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

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Message-ID: <20160415200235.cqsozl445k6evhok@treble>
Date:	Fri, 15 Apr 2016 15:02:35 -0500
From:	Josh Poimboeuf <jpoimboe@...hat.com>
To:	James Bottomley <James.Bottomley@...senPartnership.com>
Cc:	Denys Vlasenko <dvlasenk@...hat.com>,
	Himanshu Madhani <himanshu.madhani@...gic.com>,
	qla2xxx-upstream@...gic.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, Apr 15, 2016 at 12:05:26PM -0700, James Bottomley wrote:
> On Fri, 2016-04-15 at 20:56 +0200, Denys Vlasenko wrote:
> > 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.

Regardless of the outcome of the gcc bug, it seems kind of silly to
byteswap a constant value of 0xffffffffffffffff.

	uint8_t node_name[WWN_SIZE] = { 0xFF, 0xFF, 0xFF, 0xFF, \
		0xFF, 0xFF, 0xFF, 0xFF};
	u64 fabric_name = wwn_to_u64(node_name);

Similar to what Denys suggested, it can just be:

	u64 fabric_name = -1;
or
	u64 fabric_name = 0xffffffffffffffff;

Wouldn't that be an improvement to the code regardless?

-- 
Josh