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Message-ID: <df2e0758-b8ed-5aec-6adc-a18f499c0179@redhat.com>
Date: Thu, 22 Oct 2020 10:48:59 +0200
From: David Hildenbrand <david@...hat.com>
To: David Laight <David.Laight@...LAB.COM>,
Greg KH <gregkh@...uxfoundation.org>,
Al Viro <viro@...iv.linux.org.uk>,
Nick Desaulniers <ndesaulniers@...gle.com>
Cc: Christoph Hellwig <hch@....de>,
"kernel-team@...roid.com" <kernel-team@...roid.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Jens Axboe <axboe@...nel.dk>, Arnd Bergmann <arnd@...db.de>,
David Howells <dhowells@...hat.com>,
"linux-arm-kernel@...ts.infradead.org"
<linux-arm-kernel@...ts.infradead.org>,
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Subject: Re: Buggy commit tracked to: "Re: [PATCH 2/9] iov_iter: move
rw_copy_check_uvector() into lib/iov_iter.c"
On 22.10.20 10:40, David Laight wrote:
> From: David Hildenbrand
>> Sent: 22 October 2020 09:35
>>
>> On 22.10.20 10:26, Greg KH wrote:
>>> On Thu, Oct 22, 2020 at 12:39:14AM +0100, Al Viro wrote:
>>>> On Wed, Oct 21, 2020 at 06:13:01PM +0200, Greg KH wrote:
>>>>> On Fri, Sep 25, 2020 at 06:51:39AM +0200, Christoph Hellwig wrote:
>>>>>> From: David Laight <David.Laight@...LAB.COM>
>>>>>>
>>>>>> This lets the compiler inline it into import_iovec() generating
>>>>>> much better code.
>>>>>>
>>>>>> Signed-off-by: David Laight <david.laight@...lab.com>
>>>>>> Signed-off-by: Christoph Hellwig <hch@....de>
>>>>>> ---
>>>>>> fs/read_write.c | 179 ------------------------------------------------
>>>>>> lib/iov_iter.c | 176 +++++++++++++++++++++++++++++++++++++++++++++++
>>>>>> 2 files changed, 176 insertions(+), 179 deletions(-)
>>>>>
>>>>> Strangely, this commit causes a regression in Linus's tree right now.
>>>>>
>>>>> I can't really figure out what the regression is, only that this commit
>>>>> triggers a "large Android system binary" from working properly. There's
>>>>> no kernel log messages anywhere, and I don't have any way to strace the
>>>>> thing in the testing framework, so any hints that people can provide
>>>>> would be most appreciated.
>>>>
>>>> It's a pure move - modulo changed line breaks in the argument lists
>>>> the functions involved are identical before and after that (just checked
>>>> that directly, by checking out the trees before and after, extracting two
>>>> functions in question from fs/read_write.c and lib/iov_iter.c (before and
>>>> after, resp.) and checking the diff between those.
>>>>
>>>> How certain is your bisection?
>>>
>>> The bisection is very reproducable.
>>>
>>> But, this looks now to be a compiler bug. I'm using the latest version
>>> of clang and if I put "noinline" at the front of the function,
>>> everything works.
>>
>> Well, the compiler can do more invasive optimizations when inlining. If
>> you have buggy code that relies on some unspecified behavior, inlining
>> can change the behavior ... but going over that code, there isn't too
>> much action going on. At least nothing screamed at me.
>
> Apart from all the optimisations that get rid off the 'pass be reference'
> parameters and strange conditional tests.
> Plenty of scope for the compiler getting it wrong.
> But nothing even vaguely illegal.
Not the first time that people blame the compiler to then figure out
that something else is wrong ... but maybe this time is different :)
--
Thanks,
David / dhildenb
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