Re: [PATCH v2 2/2] powerpc/bug: Provide better flexibility to WARN_ON/__WARN_FLAGS() with asm goto

[Christophe Leroy]

Le 26/08/2021 à 16:45, Michael Ellerman a écrit :
> Christophe Leroy <christophe.leroy@xxxxxxxxxx> writes:
> > Le 26/08/2021 à 05:21, Michael Ellerman a écrit :
> > > Nathan Chancellor <nathan@xxxxxxxxxx> writes:
> > > > On Tue, Apr 13, 2021 at 04:38:10PM +0000, Christophe Leroy wrote:
> > > > > Using asm goto in __WARN_FLAGS() and WARN_ON() allows more
> > > > > flexibility to GCC.
> > > ...
> > > > This patch as commit 1e688dd2a3d6 ("powerpc/bug: Provide better
> > > > flexibility to WARN_ON/__WARN_FLAGS() with asm goto") cause a WARN_ON in
> > > > klist_add_tail to trigger over and over on boot when compiling with
> > > > clang:
> >
> > ...
> >
> > > This patch seems to fix it. Not sure if that's just papering over it though.
> > >
> > > diff --git a/arch/powerpc/include/asm/bug.h b/arch/powerpc/include/asm/bug.h
> > > index 1ee0f22313ee..75fcb4370d96 100644
> > > --- a/arch/powerpc/include/asm/bug.h
> > > +++ b/arch/powerpc/include/asm/bug.h
> > > @@ -119,7 +119,7 @@ __label_warn_on:						\
> > >    								\
> > >    			WARN_ENTRY(PPC_TLNEI " %4, 0",		\
> > >    				   BUGFLAG_WARNING | BUGFLAG_TAINT(TAINT_WARN),	\
> > > -				   __label_warn_on, "r" (x));	\
> > > +				   __label_warn_on, "r" (!!(x))); \
> > >    			break;					\
> > >    __label_warn_on:						\
> > >    			__ret_warn_on = true;			\
> >
> > But for a simple WARN_ON() call:
> >
> > void test(unsigned long b)
> > {
> > 	WARN_ON(b);
> > }
> >
> > Without your change with GCC you get:
> >
> > 00000000000012d0 <.test>:
> >       12d0:	0b 03 00 00 	tdnei   r3,0
> >       12d4:	4e 80 00 20 	blr
> >
> >
> > With the !! change you get:
> >
> > 00000000000012d0 <.test>:
> >       12d0:	31 23 ff ff 	addic   r9,r3,-1
> >       12d4:	7d 29 19 10 	subfe   r9,r9,r3
> >       12d8:	0b 09 00 00 	tdnei   r9,0
> >       12dc:	4e 80 00 20 	blr
>
> Yeah that's a pity.
>
> We could do something like below, which is ugly, but would be better
> than having to revert the whole thing.

Yes looks nice, we already had that kind of stuff in the past, even more ugly.

> Although this doesn't fix the strange warning in drivers/net/ethernet/sfc.

What's the warning ?


> So possibly we need a CLANG ifdef around the whole thing, and use the
> old style warn for clang.

Christophe