Re: BUG : PowerPC RCU: torture test failed with __stack_chk_fail

[Boqun Feng <boqun.feng@gmail.com>]

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