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SubjectRe: [PATCH V2 11/19] csky: Atomic operations
On Fri, Jul 06, 2018 at 07:44:03PM +0800, Guo Ren wrote:
> 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.

"test-and-set" is just the name of this type of spinlock implementation.

You _could_ use the linux test_and_set bitop, but those are defined on
unsigned long and spinlock_t is generally assumed to be of unsigned int
size.

Go with the ticket locks as per below.

> > 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?

See the email I send earlier in that thread.

> Ok, I'll try to implement ticket lock in next version patch.

If you need inspiration, look at:

git show 81bb5c6420635dfd058c210bd342c29c95ccd145^1:arch/arm64/include/asm/spinlock.h

Or look at the current version of that file and ignore the LSE version.

Note that unlock is a store half-word (u16), not having seen your arch
manual yet I don't know if you even have that.

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