[ltt-dev] [PATCH] fix the "unknown" case

[Ulrich Weigand]

"Paul E. McKenney" <paulmck at linux.vnet.ibm.com> wrote on 06/15/2010
05:07:27 PM:

> > See also the discussion in this bugzilla and the mailing list threads
> > linked from it:
> > http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42263
>
> So I cannot expect the other __sync_ primitives to be generate
> memory barriers, either, correct?  Hmmm...  From looking at
> http://gcc.gnu.org/ml/gcc-patches/2009-12/msg00198.html, I wonder
> whether I can rely on them to be using atomic instructions -- though
> I do admit that __sync_lock_release() often does not need an atomic
> instruction.

As mentioned in the bugzilla, there were bugs in __sync_synchronize
(which was just a compiler optimization barrier), and in
__sync_lock_release,
which did use a memory barrier, but at the wrong place.

With a compiler where these two are fixes (GCC 4.4.3 and above), all the
__sync_ primitives will generate memory barriers, but not directly: the
compiler will call the libgcc library function, which will call a special
ARM kernel entry point, which will then use an appropriate instruction
depending whether the kernel is compiled for SMP or UP, and depending on
the target instruction set level.  For example, the __kernel_dmb call
uses this piece of code in the kernel:

        .macro  smp_dmb
#ifdef CONFIG_SMP
#if __LINUX_ARM_ARCH__ >= 7
        dmb
#elif __LINUX_ARM_ARCH__ == 6
        mcr     p15, 0, r0, c7, c10, 5  @ dmb
#endif
#endif
        .endm

> So, should I just bite the bullet and write the usual set of asms
> myself?  People are going to want to build liburcu on old compilers,
> sad to say...

As Paolo already said, you probably do want to support older compilers.
On the other hand, if you hard-code the dmb instruction, it seems you're
tied to the ARM 7 architecture level ...   Maybe you want to directly
use the __kernel_dmb etc. calls in liburcu?  They seem to be official
kernel ABI, so should be OK to use.


Mit freundlichen Gruessen / Best Regards

Ulrich Weigand

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