Because osq has only been used for mutex/rwsem spinning logic,
we have gotten away with being rather flexible in any of the
traditional lock/unlock ACQUIRE/RELEASE minimal guarantees.
However, if wanted to be used as a _real_ lock, then it would
be in trouble. To this end, this patch provides the two
alternatives, where osq_lock/unlock() calls have the required
semantics, and a _relaxed() call, for no ordering guarantees
at all.
- node->locked is now completely without ordering for _relaxed()
(currently its under smp_load_acquire, which does not match and
the race is harmless to begin with as we just iterate again. For
the ACQUIRE flavor, it is always formed with ctr dep + smp_rmb().
- In order to avoid more code duplication via macros, the common
osq_wait_next() call is completely unordered, but the caller
can provide the necessary barriers, if required - ie the case for
osq_unlock(): similar to the node->locked case, this also relies
on ctrl dep + smp_wmb() to form RELEASE.
- If osq_lock() fails we never guarantee any ordering (obviously
same goes for _relaxed).
Both mutexes and rwsems have been updated to continue using the
relaxed versions, but this will obviously change for the later.
Signed-off-by: Davidlohr Bueso <dbueso@xxxxxxx>
---
XXX: This obviously needs a lot of testing.
include/asm-generic/barrier.h | 9 ++
include/linux/osq_lock.h | 10 ++
kernel/locking/mutex.c | 6 +-
kernel/locking/osq_lock.c | 279 +++++++++++++++++++++++-------------------
kernel/locking/rwsem-xadd.c | 4 +-
5 files changed, 177 insertions(+), 131 deletions(-)
diff --git a/include/asm-generic/barrier.h b/include/asm-generic/barrier.h
index fe297b599b0a..0036b08151c3 100644
--- a/include/asm-generic/barrier.h
+++ b/include/asm-generic/barrier.h
@@ -221,6 +221,15 @@ do { \
#endif
/**
+ * smp_release__after_ctrl_dep() - Provide RELEASE ordering after a control dependency
+ *
+ * A control dependency provides a LOAD->STORE order, the additional WMB
+ * provides STORE->STORE order, together they provide {LOAD,STORE}->STORE order,
+ * aka. (store)-RELEASE.
+ */
+#define smp_release__after_ctrl_dep() smp_wmb()
+
+/**
* smp_cond_load_acquire() - (Spin) wait for cond with ACQUIRE ordering
* @ptr: pointer to the variable to wait on
* @cond: boolean expression to wait for
diff --git a/include/linux/osq_lock.h b/include/linux/osq_lock.h
index 703ea5c30a33..a63ffa95aa70 100644
--- a/include/linux/osq_lock.h
+++ b/include/linux/osq_lock.h
@@ -29,6 +29,16 @@ static inline void osq_lock_init(struct optimistic_spin_queue *lock)
atomic_set(&lock->tail, OSQ_UNLOCKED_VAL);
}
+/*
+ * Versions of osq_lock/unlock that do not imply or guarantee (load)-ACQUIRE
+ * (store)-RELEASE barrier semantics.
+ *
+ * Note that a failed call to either osq_lock() or osq_lock_relaxed() does
+ * not imply barriers... we are next to block.
+ */
+extern bool osq_lock_relaxed(struct optimistic_spin_queue *lock);
+extern void osq_unlock_relaxed(struct optimistic_spin_queue *lock);
+
extern bool osq_lock(struct optimistic_spin_queue *lock);
extern void osq_unlock(struct optimistic_spin_queue *lock);
diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
index a70b90db3909..b1bf1e057565 100644
--- a/kernel/locking/mutex.c
+++ b/kernel/locking/mutex.c
@@ -316,7 +316,7 @@ static bool mutex_optimistic_spin(struct mutex *lock,
* 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_relaxed(&lock->osq))
goto done;
while (true) {
@@ -358,7 +358,7 @@ static bool mutex_optimistic_spin(struct mutex *lock,
}
mutex_set_owner(lock);
- osq_unlock(&lock->osq);
+ osq_unlock_relaxed(&lock->osq);
return true;
}
@@ -380,7 +380,7 @@ static bool mutex_optimistic_spin(struct mutex *lock,
cpu_relax_lowlatency();
}
- osq_unlock(&lock->osq);
+ osq_unlock_relaxed(&lock->osq);
done:
/*
* If we fell out of the spin path because of need_resched(),
diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
index 05a37857ab55..d3d1042a509c 100644
--- a/kernel/locking/osq_lock.c
+++ b/kernel/locking/osq_lock.c
@@ -28,6 +28,17 @@ static inline struct optimistic_spin_node *decode_cpu(int encoded_cpu_val)
return per_cpu_ptr(&osq_node, cpu_nr);
}
+static inline void set_node_locked_release(struct optimistic_spin_node *node)
+{
+ smp_store_release(&node->locked, 1);
+}
+
+static inline void set_node_locked_relaxed(struct optimistic_spin_node *node)
+{
+ WRITE_ONCE(node->locked, 1);
+
+}
+
/*
* Get a stable @node->next pointer, either for unlock() or unqueue() purposes.
