Questions for CONFIG_WEAK_ORDERING and CONFIG_WEAK_REORDERING_BEYOND_LLSC

Questions for CONFIG_WEAK_ORDERING and CONFIG_WEAK_REORDERING_BEYOND_LLSC

[Johannes Dickgreber]

If a cpu is WEAK_ORDERING schouldn't it do a sync independent of CONFIG_SMP ?

And if it is a SMP system schouldn't it do a sync independent of CONFIG_WEAK_ORDERING ?

And if a cpu has no sync with LLSC schouldn't it do a sync independent of CONFIG_SMP ?

All together, is the following the right thing to do ?
---
 arch/mips/include/asm/barrier.h |    5 +++--
 1 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/arch/mips/include/asm/barrier.h b/arch/mips/include/asm/barrier.h
index 8e9ac31..46b2364 100644
--- a/arch/mips/include/asm/barrier.h
+++ b/arch/mips/include/asm/barrier.h
@@ -130,12 +130,13 @@
 
 #endif /* !CONFIG_CPU_HAS_WB */
 
-#if defined(CONFIG_WEAK_ORDERING) && defined(CONFIG_SMP)
+#if defined(CONFIG_WEAK_ORDERING) || defined(CONFIG_SMP)
 #define __WEAK_ORDERING_MB	"       sync	\n"
 #else
 #define __WEAK_ORDERING_MB	"		\n"
 #endif
-#if defined(CONFIG_WEAK_REORDERING_BEYOND_LLSC) && defined(CONFIG_SMP)
+
+#if defined(CONFIG_WEAK_REORDERING_BEYOND_LLSC)
 #define __WEAK_LLSC_MB		"       sync	\n"
 #else
 #define __WEAK_LLSC_MB		"		\n"
-- 
1.6.0.2

Re: Questions for CONFIG_WEAK_ORDERING and CONFIG_WEAK_REORDERING_BEYOND_LLSC

[Ralf Baechle]

On Mon, Oct 13, 2008 at 09:01:44PM +0200, Johannes Dickgreber wrote:

> If a cpu is WEAK_ORDERING schouldn't it do a sync independent of CONFIG_SMP ?
> 
> And if it is a SMP system schouldn't it do a sync independent of CONFIG_WEAK_ORDERING ?
> 
> And if a cpu has no sync with LLSC schouldn't it do a sync independent of CONFIG_SMP ?
> 
> All together, is the following the right thing to do ?

A processor is always consistently ordered wrt. to itself, so uniprocessor
cores never need SYNCs even if that processor was weakly ordered in a
multiprocessor systems.

A while ago I walked through all mb(), rmb() and wmb() uses in the generic
code.  None of the ones I verified is actually needed on uniprocessor
kernels.  Ocasionally one of these functions is used to maintain I/O
ordering but again other mechanisms are prefered for that purpose.

  Ralf