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Re: linux says it is a bug


On Tue, Mar 04, 2014 at 10:10:21AM +0100, Richard Biener wrote:
> On Tue, Mar 4, 2014 at 7:40 AM, lin zuojian <manjian2006@gmail.com> wrote:
> > Hi,
> >     in include/linux/compiler-gcc.h :
> >
> > /* Optimization barrier */
> > /* The "volatile" is due to gcc bugs */
> > #define barrier() __asm__ __volatile__("": : :"memory")
> >
> > The comment of Linux says this is a gcc bug.But will any sane compiler
> > disable optimization without "volatile" key word?
> 
> Depends what they call an "optimization barrier".  A plain
> __asm__ ("" : : : "memory") is a memory barrier.  Adding volatile
> to the asm makes it a barrier for every other volatile instruction,
> nothing more.

This is meant to be a compiler barrier not a memory barrier and got
added by David Miller because of a problem in gcc-2.7.2:

| Add __volatile__ to barrier() definition, I convinced Linus
| to eat this patch.  The problem is that with gcc-2.7.2 derived
| compilers the instruction scheduler can move it around due to
| a bug.  This bug appears on sparc64/SMP with our old compiler
| in that is miscompiles the beginning of exit.c:release() causing
| lockups if the race is hit in the SMP specific code there.  I
| believe sparc32 gcc-2.7.2 sees this bug too, but I'm not too sure
| (Anton showed me something similar once).

Greetings,

  Hannes


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