* Can return NULL in case we were the last queued and we updated @lock instead.
@@ -50,7 +61,7 @@ osq_wait_next(struct optimistic_spin_queue *lock,
for (;;) {
if (atomic_read(&lock->tail) == curr &&
- atomic_cmpxchg_acquire(&lock->tail, curr, old) == curr) {
+ atomic_cmpxchg_relaxed(&lock->tail, curr, old) == curr) {
/*
* We were the last queued, we moved @lock back. @prev
* will now observe @lock and will complete its
@@ -70,7 +81,7 @@ osq_wait_next(struct optimistic_spin_queue *lock,
* wait for a new @node->next from its Step-C.
*/
if (node->next) {
- next = xchg(&node->next, NULL);
+ next = xchg_relaxed(&node->next, NULL);
if (next)
break;
}
@@ -81,130 +92,146 @@ osq_wait_next(struct optimistic_spin_queue *lock,
return next;
}
-bool osq_lock(struct optimistic_spin_queue *lock)
-{
- struct optimistic_spin_node *node = this_cpu_ptr(&osq_node);
- struct optimistic_spin_node *prev, *next;
- int curr = encode_cpu(smp_processor_id());
- int old;
-
- node->locked = 0;
- node->next = NULL;
- node->cpu = curr;
-
- /*
- * We need both ACQUIRE (pairs with corresponding RELEASE in
- * unlock() uncontended, or fastpath) and RELEASE (to publish
- * the node fields we just initialised) semantics when updating
- * the lock tail.
- */
- old = atomic_xchg(&lock->tail, curr);
- if (old == OSQ_UNLOCKED_VAL)
- return true;
-
- prev = decode_cpu(old);
- node->prev = prev;
- WRITE_ONCE(prev->next, node);
-
- /*
- * Normally @prev is untouchable after the above store; because at that
- * moment unlock can proceed and wipe the node element from stack.
- *
- * However, since our nodes are static per-cpu storage, we're
- * guaranteed their existence -- this allows us to apply
- * cmpxchg in an attempt to undo our queueing.
- */
-
- while (!READ_ONCE(node->locked)) {
- /*
- * If we need to reschedule bail... so we can block.
- */
- if (need_resched())
- goto unqueue;
-
- cpu_relax_lowlatency();
- }
- return true;
-
-unqueue:
- /*
- * Step - A -- stabilize @prev
- *
- * Undo our @prev->next assignment; this will make @prev's
- * unlock()/unqueue() wait for a next pointer since @lock points to us
- * (or later).
- */
-
- for (;;) {
- if (prev->next == node &&
- cmpxchg(&prev->next, node, NULL) == node)
- break;
-
- /*
- * We can only fail the cmpxchg() racing against an unlock(),
- * in which case we should observe @node->locked becomming
- * true.
- */
- if (smp_load_acquire(&node->locked))
- return true;
-
- cpu_relax_lowlatency();
-
- /*
- * Or we race against a concurrent unqueue()'s step-B, in which
- * case its step-C will write us a new @node->prev pointer.
- */
- prev = READ_ONCE(node->prev);
- }
-
- /*
- * Step - B -- stabilize @next
- *
- * Similar to unlock(), wait for @node->next or move @lock from @node
- * back to @prev.
- */
-
- next = osq_wait_next(lock, node, prev);
- if (!next)
- return false;
-
- /*
- * Step - C -- unlink
- *
- * @prev is stable because its still waiting for a new @prev->next
- * pointer, @next is stable because our @node->next pointer is NULL and
- * it will wait in Step-A.
