Commit 563adbfc authored by David Laight's avatar David Laight Committed by Linus Torvalds
Browse files

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: default avatarDavid Laight <david.laight@aculab.com>
[ Changed 'old' to 'old_cpu' by request from Waiman Long  - Linus ]
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 7c223098

kernel/locking/osq_lock.c

+9 −12
Original line number Diff line number Diff line
@@ -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);

}