Re: [patch] spinlocks: remove 'volatile'

J.A. Magallón
2006-07-06T13:40:42Z

On Thu, 06 Jul 2006 14:39:43 +0200, Arjan van de Ven <arjan@infradead.org> wrote:

> On Thu, 2006-07-06 at 08:29 -0400, linux-os (Dick Johnson) wrote:
> > On Thu, 6 Jul 2006, Arjan van de Ven wrote:
> >
> > > On Thu, 2006-07-06 at 07:59 -0400, linux-os (Dick Johnson) wrote:
> > >> On Thu, 6 Jul 2006, Ingo Molnar wrote:
> > >>
> > >>>
> > >>> * Linus Torvalds <torvalds@osdl.org> wrote:
> > >>>
> > >>>> I wonder if we should remove the "volatile". There really isn't
> > >>>> anything _good_ that gcc can do with it, but we've seen gcc code
> > >>>> generation do stupid things before just because "volatile" seems to
> > >>>> just disable even proper normal working.
> > >>
> > >> Then GCC must be fixed. The keyword volatile is correct. It should
> > >> force the compiler to read the variable every time it's used.
> > >
> > > this is not really what the C standard says.
> > >
> > >
> > >
> > >> This is not pointless. If GCC generates bad code, tell the
> > >> GCC people. The volatile keyword is essential.
> > >
> > > no the "volatile" semantics are vague, trecherous and evil. It's a LOT
> > > better to insert the well defined "barrier()" in the right places.
> >
> > Look at:
> >
> > http://en.wikipedia.org/wiki/Volatile_variable
> >
> > This is just what is needed to prevent the compiler from making non working
> > code during optimization.
>
> and an entry level document at wikipedia is more important than the C
> standard ;)
>
> >
> > Also look at:
> >
> > http://en.wikipedia.org/wiki/Memory_barrier
> >
> > This is used to prevent out-of-order execution, not at all what is
> > necessary.
>
> I did not talk about memory barriers. In fact, barrier() is NOT a memory
> barrier. It's a compiler optimization barrier!
>

// Read 10 samples from 2 A/D converters.

int* ina;
int a[10];
int* inb;
int b[10];

for (int i=0; i<10; i++)
{
a[i] = *ina;
barrier();
b[i] = *inb;
}

The barrier prevents the compiler of translating this to:

for (int i=0; i<10; i++)
{
b[i] = *inb;
a[i] = *ina;
}

or even to:

for (int i=0; i<10; i++)
a[i] = *ina;
for (int i=0; i<10; i++)
b[i] = *inb;

but does not prevent it to do this:

register int tmp_a = *ina;
register int tmp_b = *inb;

for (int i=0; i<10; i++)
{
a[i] = tmp_a;
b[i] = tmp_b;
}

because nor 'ina' nor 'inb' change under what the compiler sees inside
the loop. 'volatile' prevents the compiler of do a high level cache of
*ina or *inb.

--
J.A. Magallon <jamagallon()ono!com> \ Software is like sex:
\ It's better when it's free
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