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From: Masahiro Yamada <masahiroy@kernel.org>
To: Arnd Bergmann <arnd@arndb.de>
Cc: Joe Perches <joe@perches.com>, Will Deacon <will@kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	linux-arch <linux-arch@vger.kernel.org>,
	Android Kernel Team <kernel-team@android.com>,
	Michael Ellerman <mpe@ellerman.id.au>,
	Peter Zijlstra <peterz@infradead.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Segher Boessenkool <segher@kernel.crashing.org>,
	Christian Borntraeger <borntraeger@de.ibm.com>,
	Luc Van Oostenryck <luc.vanoostenryck@gmail.com>
Subject: Re: [RFC PATCH 1/8] compiler/gcc: Emit build-time warning for GCC prior to version 4.8
Date: Tue, 14 Jan 2020 00:35:35 +0900
Message-ID: <CAK7LNARugZUWNF_1e70ZCyK5t_e4MFXnnw1PkmaRWQioQSPkEA@mail.gmail.com> (raw)
In-Reply-To: <CAK8P3a1t9757M5CRKNX_X+T9VuLX+5=z5_845rLJGTG50RogXA@mail.gmail.com>

On Mon, Jan 13, 2020 at 11:40 PM Arnd Bergmann <arnd@arndb.de> wrote:
>
> On Fri, Jan 10, 2020 at 6:54 PM Joe Perches <joe@perches.com> wrote:
> >
> > On Fri, 2020-01-10 at 18:35 +0100, Arnd Bergmann wrote:
> > > On Fri, Jan 10, 2020 at 5:56 PM Will Deacon <will@kernel.org> wrote:
> > > > Prior to version 4.8, GCC may miscompile READ_ONCE() by erroneously
> > > > discarding the 'volatile' qualifier:
> > > >
> > > > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58145
> > > >
> > > > We've been working around this using some nasty hacks which make
> > > > READ_ONCE() both horribly complicated and also prevent us from enforcing
> > > > that it is only used on scalar types. Since GCC 4.8 is pretty old for
> > > > kernel builds now, emit a warning if we detect it during the build.
> > >
> > > No objection to recommending gcc-4.8, but I think this should either
> > > just warn once during the kernel build instead of for every file, or
> > > it should become a hard requirement.
> >
> > It might as well be a hard requirement as
> > gcc 4.8.0 is already nearly 7 years old.
> >
> > gcc 4.6.0 released 2011-03-25
> > gcc 4.8.0 released 2013-03-22
> >
> > Perhaps there are exceedingly few to zero new
> > instances using gcc compiler versions < 4.8
>
> The last time we had this discussion, the result was that gcc-4.8 is
> probably ok as a minimum version, but moving to 5.1+ (from 2015)
> was not an obvious choice:
>
> https://www.spinics.net/lists/linux-kbuild/msg23648.html
>
> If nobody complains about the move to 4.8, we can try moving to
> gcc-5.1 and GNU99/GNU11 next year  ;-)
>
>        Arnd


I agree.

"Your compiler is old and may miscompile the kernel due to
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58145 - please upgrade it."
sounds like an error rather than a warning.



-- 
Best Regards
Masahiro Yamada

  reply index

Thread overview: 32+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-01-10 16:56 [RFC PATCH 0/8] Rework READ_ONCE() to improve codegen Will Deacon
2020-01-10 16:56 ` [RFC PATCH 1/8] compiler/gcc: Emit build-time warning for GCC prior to version 4.8 Will Deacon
2020-01-10 17:35   ` Arnd Bergmann
2020-01-10 17:53     ` Joe Perches
2020-01-13 14:39       ` Arnd Bergmann
2020-01-13 15:35         ` Masahiro Yamada [this message]
2020-01-13 14:27     ` Will Deacon
2020-01-14 21:39     ` Nick Desaulniers
2020-01-15 10:35       ` David Laight
2020-01-15 10:49       ` Arnd Bergmann
2020-01-10 16:56 ` [RFC PATCH 2/8] netfilter: Avoid assigning 'const' pointer to non-const pointer Will Deacon
2020-01-10 16:56 ` [RFC PATCH 3/8] fault_inject: Don't rely on "return value" from WRITE_ONCE() Will Deacon
2020-01-10 16:56 ` [RFC PATCH 4/8] READ_ONCE: Simplify implementations of {READ,WRITE}_ONCE() Will Deacon
2020-01-10 16:56 ` [RFC PATCH 5/8] READ_ONCE: Enforce atomicity for {READ,WRITE}_ONCE() memory accesses Will Deacon
2020-01-10 19:24   ` Arnd Bergmann
2020-01-13 16:16     ` Will Deacon
2020-01-10 16:56 ` [RFC PATCH 6/8] READ_ONCE: Drop pointer qualifiers when reading from scalar types Will Deacon
2020-01-10 18:54   ` Linus Torvalds
2020-01-13 14:59     ` Will Deacon
2020-01-13 17:42       ` Luc Van Oostenryck
2020-01-13 19:31       ` Linus Torvalds
2020-01-10 16:56 ` [RFC PATCH 7/8] locking/barriers: Use '__unqual_scalar_typeof' for load-acquire macros Will Deacon
2020-01-10 19:42   ` Arnd Bergmann
2020-01-13 15:01     ` Will Deacon
2020-01-10 16:56 ` [RFC PATCH 8/8] arm64: barrier: Use '__unqual_scalar_typeof' for acquire/release macros Will Deacon
2020-01-10 17:45   ` Mark Rutland
2020-01-10 17:58 ` [RFC PATCH 0/8] Rework READ_ONCE() to improve codegen Arnd Bergmann
2020-01-10 19:46 ` Arnd Bergmann
2020-01-10 20:14   ` Linus Torvalds
2020-01-13 13:03     ` Arnd Bergmann
2020-01-13 11:20 ` David Laight
2020-01-13 12:40 ` Christian Borntraeger

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