On 4 March 2014 09:17, Hannes Frederic Sowa <han...@stressinduktion.org> wrote:
> On Tue, Mar 04, 2014 at 10:10:21AM +0100, Richard Biener wrote:
>> On Tue, Mar 4, 2014 at 7:40 AM, lin zuojian <manjian2...@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).
So the bug was probably fixed more than 15 years ago.
