linux-kernel - Re: [PATCH] scsi: fc: force inlining of wwn conversion functions

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Message-ID: <1461710218.2348.27.camel@HansenPartnership.com>
Date:	Tue, 26 Apr 2016 15:36:58 -0700
From:	James Bottomley <James.Bottomley@...senPartnership.com>
To:	Arnd Bergmann <arnd@...db.de>,
	"Martin K. Petersen" <martin.petersen@...cle.com>
Cc:	Josh Poimboeuf <jpoimboe@...hat.com>, linux-scsi@...r.kernel.org,
	linux-kernel@...r.kernel.org, Denys Vlasenko <dvlasenk@...hat.com>,
	Thomas Graf <tgraf@...g.ch>,
	Peter Zijlstra <peterz@...radead.org>,
	David Rientjes <rientjes@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>, jamborm@....gnu.org,
	Ingo Molnar <mingo@...nel.org>,
	Himanshu Madhani <himanshu.madhani@...gic.com>,
	qla2xxx-upstream@...gic.com
Subject: Re: [PATCH] scsi: fc: force inlining of wwn conversion functions

On Tue, 2016-04-26 at 17:58 +0200, Arnd Bergmann wrote:
> On Tuesday 26 April 2016 09:06:54 Martin K. Petersen wrote:
> > > > > > > "Arnd" == Arnd Bergmann <arnd@...db.de> writes:
> > 
> > Arnd> I don't think we can realistically blacklist gcc
> > -4.9.{0,1,2,3},
> > Arnd> gcc-5.{0,1,2,3}.* and gcc-6.0 and require everyone to upgrade
> > to
> > Arnd> compilers that have not been released yet in order to build a
> > Arnd> linux-4.6 kernel.
> > 
> > I agree that compiler blacklisting is problematic and I'd like to 
> > avoid it. The question is how far we go in the kernel to 
> > accommodate various levels of brokenness.
> > 
> > In any case. Sticking compiler workarounds in device driver code is 
> > akin to putting demolition orders on display on Alpha Centauri. At 
> > the very minimum the patch should put a fat comment in the code 
> > stating that these wrapper functions or #defines should not be 
> > changed in the future because that'll break builds using gcc XYZ. 
> > But that does not solve the problem for anybody else that might be 
> > doing something similar. Converting between u64 and $RANDOM_TYPE in 
> > an inline wrapper does not seem like a rare and unusual programming
> > pattern.
> 
> It's not the driver really, it's the core scsi/fc layer, which makes
> it a little dangerous that a random driver.

Well, it's libfc; that's a fibre channel transport class mostly used by
FCoE drivers ... there's few enough of those to call it driver only.

> I agree that putting a comment in would also help. What I understand
> from the bug report is that to trigger this bug you need these
> elements:
> 
> 1. an inline function marked __always_inline
> 2. another inline function that is automatically inlined (not
> __always_inline)
> 3. CONFIG_OPTIMIZE_INLINING=y to guarantee 2
> 4. __builtin_compatible_p inside that inline function
> 
> The last point is what Denys introduced in the kernel with
> bc27fb68aaad ("include/uapi/linux/byteorder, swab: force inlining of 
> some byteswap operations"). So maybe it's better after all to revert 
> that patch, to have a higher confidence in the same bug not appearing
> elsewhere. It's also really a workaround for another quirk of the
> compiler, but that one only results in duplicated functions in object
> code rather than functions that end in the middle.

Yes, I think this is my preferred option.  That patch is nothing more
than an attempt to force the compiler to do something it didn't do but
should have.  If we apply the general rule that we shouldn't work
around compiler problems in the kernel code, then that should have been
disallowed first.  Plus, as the root cause of all of this, reverting
that patch will ensure that nothing else picks up this problem (at
least from the route we got it).

James