Re: [PATCH v2] arm64: Fix task tracing

[Catalin Marinas <catalin.marinas@arm.com>]

On Mon, Apr 15, 2013 at 11:45:42AM +0100, Will Deacon wrote:
> On Mon, Apr 15, 2013 at 11:11:59AM +0100, Catalin Marinas wrote:
> > On Tue, Apr 09, 2013 at 01:33:34PM +0100, Christopher Covington wrote:
> > > For accurate accounting pass contextidr_thread_switch the prev
> > > task pointer, since cpu_switch_to has at that point changed the
> > > the stack pointer.
> > > 
> > > Signed-off-by: Christopher Covington <cov@codeaurora.org>
> > > ---
> > >  arch/arm64/kernel/process.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
> > > index 0337cdb..a49b25a 100644
> > > --- a/arch/arm64/kernel/process.c
> > > +++ b/arch/arm64/kernel/process.c
> > > @@ -315,7 +315,7 @@ struct task_struct *__switch_to(struct task_struct *prev,
> > >  	/* the actual thread switch */
> > >  	last = cpu_switch_to(prev, next);
> > >  
> > > -	contextidr_thread_switch(next);
> > > +	contextidr_thread_switch(prev);
> > 
> > The original code was indeed wrong but using prev isn't any better. For
> > a newly created thread, prev is probably 0 (if it's in a register,
> > cpu_context has been zeroed by copy_thread()) or some random stack
> > value.
> 
> Really? If prev is NULL in context_switch(...), the scheduler will implode,
> and I can't see where else switch_to is called from.
> 
> Which code path are you thinking of?

copy_thread() zeros cpu_context which is used by cpu_switch_to() to load
the next saved registers. The switch_to() function sets prev to last as
returned by __switch_to(), so this is valid but in __switch_to() we
don't have a valid prev (nor next) after cpu_switch_to() for newly
created threads.

-- 
Catalin