linux-kernel - Re: [GIT PULL] io_uring updates for 5.10-rc1

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Date:   Tue, 13 Oct 2020 22:49:00 +0200
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     Jens Axboe <axboe@...nel.dk>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     io-uring <io-uring@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [GIT PULL] io_uring updates for 5.10-rc1

On 13/10/2020 21.49, Jens Axboe wrote:
> On 10/13/20 1:46 PM, Linus Torvalds wrote:
>> On Mon, Oct 12, 2020 at 6:46 AM Jens Axboe <axboe@...nel.dk> wrote:
>>>
>>> Here are the io_uring updates for 5.10.
>>
>> Very strange. My clang build gives a warning I've never seen before:
>>
>>    /tmp/io_uring-dd40c4.s:26476: Warning: ignoring changed section
>> attributes for .data..read_mostly
>>
>> and looking at what clang generates for the *.s file, it seems to be
>> the "section" line in:
>>
>>         .type   io_op_defs,@object      # @io_op_defs
>>         .section        .data..read_mostly,"a",@progbits
>>         .p2align        4
>>
>> I think it's the combination of "const" and "__read_mostly".
>>
>> I think the warning is sensible: how can a piece of data be both
>> "const" and "__read_mostly"? If it's "const", then it's not "mostly"
>> read - it had better be _always_ read.
>>
>> I'm letting it go, and I've pulled this (gcc doesn't complain), but
>> please have a look.
> 
> Huh weird, I'll take a look. FWIW, the construct isn't unique across
> the kernel.

Citation needed. There's lots of "pointer to const foo" stuff declared
as __read_mostly, but I can't find any objects that are themselves both
const and __read_mostly. Other than that io_op_defs and io_uring_fops now.

But... there's something a little weird:

$ grep read_most -- fs/io_uring.s
        .section        .data..read_mostly,"a",@progbits
$ readelf --wide -S fs/io_uring.o | grep read_most
  [32] .data..read_mostly PROGBITS        0000000000000000 01b4e0 000188
00  WA  0   0 32

(this is with gcc/gas). So despite that .section directive not saying
"aw", the section got the W flag anyway. There are lots of

        .section "__tracepoints_ptrs", "a"
      .pushsection .smp_locks,"a"

in the .s file, and those sections do end up with just the A bit in the
.o file. Does gas maybe somehow special-case a section name starting
with .data?

Rasmus