linux-kernel - Re: [PATCH 1/3] stacktrace: Remove superfluous WARN_ONCE() from save_stack_trace_tsk

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Date:   Fri, 31 May 2019 14:27:51 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Miroslav Benes <mbenes@...e.cz>
Cc:     Petr Mladek <pmladek@...e.com>, Jiri Kosina <jikos@...nel.org>,
        Joe Lawrence <joe.lawrence@...hat.com>,
        Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>,
        live-patching@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] stacktrace: Remove superfluous WARN_ONCE() from
 save_stack_trace_tsk_reliable()

On Fri, May 31, 2019 at 02:25:15PM +0200, Miroslav Benes wrote:
> On Fri, 31 May 2019, Petr Mladek wrote:
> 
> > WARN_ONCE() in the generic save_stack_trace_tsk_reliable() is superfluous.
> > 
> > The information is passed also via the return value. The only current
> > user klp_check_stack() writes its own warning when the reliable stack
> > traces are not supported. Other eventual users might want its own error
> > handling as well.
> > 
> > Signed-off-by: Petr Mladek <pmladek@...e.com>
> > Acked-by: Miroslav Benes <mbenes@...e.cz>
> > Reviewed-by: Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>
> > ---
> >  kernel/stacktrace.c | 1 -
> >  1 file changed, 1 deletion(-)
> > 
> > diff --git a/kernel/stacktrace.c b/kernel/stacktrace.c
> > index 5667f1da3ede..8d088408928d 100644
> > --- a/kernel/stacktrace.c
> > +++ b/kernel/stacktrace.c
> > @@ -259,7 +259,6 @@ __weak int
> >  save_stack_trace_tsk_reliable(struct task_struct *tsk,
> >  			      struct stack_trace *trace)
> >  {
> > -	WARN_ONCE(1, KERN_INFO "save_stack_tsk_reliable() not implemented yet.\n");
> >  	return -ENOSYS;
> >  }
> 
> Do we even need the weak function now after Thomas' changes to 
> kernel/stacktrace.c?
> 
> - livepatch is the only user and it calls stack_trace_save_tsk_reliable()
> - x86 defines CONFIG_ARCH_STACKWALK and CONFIG_HAVE_RELIABLE_STACKTRACE, 
>   so it has stack_trace_save_tsk_reliable() implemented and it calls 
>   arch_stack_walk_reliable()
> - powerpc defines CONFIG_HAVE_RELIABLE_STACKTRACE and does not have 
>   CONFIG_ARCH_STACKWALK. It also has stack_trace_save_tsk_reliable() 
>   implemented and it calls save_stack_trace_tsk_reliable(), which is 
>   implemented in arch/powerpc/
> - all other archs do not have CONFIG_HAVE_RELIABLE_STACKTRACE and there is 
>   stack_trace_save_tsk_reliable() returning ENOSYS for these cases in 
>   include/linux/stacktrace.c

I think you're right.  stack_trace_save_tsk_reliable() in stacktrace.h
returning -ENOSYS serves the same purpose as the old weak version of
save_stack_trace_tsk_reliable() which is no longer called directly.

-- 
Josh