[PATCH next 3/5] locking/osq_lock: Clarify osq_wait_next()

[David Laight]

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.

Also directly return NULL or 'next' instead of breaking the loop.

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.lai...@aculab.com>
---
 kernel/locking/osq_lock.c | 23 ++++++++++-------------
 1 file changed, 10 insertions(+), 13 deletions(-)

diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
index 55f5db896c02..9bb3a077ba92 100644
--- a/kernel/locking/osq_lock.c
+++ b/kernel/locking/osq_lock.c
@@ -48,18 +48,17 @@ static inline struct optimistic_spin_node *decode_cpu(int 
encoded_cpu_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)
 {
-       struct optimistic_spin_node *next = NULL;
+       struct optimistic_spin_node *next;
        int curr = node->cpu;
-       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.
+        * If osq_lock() is being cancelled there must be a previous node
+        * and 'old' is its CPU #.
+        * For osq_unlock() there is never a previous node and old is set
+        * to OSQ_UNLOCKED_VAL.
         */
-       old = prev ? prev->cpu : OSQ_UNLOCKED_VAL;
 
        for (;;) {
                if (atomic_read(&lock->tail) == curr &&
@@ -69,7 +68,7 @@ osq_wait_next(struct optimistic_spin_queue *lock,
                         * will now observe @lock and will complete its
                         * unlock()/unqueue().
                         */
-                       break;
+                       return NULL;
                }
 
                /*
@@ -85,13 +84,11 @@ osq_wait_next(struct optimistic_spin_queue *lock,
                if (node->next) {
                        next = xchg(&node->next, NULL);
                        if (next)
-                               break;
+                               return next;
                }
 
                cpu_relax();
        }
-
-       return next;
 }
 
 bool osq_lock(struct optimistic_spin_queue *lock)
@@ -192,7 +189,7 @@ 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;
 
@@ -232,7 +229,7 @@ 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);
 }
-- 
2.17.1

-
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