Re: Buggy commit tracked to: "Re: [PATCH 2/9] iov_iter: move rw_copy_check_uvector() into lib/iov_iter.c"

David Hildenbrand
2020-10-22T08:35:38Z

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@ACULAB.COM>
>>>>
>>>> This lets the compiler inline it into import_iovec() generating
>>>> much better code.
>>>>
>>>> Signed-off-by: David Laight <david.laight@aculab.com>
>>>> Signed-off-by: Christoph Hellwig <hch@lst.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.

--
Thanks,

David / dhildenb