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

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Date:	Mon, 18 Apr 2016 08:22:33 -0500
From:	Josh Poimboeuf <jpoimboe@...hat.com>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	Ingo Molnar <mingo@...nel.org>,
	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,
	linux-scsi <linux-scsi@...r.kernel.org>
Subject: Re: This patch triggers a bad gcc bug (was Re: [PATCH] force
 inlining of some byteswap operations)

On Sat, Apr 16, 2016 at 09:42:37AM +0200, Arnd Bergmann wrote:
> On Friday 15 April 2016 07:45:19 Ingo Molnar wrote:
> > 
> > * Denys Vlasenko <dvlasenk@...hat.com> 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?
> > 
> > Which, by the way, is pretty cool!
> 
> I have done a patch for the asm-generic/unaligned handling recently that
> reworks the implementation to avoid an ARM specific bug (gcc uses certain
> CPU instructions that require aligned data when we tell it that unaligned
> data is not).
> 
> It changes the code enough that the gcc bug might not be triggered any more,
> aside from generating far superior code in some cases.

I tried this patch, but unfortunately it doesn't make the gcc bug go
away.

-- 
Josh