linux-kernel - Re: This patch triggers a bad gcc bug (was Re: [PATCH] force inlining of some byteswap operations)

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Message-ID: <20160419085221.GA29087@gmail.com>
Date:	Tue, 19 Apr 2016 10:52:21 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Josh Poimboeuf <jpoimboe@...hat.com>
Cc:	Denys Vlasenko <dvlasenk@...hat.com>,
	James Bottomley <James.Bottomley@...senPartnership.com>,
	Thomas Graf <tgraf@...g.ch>,
	Peter Zijlstra <peterz@...radead.org>,
	David Rientjes <rientjes@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, Arnd Bergmann <arnd@...db.de>,
	linux-scsi <linux-scsi@...r.kernel.org>, jamborm@....gnu.org
Subject: Re: This patch triggers a bad gcc bug (was Re: [PATCH] force
 inlining of some byteswap operations)


* Josh Poimboeuf <jpoimboe@...hat.com> wrote:

> On Sat, Apr 16, 2016 at 11:03:32AM +0200, Ingo Molnar wrote:
> > 
> > * Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> > 
> > > > 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.
> > 
> > So it appears to me that due to the hard to detect nature of the GCC bug the fix 
> > will probably be backported by them, so I think we should be fine with relying on 
> > objtool to detect weird code sequences in the kernel, and should work around 
> > specific instances of the bug.
> 
> I agree.  So how should we work around the bug in this case?  There have
> been several suggestions:
> 
> - change wwn_to_u64() to __always_inline
> 
> - change qla2x00_get_host_fabric_name() to skip the unnecessary call to
>   wwn_to_u64()
> 
> - revert one of the two commits:
>   bc27fb68aaad ("include/uapi/linux/byteorder, swab: force inlining of some byteswap operations")
>   ef3fb2422ffe ("scsi: fc: use get/put_unaligned64 for wwn access")

The first option sounds like the best one by far: it does a change that is related 
to the GCC bug (tweaks inlining), has near zero impact and does not revert other 
useful progress.

Thanks,

	Ingo