linux-kernel - Re: [PATCH 2/3] asm-generic, x86: wrap atomic operations

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Date:   Wed, 22 Mar 2017 11:42:48 +0100
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Mark Rutland <mark.rutland@....com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Will Deacon <will.deacon@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        kasan-dev <kasan-dev@...glegroups.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "x86@...nel.org" <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/3] asm-generic, x86: wrap atomic operations

On Tue, Mar 21, 2017 at 10:20 PM, Arnd Bergmann <arnd@...db.de> wrote:
> On Tue, Mar 21, 2017 at 7:06 PM, Dmitry Vyukov <dvyukov@...gle.com> wrote:
>> On Tue, Mar 21, 2017 at 11:41 AM, Mark Rutland <mark.rutland@....com> wrote:
>>> On Tue, Mar 21, 2017 at 12:25:06PM +0300, Andrey Ryabinin wrote:
>>
>> I don't mind changing READ_ONCE_NOCHECK to READ_ONCE. But I don't have
>> strong preference either way.
>>
>> We could do:
>> #define arch_atomic_read_is_already_instrumented 1
>> and then skip instrumentation in asm-generic if it's defined. But I
>> don't think it's worth it.
>>
>> There is no functional difference, it's only an optimization (now
>> somewhat questionable). As Andrey said, one can get a splash of
>> reports anyway, and it's the first one that is important. We use KASAN
>> with panic_on_warn=1 so we don't even see the rest.
>
> I'm getting couple of new stack size warnings that are all the result
> of the _NOCHECK.
>
> /git/arm-soc/mm/page_alloc.c: In function 'show_free_areas':
> /git/arm-soc/mm/page_alloc.c:4685:1: error: the frame size of 3368
> bytes is larger than 3072 bytes [-Werror=frame-larger-than=]
>  }
> /git/arm-soc/lib/atomic64_test.c: In function 'test_atomic':
> /git/arm-soc/lib/atomic64_test.c:148:1: error: the frame size of 6528
> bytes is larger than 3072 bytes [-Werror=frame-larger-than=]
>  }
>  ^
> /git/arm-soc/lib/atomic64_test.c: In function 'test_atomic64':
> /git/arm-soc/lib/atomic64_test.c:243:1: error: the frame size of 7112
> bytes is larger than 3072 bytes [-Werror=frame-larger-than=]
>
> This is with my previous set of patches already applied, so
> READ_ONCE should not cause problems. Reverting
> the READ_ONCE_NOCHECK() in atomic_read() and atomic64_read()
> back to READ_ONCE()
>
> I also get a build failure as a result of your patch, but this one is
> not addressed by using READ_ONCE():
>
> In file included from /git/arm-soc/arch/x86/include/asm/atomic.h:7:0,
>                  from /git/arm-soc/include/linux/atomic.h:4,
>                  from /git/arm-soc/arch/x86/include/asm/thread_info.h:53,
>                  from /git/arm-soc/include/linux/thread_info.h:25,
>                  from /git/arm-soc/arch/x86/include/asm/preempt.h:6,
>                  from /git/arm-soc/include/linux/preempt.h:80,
>                  from /git/arm-soc/include/linux/spinlock.h:50,
>                  from /git/arm-soc/include/linux/mmzone.h:7,
>                  from /git/arm-soc/include/linux/gfp.h:5,
>                  from /git/arm-soc/include/linux/mm.h:9,
>                  from /git/arm-soc/mm/slub.c:12:
> /git/arm-soc/mm/slub.c: In function '__slab_free':
> /git/arm-soc/arch/x86/include/asm/cmpxchg.h:174:2: error: 'asm'
> operand has impossible constraints
>   asm volatile(pfx "cmpxchg%c4b %2; sete %0"   \
>   ^
> /git/arm-soc/arch/x86/include/asm/cmpxchg.h:183:2: note: in expansion
> of macro '__cmpxchg_double'
>   __cmpxchg_double(LOCK_PREFIX, p1, p2, o1, o2, n1, n2)
>   ^~~~~~~~~~~~~~~~
> /git/arm-soc/include/asm-generic/atomic-instrumented.h:236:2: note: in
> expansion of macro 'arch_cmpxchg_double'
>   arch_cmpxchg_double(____p1, (p2), (o1), (o2), (n1), (n2)); \
>   ^~~~~~~~~~~~~~~~~~~
> /git/arm-soc/mm/slub.c:385:7: note: in expansion of macro 'cmpxchg_double'
>    if (cmpxchg_double(&page->freelist, &page->counters,
>        ^~~~~~~~~~~~~~
> /git/arm-soc/scripts/Makefile.build:308: recipe for target 'mm/slub.o' failed
>
> http://pastebin.com/raw/qXVpi9Ev has the defconfig file I used, and I get the
> error with any gcc version I tried (4.9 through 7.0.1).


Initially I've tested with my stock gcc 4.8.4 (Ubuntu
4.8.4-2ubuntu1~14.04.3) and amusingly it works. But I can reproduce
the bug with 7.0.1.
Filed https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80148
Will think about kernel fix.
Thanks!