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Re: [PATCH] [RFC] nptl: use compare and exchange for lll_cond_lock

On Tue, 2014-09-23 at 14:45 -0300, Adhemerval Zanella wrote:
> While checking the generated code and macros used in generic lowlevellock.h,
> I noted powerpc and other arch uses uses a compare and swap instead of a plain
> exchange value on lll_cond_lock.
> I am not really sure which behavior would be desirable, since as far I could
> they will have both the same side effects (since lll_cond_lock, different
> from lll_lock, does not hold value of '1').

What do you mean by "[the function] does not hold value of '1'"?

> So I am proposing this patch to sync default implementation for what mostly
> architectures (ia64, ppc, s390, sparc, x86, hppa) uses for lll_cond_lock.  I see
> that only microblaze and sh (I am not sure about this one, I not well versed in
> its assembly and I'm being guided by its comment) used the atomic_exchange_acq
> instead.

I think both versions work from a correctness POV, but doing an
unconditional exchange should be the right thing to do.

The default implementation of __lll_lock_wait will test if the futex
variable equals 2, and if not, do an exchange right away before running
the FUTEX_WAIT syscall.  So if the CAS that you propose fails, the next
thing that will happen is an exchange.  Thus, it seems that we should do
the exchange right away.


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