From 563adbfc351b2af9f1406b809ba60b9f1673a882 Mon Sep 17 00:00:00 2001 From: David Laight Date: Fri, 29 Dec 2023 20:56:03 +0000 Subject: 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 [ Changed 'old' to 'old_cpu' by request from Waiman Long - Linus ] Signed-off-by: Linus Torvalds --- kernel/locking/osq_lock.c | 21 +++++++++------------ 1 file changed, 9 insertions(+), 12 deletions(-) (limited to 'kernel/locking') diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c index d414eef4bec6..15955ce35c53 100644 --- 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); } -- cgit