locking/osq_lock: Clarify osq_wait_next() calling convention

osq_wait_next() is passed 'prev' from osq_lock() and NULL from
osq_unlock() but only needs the 'cpu' value to write to lock->tail.

Just pass prev->cpu or OSQ_UNLOCKED_VAL instead.

Should have no effect on the generated code since gcc manages to assume
that 'prev != NULL' due to an earlier dereference.

Signed-off-by: David Laight <david.laight@aculab.com>
[ Changed 'old' to 'old_cpu' by request from Waiman Long  - Linus ]
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
index d414eef4bec60fc282166461085f7e91a09bebeb..15955ce35c53409520da5d7a74f9855da77b7801 100644 (file)
--- a/kernel/locking/osq_lock.c
+++ b/kernel/locking/osq_lock.c
@@ -44,26 +44,23 @@ static inline struct optimistic_spin_node *decode_cpu(int encoded_cpu_val)
 /*
  * 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.
+ *
+ * If osq_lock() is being cancelled there must be a previous node
+ * and 'old_cpu' is its CPU #.
+ * For osq_unlock() there is never a previous node and old_cpu is
+ * set to OSQ_UNLOCKED_VAL.
  */
 static inline struct optimistic_spin_node *
 osq_wait_next(struct optimistic_spin_queue *lock,
              struct optimistic_spin_node *node,
-             struct optimistic_spin_node *prev)
+             int old_cpu)
 {
        struct optimistic_spin_node *next = NULL;
        int curr = encode_cpu(smp_processor_id());
-       int old;
-
-       /*
-        * If there is a prev node in queue, then the 'old' value will be
-        * the prev node's CPU #, else it's set to OSQ_UNLOCKED_VAL since if
-        * we're currently last in queue, then the queue will then become empty.
-        */
-       old = prev ? prev->cpu : OSQ_UNLOCKED_VAL;
 
        for (;;) {
                if (atomic_read(&lock->tail) == curr &&
-                   atomic_cmpxchg_acquire(&lock->tail, curr, old) == curr) {
+                   atomic_cmpxchg_acquire(&lock->tail, curr, old_cpu) == curr) {
                        /*
                         * We were the last queued, we moved @lock back. @prev
                         * will now observe @lock and will complete its
@@ -193,7 +190,7 @@ bool osq_lock(struct optimistic_spin_queue *lock)
         * back to @prev.
         */
 
-       next = osq_wait_next(lock, node, prev);
+       next = osq_wait_next(lock, node, prev->cpu);
        if (!next)
                return false;
 
@@ -233,7 +230,7 @@ void osq_unlock(struct optimistic_spin_queue *lock)
                return;
        }
 
-       next = osq_wait_next(lock, node, NULL);
+       next = osq_wait_next(lock, node, OSQ_UNLOCKED_VAL);
        if (next)
                WRITE_ONCE(next->locked, 1);
 }