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SubjectRe: [PATCH V3 11/27] csky: Atomic operations
On Mon, Sep 17, 2018 at 10:17:55AM +0200, Peter Zijlstra wrote:
> On Sat, Sep 15, 2018 at 10:55:13PM +0800, Guo Ren wrote:
> > > > +#define ATOMIC_OP_RETURN(op, c_op) \
>
> > > > +#define ATOMIC_FETCH_OP(op, c_op) \
>
> > > For these you could generate _relaxed variants and not provide smp_mb()
> > > inside them.
> > Ok, but I'll modify it in next commit.
>
> That's fine. Just wanted to let you know about _relaxed() since it will
> benefit your platform.
Thank you.

> > > > +#define ATOMIC_OP(op, c_op) \
> > > > +static inline void atomic_##op(int i, atomic_t *v) \
> > > > +{ \
> > > > + unsigned long tmp, flags; \
> > > > + \
> > > > + raw_local_irq_save(flags); \
> > > > + \
> > > > + asm volatile ( \
> > > > + " ldw %0, (%2) \n" \
> > > > + " " #op " %0, %1 \n" \
> > > > + " stw %0, (%2) \n" \
> > > > + : "=&r" (tmp) \
> > > > + : "r" (i), "r"(&v->counter) \
> > > > + : "memory"); \
> > > > + \
> > > > + raw_local_irq_restore(flags); \
> > > > +}
> > >
> > > Is this really 'better' than the generic UP fallback implementation?
> > There is a lock irq instruction "idly4" with out irq_save. eg:
> > asm volatile ( \
> > " idly4 \n" \
> > " ldw %0, (%2) \n" \
> > " " #op " %0, %1 \n" \
> > " stw %0, (%2) \n" \
> > I'll change to that after full tested.
>
> That is pretty nifty, could you explain (or reference me to a arch doc
> that does) the exact semantics of that "idly4" instruction?
The idly4 allows the 4 instructions behind it to not respond to interrupts.
When ldw got exception, it will cause the carry to be 1. So I need
prepare the assemble like this:
1: cmpne r0, r0
idly4
ldw %0, (%2)
bt 1b
" #op " ...
stw ...

I need more stress test on it and then I'll change to it.

> > > > +static inline void arch_spin_lock(arch_spinlock_t *lock)
> > > > +{
> > > > + arch_spinlock_t lockval;
> > > > + u32 ticket_next = 1 << TICKET_NEXT;
> > > > + u32 *p = &lock->lock;
> > > > + u32 tmp;
> > > > +
> > > > + smp_mb();
> > >
> > > spin_lock() doesn't need smp_mb() before.
> > read_lock and write_lock also needn't smp_mb() before, isn't it?
>
> Correct. The various *_lock() functions only need imply an ACQUIRE
> barrier, such that the critical section happens after the lock is taken.
>
> > > > +
> > > > +static inline void arch_spin_unlock(arch_spinlock_t *lock)
> > > > +{
> > > > + smp_mb();
> > > > + lock->tickets.owner++;
> > > > + smp_mb();
> > >
> > > spin_unlock() doesn't need smp_mb() after.
> > read_unlock and write_unlock also needn't smp_mb() after, isn't it?
>
> Indeed so, the various *_unlock() functions only need imply a RELEASE
> barrier, such that the critical section happend before the lock is
> released.
>
> In both cases (lock and unlock) there is a great amount of subtle
> details, but most of that is irrelevant if all you have is smp_mb().
Got it, Thx for the explanation.

>
>
> > > > +/*
> > > > + * Test-and-set spin-locking.
> > > > + */
> > >
> > > Why retain that?
> > >
> > > same comments; it has far too many smp_mb()s in.
> > I'm not sure about queued_rwlocks and just for 2-cores-smp test-and-set is
> > faster and simpler, isn't it?
>
> Even on 2 cores I think you can create starvation cases with
> test-and-set spinlocks. And the maintenace overhead of carrying two lock
> implementations is non trivial.
>
> As to performance; I cannot say, but the ticket lock isn't very
> expensive, you could benchmark of course.
Ticket lock is good.
But How about queued_rwlocks v.s my_test_set_rwlock?
I'm not sure about the queued_rwlocks. I just implement the ticket-spinlock.

Best Regards
Guo Ren

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