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SubjectRe: [PATCH V2 11/19] csky: Atomic operations
On Thu, Jul 05, 2018 at 07:59:02PM +0200, Peter Zijlstra wrote:
> On Mon, Jul 02, 2018 at 01:30:14AM +0800, Guo Ren wrote:
>
> > +static inline void arch_spin_lock(arch_spinlock_t *lock)
> > +{
> > + unsigned int *p = &lock->lock;
> > + unsigned int tmp;
> > +
> > + asm volatile (
> > + "1: ldex.w %0, (%1) \n"
> > + " bnez %0, 1b \n"
> > + " movi %0, 1 \n"
> > + " stex.w %0, (%1) \n"
> > + " bez %0, 1b \n"
> > + : "=&r" (tmp)
> > + : "r"(p)
> > + : "memory");
> > + smp_mb();
> > +}
>
> Test-and-set with MB acting as ACQUIRE, ok.
Em ... Ok, I'll try to use test-and-set function instead of it.

> > +static inline void arch_spin_unlock(arch_spinlock_t *lock)
> > +{
> > + unsigned int *p = &lock->lock;
> > + unsigned int tmp;
> > +
> > + smp_mb();
> > + asm volatile (
> > + "1: ldex.w %0, (%1) \n"
> > + " movi %0, 0 \n"
> > + " stex.w %0, (%1) \n"
> > + " bez %0, 1b \n"
> > + : "=&r" (tmp)
> > + : "r"(p)
> > + : "memory");
> > +}
>
> MB acting for RELEASE, but _why_ are you using a LDEX/STEX to clear the
> lock word? Would not a normal store work?
Normal store is enough, I'll fixup it in next version patch.

> Also, the fact that you need MB for release implies your LDEX does not
> in fact imply anything and your xchg/cmpxchg implementation is broken.
xchg/cmxchg broken without 1th smp_mb()? Why we need protect the
instructions flow before the ldex.w?

> > +static inline int arch_spin_trylock(arch_spinlock_t *lock)
> > +{
> > + unsigned int *p = &lock->lock;
> > + unsigned int tmp;
> > +
> > + asm volatile (
> > + "1: ldex.w %0, (%1) \n"
> > + " bnez %0, 2f \n"
> > + " movi %0, 1 \n"
> > + " stex.w %0, (%1) \n"
> > + " bez %0, 1b \n"
> > + " movi %0, 0 \n"
> > + "2: \n"
> > + : "=&r" (tmp)
> > + : "r"(p)
> > + : "memory");
> > + smp_mb();
> > +
> > + return !tmp;
> > +}
>
> Strictly speaking you can avoid the MB on failure. You only need to
> provide ACQUIRE semantics on success.
>
> That said, I would really suggest you implement a ticket lock instead of
> a test-and-set lock. They're not really all that complicated and do
> provide better worst case behaviour.
Ok, I'll try to implement ticket lock in next version patch.

>
>
> > +/****** read lock/unlock/trylock ******/
>
> Please have a look at using qrwlock -- esp. if you implement a ticket
> lock, then the rwlock comes for 'free'.
Ok, I'll try it.

Guo Ren

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