Re: [PATCH v2 02/43] powerpc/traps: Declare unrecoverable_exception() as __noreturn

[Christophe Leroy]

Le 10/03/2021 à 02:22, Nicholas Piggin a écrit :
> Excerpts from Christophe Leroy's message of March 9, 2021 10:09 pm:
> > unrecoverable_exception() is never expected to return, most callers
> > have an infiniteloop in case it returns.
> >
> > Ensure it really never returns by terminating it with a BUG(), and
> > declare it __no_return.
> >
> > It always GCC to really simplify functions calling it. In the exemple
> > below, it avoids the stack frame in the likely fast path and avoids
> > code duplication for the exit.
> >
> > With this patch:
>
> [snip]
>
> Nice.
>
> > diff --git a/arch/powerpc/kernel/traps.c b/arch/powerpc/kernel/traps.c
> > index a44a30b0688c..d5c9d9ddd186 100644
> > --- a/arch/powerpc/kernel/traps.c
> > +++ b/arch/powerpc/kernel/traps.c
> > @@ -2170,11 +2170,15 @@ DEFINE_INTERRUPT_HANDLER(SPEFloatingPointRoundException)
> >    * in the MSR is 0.  This indicates that SRR0/1 are live, and that
> >    * we therefore lost state by taking this exception.
> >    */
> > -void unrecoverable_exception(struct pt_regs *regs)
> > +void __noreturn unrecoverable_exception(struct pt_regs *regs)
> >   {
> >   	pr_emerg("Unrecoverable exception %lx at %lx (msr=%lx)\n",
> >   		 regs->trap, regs->nip, regs->msr);
> >   	die("Unrecoverable exception", regs, SIGABRT);
> > +	/* die() should not return */
> > +	WARN(true, "die() unexpectedly returned");
> > +	for (;;)
> > +		;
> >   }
>
> I don't think the WARN should be added because that will cause another
> interrupt after something is already badly wrong, so this might just
> make it harder to debug.
>
> For example if die() is falling through for some reason, we warn and
> cause a program check here, and that might also be unrecoverable so it
> might come through here and fall through again and warn again, etc.
>
> Putting the infinite loop is good enough I think (and better than there
> was previously).

Ok, dropped the WARN()

> Otherwise
>
> Reviewed-by: Nicholas Piggin <npiggin@xxxxxxxxx>
>
> Thanks,
> Nick