linux-kernel - Re: [PATCH] qla2xxx: rewrite code to avoid hitting gcc bug 70646

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Date:	Fri, 15 Apr 2016 12:05:26 -0700
From:	James Bottomley <James.Bottomley@...senPartnership.com>
To:	Denys Vlasenko <dvlasenk@...hat.com>,
	Himanshu Madhani <himanshu.madhani@...gic.com>
Cc:	qla2xxx-upstream@...gic.com, Josh Poimboeuf <jpoimboe@...hat.com>,
	linux-scsi@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] qla2xxx: rewrite code to avoid hitting gcc bug 70646

On Fri, 2016-04-15 at 20:56 +0200, Denys Vlasenko wrote:
> On 04/15/2016 04:40 PM, James Bottomley wrote:
> > On Fri, 2016-04-15 at 12:36 +0200, Denys Vlasenko wrote:
> > > More info here:
> > > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70646
> > 
> > This bug is under investigation, so I'd rather not alter code for a
> > gcc
> > bug until we know if we can supply options to fix it rather than
> > changing code.
> 
> 
> Background. The bug exists in gcc for 2 years, but it is rather
> hard to trigger, so nobody noticed.

We know this ... linux-scsi is on the cc for the other thread on this.

> Unfortunately for kernel, these two commits landed in Linus tree
> in March 16 and 17:
> 
> 
> On 04/13/2016 05:36 AM, Josh Poimboeuf wrote:
> > It occurs with the combination of the following two recent commits:
> > 
> > - bc27fb68aaad ("include/uapi/linux/byteorder, swab: force inlining
> > of some byteswap operations")
> > - ef3fb2422ffe ("scsi: fc: use get/put_unaligned64 for wwn access")
> 
> 
> and now *many* users of qla2x00 and new-ish gcc are going to
> very much notice it, as their kernels will start crashing reliably.
> 
> The commits can be reverted, sure, but they per se do not contain
> anything unusual. They, together with not very typical construct
> in qla2x00_get_host_fabric_name, one
> which boils down to "swab64p(constant_array_of_8_bytes)",
> just happen to nudge gcc in a right way to finally trigger the bug.
> 
> So I came with another idea how to forestall the imminent deluge of
> qla2x00 oops reports - this patch.

There are actually a raft of checkers that run the upstream code which
aren't seeing any problem; likely because the code is harder to trigger
than you think.  So, lets wait until the resolution of the other thread
before we panic, especially since we're only at -rc3.

James