From: Martin Jambor <mjambor@suse.cz>
To: Arnd Bergmann <arnd@arndb.de>
Cc: "Martin K. Petersen" <martin.petersen@oracle.com>,
James Bottomley <James.Bottomley@hansenpartnership.com>,
Josh Poimboeuf <jpoimboe@redhat.com>,
linux-scsi@vger.kernel.org, linux-kernel@vger.kernel.org,
Denys Vlasenko <dvlasenk@redhat.com>, Thomas Graf <tgraf@suug.ch>,
Peter Zijlstra <peterz@infradead.org>,
David Rientjes <rientjes@google.com>,
Andrew Morton <akpm@linux-foundation.org>,
Ingo Molnar <mingo@kernel.org>,
Himanshu Madhani <himanshu.madhani@qlogic.com>,
qla2xxx-upstream@qlogic.com, Jan Hubicka <hubicka@ucw.cz>
Subject: Re: [PATCH] scsi: fc: force inlining of wwn conversion functions
Date: Wed, 27 Apr 2016 13:05:03 +0200 [thread overview]
Message-ID: <20160427110503.GB24887@virgil.suse.cz> (raw)
In-Reply-To: <5298237.1Guzp0G04x@wuerfel>
Hi,
On Tue, Apr 26, 2016 at 05:58:20PM +0200, Arnd Bergmann wrote:
> On Tuesday 26 April 2016 09:06:54 Martin K. Petersen wrote:
> > >>>>> "Arnd" == Arnd Bergmann <arnd@arndb.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.
>
> 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 __always_inline requirement is not true. In fact, if you look at
the example testcase filed in
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70646#c7 you'll see it
uses __builtin_compatible_p in an __always inline function that is
called from one that is not tagged with that attribute.
And generally speaking, always inline is never a requirement, any call
or chain of calls that the inliner can decide to inline can lead to
the bug (if it complies with the condition below).
What is a requirement, though, is that __builtin_compatible_p is
called on something passed in an argument by reference or in an
aggregate (i.e. struct or array) argument.
So,
int foo1 (unsigned long *ref)
{
if (__builtin_constant (*ref))
...
else
/* wrongly unreachable code */
}
can lead to this issue, as can
int foo2 (struct S s)
{
if ((__builtin_constant (s.l))
...
else
/* wrongly unreachable code */
}
but
int foo3 (unsigned long val)
{
if (__builtin_constant (val))
...
else
/* all OK */
}
cannot, and is fine. But please note that wrapping a foo[12]-like
function into a dereferencing wrapper might not help if foo[12] would
be early-inlined into such wrapper (GCC has two inliners, a very
simple early-inliner that only handles simple cases and a full-blown
IPA inliner that contains the bug). I believe this can be ensured by
making the wrapper always_inline and never calling it indirectly (via
a pointer). Honza (CCed), you know inlining heuristics better, please
correct me if my last statement is somehow inaccurate (or indeed if
you have a better idea how kernel developers can make sure they do not
hit the bug).
Thanks,
Martin
>
> 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.
>
> Arnd
>
next prev parent reply other threads:[~2016-04-27 11:05 UTC|newest]
Thread overview: 45+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-02-04 19:45 [PATCH] asm-generic: force inlining of some atomic_long operations Denys Vlasenko
2016-02-04 19:45 ` [PATCH] force inlining of some byteswap operations Denys Vlasenko
2016-02-05 7:28 ` Ingo Molnar
2016-04-13 3:36 ` This patch triggers a bad gcc bug (was Re: [PATCH] force inlining of some byteswap operations) Josh Poimboeuf
2016-04-13 12:12 ` Denys Vlasenko
2016-04-13 12:36 ` Josh Poimboeuf
2016-04-13 15:15 ` Josh Poimboeuf
2016-04-13 16:55 ` James Bottomley
2016-04-13 17:10 ` Josh Poimboeuf
2016-04-14 15:29 ` Denys Vlasenko
2016-04-14 15:57 ` Josh Poimboeuf
2016-04-14 17:09 ` Denys Vlasenko
2016-04-15 5:45 ` Ingo Molnar
2016-04-15 13:47 ` Josh Poimboeuf
2016-04-15 22:20 ` Josh Poimboeuf
2016-04-16 9:03 ` Ingo Molnar
2016-04-18 13:39 ` Josh Poimboeuf
2016-04-18 14:07 ` Arnd Bergmann
2016-04-18 14:12 ` Josh Poimboeuf
2016-04-18 14:21 ` Arnd Bergmann
2016-04-19 8:52 ` Ingo Molnar
2016-04-19 13:56 ` [PATCH] scsi: fc: force inlining of wwn conversion functions Josh Poimboeuf
2016-04-22 23:17 ` Quinn Tran
2016-04-25 16:07 ` Josh Poimboeuf
2016-04-26 2:40 ` Martin K. Petersen
2016-04-26 3:37 ` James Bottomley
2016-04-26 7:22 ` Arnd Bergmann
2016-04-26 8:35 ` Christoph Hellwig
2016-04-26 10:05 ` Arnd Bergmann
2016-04-26 13:06 ` Martin K. Petersen
2016-04-26 15:58 ` Arnd Bergmann
2016-04-26 22:36 ` James Bottomley
2016-04-27 0:44 ` Martin K. Petersen
2016-04-27 11:05 ` Martin Jambor [this message]
2016-04-27 21:34 ` Arnd Bergmann
2016-04-28 14:58 ` Chris Metcalf
2016-04-28 15:23 ` Arnd Bergmann
2016-04-28 15:48 ` Chris Metcalf
2016-04-27 22:00 ` [PATCH, RFT] byteswap: try to avoid __builtin_constant_p gcc bug Arnd Bergmann
2016-04-27 22:11 ` Josh Poimboeuf
2016-04-28 16:27 ` Quinn Tran
2016-04-16 7:42 ` This patch triggers a bad gcc bug (was Re: [PATCH] force inlining of some byteswap operations) Arnd Bergmann
2016-04-18 13:22 ` Josh Poimboeuf
2016-02-04 19:45 ` [PATCH] force inlining of unaligned byteswap operations Denys Vlasenko
2016-02-05 7:28 ` Ingo Molnar
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