ctx *ww_ctx,
* acquire the mutex all at once, the spinners need to take a
* MCS (queued) lock first before spinning on the owner field.
*/
- if (!osq_lock(&lock->osq))
+ if (!osq_lock(&lock->osq, NULL, NULL))
goto fail;
}

diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
index 6ef600aa0f47..40c94380a485 100644
--- a/kernel/locking/osq_lock.c
+++ b/kernel/locking/osq_lock.c
@@ -11,6 +11,12 @@
* called from interrupt context and we have preemption disabled while
* spinning.
*/
+struct optimistic_spin_node {
+ struct optimistic_spin_node *next, *prev;
+ int locked; /* 1 if lock acquired */
+ int cpu; /* encoded CPU # + 1 value */
+};
+
static DEFINE_PER_CPU_SHARED_ALIGNED(struct optimistic_spin_node, osq_node);

/*
@@ -87,7 +93,8 @@ osq_wait_next(struct optimistic_spin_queue *lock,
return next;
}

-bool osq_lock(struct optimistic_spin_queue *lock)
+bool osq_lock(struct optimistic_spin_queue *lock,
+ bool (*break_fn)(void *), void *break_arg)
{
struct optimistic_spin_node *node = this_cpu_ptr(&osq_node);
struct optimistic_spin_node *prev, *next;
@@ -143,6 +150,9 @@ bool osq_lock(struct optimistic_spin_queue *lock)
if (need_resched() || vcpu_is_preempted(node_cpu(node->prev)))
goto unqueue;

+ if (unlikely(break_fn) && break_fn(break_arg))
+ goto unqueue;
+
cpu_relax();
}
return true;
diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c
index 49f052d68404..c15926ecb21e 100644
--- a/kernel/locking/rwsem.c
+++ b/kernel/locking/rwsem.c
@@ -807,7 +807,7 @@ static bool rwsem_optimistic_spin(struct rw_semaphore *sem, bool wlock,
preempt_disable();

/* sem->wait_lock should not be held when doing optimistic spinning */
- if (!osq_lock(&sem->osq))
+ if (!osq_lock(&sem->osq, NULL, NULL))
goto done;

curtime = timeout ? sched_clock() : 0;
--
2.18.1