Re: [PATCH v3 0/6] powerpc: queued spinlocks and rwlocks

Nicholas Piggin
2020-07-23T13:30:16Z

Excerpts from Waiman Long's message of July 22, 2020 12:36 am:
> On 7/21/20 7:08 AM, Nicholas Piggin wrote:
>> diff --git a/arch/powerpc/include/asm/qspinlock.h b/arch/powerpc/include/asm/qspinlock.h
>> index b752d34517b3..26d8766a1106 100644
>> --- a/arch/powerpc/include/asm/qspinlock.h
>> +++ b/arch/powerpc/include/asm/qspinlock.h
>> @@ -31,16 +31,57 @@ static inline void queued_spin_unlock(struct qspinlock *lock)
>>
>> #else
>> extern void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val);
>> +extern void queued_spin_lock_slowpath_queue(struct qspinlock *lock);
>> #endif
>>
>> static __always_inline void queued_spin_lock(struct qspinlock *lock)
>> {
>> - u32 val = 0;
>> -
>> - if (likely(atomic_try_cmpxchg_lock(&lock->val, &val, _Q_LOCKED_VAL)))
>> + atomic_t *a = &lock->val;
>> + u32 val;
>> +
>> +again:
>> + asm volatile(
>> +"1:\t" PPC_LWARX(%0,0,%1,1) " # queued_spin_lock \n"
>> + : "=&r" (val)
>> + : "r" (&a->counter)
>> + : "memory");
>> +
>> + if (likely(val == 0)) {
>> + asm_volatile_goto(
>> + " stwcx. %0,0,%1 \n"
>> + " bne- %l[again] \n"
>> + "\t" PPC_ACQUIRE_BARRIER " \n"
>> + :
>> + : "r"(_Q_LOCKED_VAL), "r" (&a->counter)
>> + : "cr0", "memory"
>> + : again );
>> return;
>> -
>> - queued_spin_lock_slowpath(lock, val);
>> + }
>> +
>> + if (likely(val == _Q_LOCKED_VAL)) {
>> + asm_volatile_goto(
>> + " stwcx. %0,0,%1 \n"
>> + " bne- %l[again] \n"
>> + :
>> + : "r"(_Q_LOCKED_VAL | _Q_PENDING_VAL), "r" (&a->counter)
>> + : "cr0", "memory"
>> + : again );
>> +
>> + atomic_cond_read_acquire(a, !(VAL & _Q_LOCKED_MASK));
>> +// clear_pending_set_locked(lock);
>> + WRITE_ONCE(lock->locked_pending, _Q_LOCKED_VAL);
>> +// lockevent_inc(lock_pending);
>> + return;
>> + }
>> +
>> + if (val == _Q_PENDING_VAL) {
>> + int cnt = _Q_PENDING_LOOPS;
>> + val = atomic_cond_read_relaxed(a,
>> + (VAL != _Q_PENDING_VAL) || !cnt--);
>> + if (!(val & ~_Q_LOCKED_MASK))
>> + goto again;
>> + }
>> + queued_spin_lock_slowpath_queue(lock);
>> }
>> #define queued_spin_lock queued_spin_lock
>>
>
> I am fine with the arch code override some part of the generic code.

Cool.

>> diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
>> index b9515fcc9b29..ebcc6f5d99d5 100644
>> --- a/kernel/locking/qspinlock.c
>> +++ b/kernel/locking/qspinlock.c
>> @@ -287,10 +287,14 @@ static __always_inline u32 __pv_wait_head_or_lock(struct qspinlock *lock,
>>
>> #ifdef CONFIG_PARAVIRT_SPINLOCKS
>> #define queued_spin_lock_slowpath native_queued_spin_lock_slowpath
>> +#define queued_spin_lock_slowpath_queue native_queued_spin_lock_slowpath_queue
>> #endif
>>
>> #endif /* _GEN_PV_LOCK_SLOWPATH */
>>
>> +void queued_spin_lock_slowpath_queue(struct qspinlock *lock);
>> +static void __queued_spin_lock_slowpath_queue(struct qspinlock *lock);
>> +
>> /**
>> * queued_spin_lock_slowpath - acquire the queued spinlock
>> * @lock: Pointer to queued spinlock structure
>> @@ -314,12 +318,6 @@ static __always_inline u32 __pv_wait_head_or_lock(struct qspinlock *lock,
>> */
>> void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
>> {
>> - struct mcs_spinlock *prev, *next, *node;
>> - u32 old, tail;
>> - int idx;
>> -
>> - BUILD_BUG_ON(CONFIG_NR_CPUS >= (1U << _Q_TAIL_CPU_BITS));
>> -
>> if (pv_enabled())
>> goto pv_queue;
>>
>> @@ -397,6 +395,26 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
>> queue:
>> lockevent_inc(lock_slowpath);
>> pv_queue:
>> + __queued_spin_lock_slowpath_queue(lock);
>> +}
>> +EXPORT_SYMBOL(queued_spin_lock_slowpath);
>> +
>> +void queued_spin_lock_slowpath_queue(struct qspinlock *lock)
>> +{
>> + lockevent_inc(lock_slowpath);
>> + __queued_spin_lock_slowpath_queue(lock);
>> +}
>> +EXPORT_SYMBOL(queued_spin_lock_slowpath_queue);
>> +
>> +static void __queued_spin_lock_slowpath_queue(struct qspinlock *lock)
>> +{
>> + struct mcs_spinlock *prev, *next, *node;
>> + u32 old, tail;
>> + u32 val;
>> + int idx;
>> +
>> + BUILD_BUG_ON(CONFIG_NR_CPUS >= (1U << _Q_TAIL_CPU_BITS));
>> +
>> node = this_cpu_ptr(&qnodes[0].mcs);
>> idx = node->count++;
>> tail = encode_tail(smp_processor_id(), idx);
>> @@ -559,7 +577,6 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
>> */
>> __this_cpu_dec(qnodes[0].mcs.count);
>> }
>> -EXPORT_SYMBOL(queued_spin_lock_slowpath);
>>
>> /*
>> * Generate the paravirt code for queued_spin_unlock_slowpath().
>>
> I would prefer to extract out the pending bit handling code out into a
> separate helper function which can be overridden by the arch code
> instead of breaking the slowpath into 2 pieces.

You mean have the arch provide a queued_spin_lock_slowpath_pending
function that the slow path calls?

I would actually prefer the pending handling can be made inline in
the queued_spin_lock function, especially with out-of-line locks it
makes sense to put it there.

We could ifdef out queued_spin_lock_slowpath_queue if it's not used,
then __queued_spin_lock_slowpath_queue would be inlined into the
caller so there would be no split?

Thanks,
Nick