netdev - Re: [PATCH 1/3] io_uring: add support for async work inheriting files table

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Date:   Fri, 18 Oct 2019 08:43:48 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Jann Horn <jannh@...gle.com>
Cc:     linux-block@...r.kernel.org,
        "David S. Miller" <davem@...emloft.net>,
        Network Development <netdev@...r.kernel.org>
Subject: Re: [PATCH 1/3] io_uring: add support for async work inheriting files
 table

On 10/18/19 8:40 AM, Jann Horn wrote:
> On Fri, Oct 18, 2019 at 4:37 PM Jens Axboe <axboe@...nel.dk> wrote:
>>
>> On 10/18/19 8:34 AM, Jann Horn wrote:
>>> On Fri, