LDREX/STREX and pre-emption on SMP hardware
Catalin Marinas
catalin.marinas at arm.com
Mon Sep 14 06:06:13 EDT 2009
On Mon, 2009-09-14 at 11:00 +0100, Russell King - ARM Linux wrote:
> On Mon, Sep 14, 2009 at 02:43:53AM +0100, Jamie Lokier wrote:
> > Catalin Marinas wrote:
> > > With interrupts (I1, I2 interrupt handlers)
> > >
> > > I1 I2
> > > LDREX
> > > LDREX
> > > STREX (succeeds)
> > > STREX (fails)
> > >
> > > In the interrupt case, they are nested so the STREX in I2 is always
> > > executed before STREX in I1 (you can extrapolate with several nested
> > > interrupts).
> >
> > This assumes LDREX/STREX are always called in pairs. But this is in
> > fact _not_ the case. Take a look at atomic_cmpxchg:
> >
> > do {
> > __asm__ __volatile__("@ atomic_cmpxchg\n"
> > "ldrex %1, [%2]\n"
> > "mov %0, #0\n"
> > "teq %1, %3\n"
> > "strexeq %0, %4, [%2]\n"
> > : "=&r" (res), "=&r" (oldval)
> > : "r" (&ptr->counter), "Ir" (old), "r" (new)
> > : "cc");
> > } while (res);
> >
> > In the case where ptr->counter != old, STREX is not executed, and the
> > do{...}while loop does not loop. Thus LDREX/STREX aren't paired.
>
> It doesn't matter though - consider two threads using LDREX on the same
> location:
>
> T1 T2
> LDREX
> LDREX
> STREXEQ (not satsified)
> STREX
As I replied to Jamie on a similar issue, you can have:
T1 T2
LDREX
LDREX
STREXEQ (satisfied, succeeds)
LDREX
STREXEQ (not satisfied)
STREX (succeeds)
Though this may be an unlikely sequence.
--
Catalin
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