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

[paulmck@linux.vnet.ibm.com (Paul E. McKenney)]

On Tue, Jun 15, 2010 at 06:07:20PM +0200, Ulrich Weigand wrote:
> "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

OK, so GCC 4.4.3 and above can be trusted.

> > 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.

My current autoconf code checks for "armv7l", so I am OK being ARMv7
specific.  But it would be really good to cover the rest of the ARM
family, and __kernel_dmb and friends might be a way to do that!

							Thanx, Paul