[PATCH 01/12] powerpc: qspinlock: Fix qnode->locked value interpretation

[Rohan McLure]

> On 9 May 2023, at 12:01 pm, Nicholas Piggin <npiggin at gmail.com> wrote:
> 
> On Mon May 8, 2023 at 12:01 PM AEST, Rohan McLure wrote:
>> A comment accompanying the locked attribute of a qnode assigns a value
>> of 1 to mean that the lock has been acquired. The usages of this
>> variable however assume opposite semantics. Update usages so that the
>> assertions of this comment are reflected in this file.
> 
> 1 actually means if the lock is acquired for this waiter. The
> previous owner sets it to 1 which means we now have the lock.
> It's slightly confusing but that's how generic qspinlock calls
> it too.
> 
> It actually doesn't even really mean we have acquired the lock
> though, it means we got through the MCS queue. "Waiting" or
> "released" or something like that might be a better name.

This makes more sense. Seemed pretty unlikely to me that swapped
polarity have gone unnoticed, so glad to have that cleared up.

> 
> Could change the name or comment to make that a bit clearer, but
> while it'the same as kernel/locking/qspinlock.c then better
> keep polarity the same.

Yeah since ‘locked’ is an mcs intrinsic I think I’d rather keep
the name from kernel/locking/qspinlock.c.

> 
> Thanks,
> Nick
> 
>> 
>> Signed-off-by: Rohan McLure <rmclure at linux.ibm.com>
>> ---
>> arch/powerpc/lib/qspinlock.c | 10 +++++-----
>> 1 file changed, 5 insertions(+), 5 deletions(-)
>> 
>> diff --git a/arch/powerpc/lib/qspinlock.c b/arch/powerpc/lib/qspinlock.c
>> index e4bd145255d0..9cf93963772b 100644
>> --- a/arch/powerpc/lib/qspinlock.c
>> +++ b/arch/powerpc/lib/qspinlock.c
>> @@ -435,7 +435,7 @@ static __always_inline bool yield_to_prev(struct qspinlock *lock, struct qnode *
>> 
>> smp_rmb(); /* See __yield_to_locked_owner comment */
>> 
>> - if (!node->locked) {
>> + if (node->locked) {
>> yield_to_preempted(prev_cpu, yield_count);
>> spin_begin();
>> return preempted;
>> @@ -566,7 +566,7 @@ static __always_inline void queued_spin_lock_mcs_queue(struct qspinlock *lock, b
>> node->lock = lock;
>> node->cpu = smp_processor_id();
>> node->yield_cpu = -1;
>> - node->locked = 0;
>> + node->locked = 1;
>> 
>> tail = encode_tail_cpu(node->cpu);
>> 
>> @@ -584,7 +584,7 @@ static __always_inline void queued_spin_lock_mcs_queue(struct qspinlock *lock, b
>> 
>> /* Wait for mcs node lock to be released */
>> spin_begin();
>> - while (!node->locked) {
>> + while (node->locked) {
>> spec_barrier();
>> 
>> if (yield_to_prev(lock, node, old, paravirt))
>> @@ -693,13 +693,13 @@ static __always_inline void queued_spin_lock_mcs_queue(struct qspinlock *lock, b
>> */
>> if (paravirt && pv_prod_head) {
>> int next_cpu = next->cpu;
>> - WRITE_ONCE(next->locked, 1);
>> + WRITE_ONCE(next->locked, 0);
>> if (_Q_SPIN_MISO)
>> asm volatile("miso" ::: "memory");
>> if (vcpu_is_preempted(next_cpu))
>> prod_cpu(next_cpu);
>> } else {
>> - WRITE_ONCE(next->locked, 1);
>> + WRITE_ONCE(next->locked, 0);
>> if (_Q_SPIN_MISO)
>> asm volatile("miso" ::: "memory");
>> }
>> -- 
>> 2.37.2
>