linux-kernel - Re: [PATCH] powerpc/bug: Remove specific powerpc BUG

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Date:   Thu, 11 Feb 2021 08:30:59 -0600
From:   Segher Boessenkool <segher@...nel.crashing.org>
To:     Christophe Leroy <christophe.leroy@...roup.eu>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>, npiggin@...il.com,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] powerpc/bug: Remove specific powerpc BUG_ON()

On Thu, Feb 11, 2021 at 03:09:43PM +0100, Christophe Leroy wrote:
> Le 11/02/2021 à 12:49, Segher Boessenkool a écrit :
> >On Thu, Feb 11, 2021 at 07:41:52AM +0000, Christophe Leroy wrote:
> >>powerpc BUG_ON() is based on using twnei or tdnei instruction,
> >>which obliges gcc to format the condition into a 0 or 1 value
> >>in a register.
> >
> >Huh?  Why is that?
> >
> >Will it work better if this used __builtin_trap?  Or does the kernel only
> >detect very specific forms of trap instructions?
> 
> We already made a try with __builtin_trap() 1,5 year ago, see 
> https://patchwork.ozlabs.org/project/linuxppc-dev/patch/20510ce03cc9463f1c9e743c1d93b939de501b53.1566219503.git.christophe.leroy@c-s.fr/
> 
> The main problems encountered are:
> - It is only possible to use it for BUG_ON, not for WARN_ON because GCC 
> considers it as noreturn. Is there any workaround ?

A trap is noreturn by definition:

 -- Built-in Function: void __builtin_trap (void)
     This function causes the program to exit abnormally.

> - The kernel (With CONFIG_DEBUG_BUGVERBOSE) needs to be able to identify 
> the source file and line corresponding to the trap. How can that be done 
> with __builtin_trap() ?

The DWARF debug info should be sufficient.  Perhaps you can post-process
some way?

You can create a trap that falls through yourself (by having a trap-on
condition with a condition that is always true, but make the compiler
not see that).  This isn't efficient though.

Could you file a feature request (in bugzilla)?  It is probably useful
for generic code as well, but we could implement this for powerpc only
if needed.


Segher