Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752607AbcDOWUo (ORCPT ); Fri, 15 Apr 2016 18:20:44 -0400 Received: from mx1.redhat.com ([209.132.183.28]:51641 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751878AbcDOWUm (ORCPT ); Fri, 15 Apr 2016 18:20:42 -0400 Date: Fri, 15 Apr 2016 17:20:37 -0500 From: Josh Poimboeuf To: Ingo Molnar Cc: Denys Vlasenko , James Bottomley , Thomas Graf , Peter Zijlstra , David Rientjes , Andrew Morton , linux-kernel@vger.kernel.org, Arnd Bergmann , linux-scsi , jamborm@gcc.gnu.org Subject: Re: This patch triggers a bad gcc bug (was Re: [PATCH] force inlining of some byteswap operations) Message-ID: <20160415222037.wu4n6w2yedxdzfle@treble> References: <570E37A9.2050503@redhat.com> <20160413123607.3y3v5esq3myto4kc@treble> <20160413151500.tate3u6trg4agugo@treble> <1460566509.2322.14.camel@HansenPartnership.com> <20160413171058.24tnvvquvnipnwnd@treble> <570FB742.2010309@redhat.com> <20160414155708.upka6phfvwpbp6zw@treble> <570FCEB7.1070200@redhat.com> <20160415054518.GA28079@gmail.com> <20160415134745.wxb73xemv2tqib5m@treble> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20160415134745.wxb73xemv2tqib5m@treble> User-Agent: Mutt/1.6.0.1 (2016-04-01) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2495 Lines: 63 On Fri, Apr 15, 2016 at 08:47:45AM -0500, Josh Poimboeuf wrote: > On Fri, Apr 15, 2016 at 07:45:19AM +0200, Ingo Molnar wrote: > > > > * Denys Vlasenko wrote: > > > > > > In fact, the following patch seems to fix it: > > > > > > > > diff --git a/include/scsi/scsi_transport_fc.h b/include/scsi/scsi_transport_fc.h > > > > index bf66ea6..56b9e81 100644 > > > > --- a/include/scsi/scsi_transport_fc.h > > > > +++ b/include/scsi/scsi_transport_fc.h > > > > @@ -796,7 +796,7 @@ fc_remote_port_chkready(struct fc_rport *rport) > > > > return result; > > > > } > > > > > > > > -static inline u64 wwn_to_u64(u8 *wwn) > > > > +static __always_inline u64 wwn_to_u64(u8 *wwn) > > > > { > > > > return get_unaligned_be64(wwn); > > > > } > > > > > > It is not a guarantee. > > > > Of course it's a workaround - but is there any deterministic way to turn off this > > GCC bug (by activating some GCC command line switch), or do we have to live with > > objtool warning about this GCC? > > I don't think we know yet if there's a reliable way to turn the bug off. > > Also, according to the gcc guys, this bug won't always result in a > truncated function, and may sometimes just make some inline function > call sites disappear: > > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70646#c14 > > though I haven't been able to confirm that experimentally. But if it's > true, that means that objtool won't be able to detect all cases of the > bug and some function calls may just silently disappear! > > There's a lot of activity in the bug now, so hopefully they'll be able > to tell us soon if there's a reliable way to avoid it and/or detect it. > > BTW, Denys posted a workaround patch for the qla2xxxx code: > > https://lkml.kernel.org/r/1460716583-15673-1-git-send-email-dvlasenk@redhat.com Martin Jambor wrote a succinct summary of the conditions needed for this bug: "This bug can occur when an inlineable function containing a call to __builtin_constant_p, which checks a parameter or a value it references and a (possibly indirect) caller of the function actually passes a constant, but stores it using a type of a different size." So to prevent it from happening elsewhere in the kernel, it sounds like we'd have to either remove all uses of __builtin_constant_p() or disable inlining completely. There's also no reliable way to detect the bug has occurred, though objtool will detect it in cases when the function gets truncated. -- Josh