On Mon, Apr 24, 2023 at 12:29:00PM -0500, Segher Boessenkool wrote:
> On Mon, Apr 24, 2023 at 08:28:55AM -0700, Boqun Feng wrote:
> > On Mon, Apr 24, 2023 at 10:13:51AM -0500, Segher Boessenkool wrote:
> > > At what points can r13 change? Only when some particular functions are
> > > called?
> >
> > r13 is the local paca:
> >
> > register struct paca_struct *local_paca asm("r13");
> >
> > , which is a pointer to percpu data.
>
> Yes, it is a global register variable.
>
> > So if a task schedule from one CPU to anotehr CPU, the value gets
> > changed.
>
> But the compiler does not see that something else changes local_paca (or
It's more like this, however, in this case r13 is not changed:
CPU 0 CPU 1
{r13 = 0x00} {r13 = 0x04}
<thread 1>
<in interrupt>
_switch():
<switch to the stack of thread 2>
<no need to change r13>
<in thread 2>
<thread 2>
<thread 3>
_switch():
<switch to the stack of thread 1>
<no need to change r13>
<in thread 1>
<thread 1>
as you can see thread 1 schedules from CPU 0 to CPU 1 and neither CPU
changes its r13, but in the point of view for thread 1, its r13 changes.
> r13 some other way, via assembler code perhaps)? Or is there a compiler
> bug?
>
This looks to me a compiler bug, but I'm not 100% sure.
Regards,
Boqun
> If the latter is true:
>
> Can you make a reproducer and open a GCC PR? <https://gcc.gnu.org/bugs/>
> for how to get started doing that. We need *exact* code that shows the
> problem, together with a compiler command line. So that we can
> reproduce the problem. That is step 0 in figuring out what is going on,
> and then maybe fixing the problem :-)
>
>
> Segher
