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locking/osq_lock: Clarify osq_wait_next() calling convention

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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>
This commit is contained in:
David Laight 2023-12-29 20:56:03 +00:00 • committed by Linus Torvalds
parent 7c22309821
commit 563adbfc35
1 changed files with 9 additions and 12 deletions
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21
kernel/locking/osq_lock.c
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@ -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. * 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. * 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 * static inline struct optimistic_spin_node *
osq_wait_next(struct optimistic_spin_queue *lock, osq_wait_next(struct optimistic_spin_queue *lock,
struct optimistic_spin_node *node, struct optimistic_spin_node *node,
struct optimistic_spin_node *prev) int old_cpu)
{ {
struct optimistic_spin_node *next = NULL; struct optimistic_spin_node *next = NULL;
int curr = encode_cpu(smp_processor_id()); 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 (;;) { for (;;) {
if (atomic_read(&lock->tail) == curr && 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 * We were the last queued, we moved @lock back. @prev
* will now observe @lock and will complete its * will now observe @lock and will complete its
@ -193,7 +190,7 @@ bool osq_lock(struct optimistic_spin_queue *lock)
* back to @prev. * back to @prev.
*/ */
next = osq_wait_next(lock, node, prev); next = osq_wait_next(lock, node, prev->cpu);
if (!next) if (!next)
return false; return false;
@ -233,7 +230,7 @@ void osq_unlock(struct optimistic_spin_queue *lock)
return; return;
} }
next = osq_wait_next(lock, node, NULL); next = osq_wait_next(lock, node, OSQ_UNLOCKED_VAL);
if (next) if (next)
WRITE_ONCE(next->locked, 1); WRITE_ONCE(next->locked, 1);
} }