[PATCH next v2 3/5] locking/osq_lock: Use node->prev_cpu instead of saving node->prev.

David Laight
2023-12-31T21:54:27Z

node->prev is only used to update 'prev' in the unlikely case
of concurrent unqueues.
This can be replaced by a check for node->prev_cpu changing
and then calling decode_cpu() to get the changed 'prev' pointer.

node->cpu (or more particularly) prev->cpu is only used for the
osq_wait_next() call in the unqueue path.
Normally this is exactly the value that the initial xchg() read
from lock->tail (used to obtain 'prev'), but can get updated
by concurrent unqueues.

Both the 'prev' and 'cpu' members of optimistic_spin_node are
now unused and can be deleted.

Signed-off-by: David Laight <david.laight@aculab.com>
---
kernel/locking/osq_lock.c | 31 +++++++++++++++++--------------
1 file changed, 17 insertions(+), 14 deletions(-)

diff --git a/kernel/locking/osq_lock.c b/kernel/locking/osq_lock.c
index eb8a6dfdb79d..27324b509f68 100644
--- a/kernel/locking/osq_lock.c
+++ b/kernel/locking/osq_lock.c
@@ -13,9 +13,8 @@
*/

struct optimistic_spin_node {
- struct optimistic_spin_node *next, *prev;
+ struct optimistic_spin_node *next;
int locked; /* 1 if lock acquired */
- int cpu; /* encoded CPU # + 1 value */
int prev_cpu; /* encoded CPU # + 1 value */
};

@@ -91,10 +90,9 @@ bool osq_lock(struct optimistic_spin_queue *lock)
struct optimistic_spin_node *node = this_cpu_ptr(&osq_node);
struct optimistic_spin_node *prev, *next;
int curr = encode_cpu(smp_processor_id());
- int old;
+ int prev_cpu;

node->next = NULL;
- node->cpu = curr;

/*
* We need both ACQUIRE (pairs with corresponding RELEASE in
@@ -102,13 +100,12 @@ bool osq_lock(struct optimistic_spin_queue *lock)
* the node fields we just initialised) semantics when updating
* the lock tail.
*/
- old = atomic_xchg(&lock->tail, curr);
- if (old == OSQ_UNLOCKED_VAL)
+ prev_cpu = atomic_xchg(&lock->tail, curr);
+ if (prev_cpu == OSQ_UNLOCKED_VAL)
return true;

- node->prev_cpu = old;
- prev = decode_cpu(old);
- node->prev = prev;
+ node->prev_cpu = prev_cpu;
+ prev = decode_cpu(prev_cpu);
node->locked = 0;

/*
@@ -174,9 +171,16 @@ bool osq_lock(struct optimistic_spin_queue *lock)

/*
* Or we race against a concurrent unqueue()'s step-B, in which
- * case its step-C will write us a new @node->prev pointer.
+ * case its step-C will write us a new @node->prev_cpu value.
*/
- prev = READ_ONCE(node->prev);
+ {
+ int new_prev_cpu = READ_ONCE(node->prev_cpu);
+
+ if (new_prev_cpu == prev_cpu)
+ continue;
+ prev_cpu = new_prev_cpu;
+ prev = decode_cpu(prev_cpu);
+ }
}

/*
@@ -186,7 +190,7 @@ bool osq_lock(struct optimistic_spin_queue *lock)
* back to @prev.
*/

- next = osq_wait_next(lock, node, prev->cpu);
+ next = osq_wait_next(lock, node, prev_cpu);
if (!next)
return false;

@@ -198,8 +202,7 @@ bool osq_lock(struct optimistic_spin_queue *lock)
* it will wait in Step-A.
*/

- WRITE_ONCE(next->prev_cpu, prev->cpu);
- WRITE_ONCE(next->prev, prev);
+ WRITE_ONCE(next->prev_cpu, prev_cpu);
WRITE_ONCE(prev->next, next);

return false;
--
2.17.1

-
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