- */
-
- WRITE_ONCE(next->prev, prev);
- WRITE_ONCE(prev->next, next);
-
- return false;
+#define OSQ_LOCK(EXT, FENCECB) \
+bool osq_lock##EXT(struct optimistic_spin_queue *lock) \
+{ \
+ struct optimistic_spin_node *node = this_cpu_ptr(&osq_node); \
+ struct optimistic_spin_node *prev, *next; \
+ int old, curr = encode_cpu(smp_processor_id()); \
+ \
+ node->locked = 0; \
+ node->next = NULL; \
+ node->cpu = curr; \
+ \
+ /* \
+ * At the very least we need RELEASE semantics to initialize \
+ * the node fields _before_ publishing it to the the lock tail. \
+ */ \
+ old = atomic_xchg_release(&lock->tail, curr); \
+ if (old == OSQ_UNLOCKED_VAL) { \
+ FENCECB; \
+ return true; \
+ } \
+ \
+ prev = decode_cpu(old); \
+ node->prev = prev; \
+ WRITE_ONCE(prev->next, node); \
+ \
+ /* \
+ * Normally @prev is untouchable after the above store; because \
+ * at that moment unlock can proceed and wipe the node element \
+ * from stack. \
+ * \
+ * However, since our nodes are static per-cpu storage, we're \
+ * guaranteed their existence -- this allows us to apply \
+ * cmpxchg in an attempt to undo our queueing. \
+ */ \
+ while (!READ_ONCE(node->locked)) { \
+ /* \
+ * If we need to reschedule bail... so we can block. \
+ */ \
+ if (need_resched()) \
+ goto unqueue; \
+ \
+ cpu_relax_lowlatency(); \
+ } \
+ FENCECB; \
+ return true; \
+ \
+unqueue: \
+ /* \
+ * Step - A -- stabilize @prev \
+ * \
+ * Undo our @prev->next assignment; this will make @prev's \
+ * unlock()/unqueue() wait for a next pointer since @lock \
+ * points to us (or later). \
+ */ \
+ for (;;) { \
+ /* \
+ * Failed calls to osq_lock() do not guarantee any \
+ * ordering, thus always rely on RELAXED semantics. \
+ * This also applies below, in Step - B. \
+ */ \
+ if (prev->next == node && \
+ cmpxchg_relaxed(&prev->next, node, NULL) == node) \
+ break; \
+ \
+ /* \
+ * We can only fail the cmpxchg() racing against an \
+ * unlock(), in which case we should observe \
+ * @node->locked becoming true. \
+ */ \
+ if (READ_ONCE(node->locked)) { \
+ FENCECB; \
+ return true; \
+ } \
+ \
+ cpu_relax_lowlatency(); \
+ \
+ /* \
+ * Or we race against a concurrent unqueue()'s step-B, \
+ * in which case its step-C will write us a new \
+ * @node->prev pointer. \
+ */ \
+ prev = READ_ONCE(node->prev); \
+ } \
+ \
+ /* \
+ * Step - B -- stabilize @next \
+ * \
+ * Similar to unlock(), wait for @node->next or move @lock \
+ * from @node back to @prev. \
+ */ \
+ next = osq_wait_next(lock, node, prev); \
+ if (!next) \
+ return false; \
+ \
+ /* \
+ * Step - C -- unlink \
+ * \
+ * @prev is stable because its still waiting for a new \
+ * @prev->next pointer, @next is stable because our \
+ * @node->next pointer is NULL and it will wait in Step-A. \
+ */ \
+ WRITE_ONCE(next->prev, prev); \
+ WRITE_ONCE(prev->next, next); \
+ \
+ return false; \
}
-void osq_unlock(struct optimistic_spin_queue *lock)
-{
- struct optimistic_spin_node *node, *next;
- int curr = encode_cpu(smp_processor_id());
-
- /*
- * Fast path for the uncontended case.
- */
- if (likely(atomic_cmpxchg_release(&lock->tail, curr,
- OSQ_UNLOCKED_VAL) == curr))
- return;
-
- /*
- * Second most likely case.
- */
- node = this_cpu_ptr(&osq_node);
- next = xchg(&node->next, NULL);
- if (next) {
- WRITE_ONCE(next->locked, 1);
- return;
- }
-
- next = osq_wait_next(lock, node, NULL);
- if (next)
- WRITE_ONCE(next->locked, 1);
+OSQ_LOCK(, smp_acquire__after_ctrl_dep())
+OSQ_LOCK(_relaxed, )
+
+#define OSQ_UNLOCK(EXT, FENCE, FENCECB) \
+void osq_unlock##EXT(struct optimistic_spin_queue *lock) \
+{ \
+ struct optimistic_spin_node *node, *next; \
+ int curr = encode_cpu(smp_processor_id()); \
+ \
+ /* \
+ * Fast path for the uncontended case. \
+ */ \
+ if (likely(atomic_cmpxchg_##FENCE(&lock->tail, curr, \
+ OSQ_UNLOCKED_VAL) == curr)) \
+ return; \
+ \
+ /* \
+ * Second most likely case. \
+ */ \
+ node = this_cpu_ptr(&osq_node); \
+ next = xchg(&node->next, NULL); \
+ if (next) \
+ goto done_setlocked; \
+ \
+ next = osq_wait_next(lock, node, NULL); \
+ if (!next) { \
+ FENCECB; \
+ return; \
+ } \
+ \
+done_setlocked: \
+ set_node_locked_##FENCE(next); \
}
+
+OSQ_UNLOCK(, release, smp_release__after_ctrl_dep())
+OSQ_UNLOCK(_relaxed, relaxed, )
diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
index 2337b4bb2366..88e95b114392 100644
--- a/kernel/locking/rwsem-xadd.c
+++ b/kernel/locking/rwsem-xadd.c
@@ -389,7 +389,7 @@ static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
if (!rwsem_can_spin_on_owner(sem))
goto done;
- if (!osq_lock(&sem->osq))
+ if (!osq_lock_relaxed(&sem->osq))
goto done;
/*
@@ -425,7 +425,7 @@ static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
*/
cpu_relax_lowlatency();
}
- osq_unlock(&sem->osq);
+ osq_unlock_relaxed(&sem->osq);
done:
preempt_enable();
return taken;
--
2.6.6
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