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

[Adhemerval Zanella <azanella at linux dot vnet dot ibm dot com>]

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').

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.

Checked on powerpc32 and powercp64 with my previous lowlevellock.h removal
patch.

--

	* sysdeps/nptl/lowlevellock.h (__lll_cond_lock): Use
	atomic_compare_and_exchange_val_acq instead of atomic_exchange_acq.

---

diff --git a/sysdeps/nptl/lowlevellock.h b/sysdeps/nptl/lowlevellock.h
index 28f4ba3..ba22734 100644
--- a/sysdeps/nptl/lowlevellock.h
+++ b/sysdeps/nptl/lowlevellock.h
@@ -73,7 +73,8 @@ extern int __lll_robust_lock_wait (int *futex, int private) attribute_hidden;
   ((void)                                                               \
    ({                                                                   \
      int *__futex = (futex);                                            \
-     if (__glibc_unlikely (atomic_exchange_acq (__futex, 2) != 0))      \
+     if (__glibc_unlikely (						\
+	 atomic_compare_and_exchange_val_acq (__futex, 2, 0) != 0))	\
        __lll_lock_wait (__futex, private);                              \
    }))
 #define lll_cond_lock(futex, private) __lll_cond_lock (&(futex), private)