Re: locking problems with mips atomicity ?

[Daniel Jacobowitz <dan@debian.org>]

On Wed, Apr 21, 2004 at 12:53:56AM +0200, Harm Verhagen wrote:
> On Wed, 2004-04-21 at 00:49, Daniel Jacobowitz wrote:
> > On Wed, Apr 21, 2004 at 12:44:34AM +0200, Harm Verhagen wrote:
> > > The code from linux 2.4.26 arch-mips/atomic.h looks _very_ similar to
> > > the code described in the thread that has a BUG.
> > > 
> > > static __inline__ void atomic_add(int i, atomic_t * v)
> > > {
> > > 	unsigned long temp;
> > > 
> > > 	__asm__ __volatile__(
> > > 		"1:   ll      %0, %1      # atomic_add\n"
> > > 		"     addu    %0, %2                  \n"
> > > 		"     sc      %0, %1                  \n"
> > > 		"     beqz    %0, 1b                  \n"
> > > 		: "=&r" (temp), "=m" (v->counter)
> > > 		: "Ir" (i), "m" (v->counter));
> > > }
> > > 
> > > So I wonder if there is a bug here. 
> > > Can some MIPS guru check ? :)
> > 
> > It won't be a problem in the kernel.  The problem only happens when the
> > assembler expands a macro to multiple instructions including a load,
> > and that only happens for position-independent code; the kernel is not
> > PIC.
> Sorry for not understanding completely. What makes the gcc code PIC
> then? The code looks similar, an inline function with inline assembly.
> Could you elaborate ?

You may want to look for documentation describing the MIPS PIC
conventions.  Good luck; they're bizarre.

The problem is that in userspace "ll $2, symbol_name" may expand to a
load of the address of symbol_name from the GOT (global offset table).

-- 
Daniel Jacobowitz
MontaVista Software                         Debian GNU/Linux Developer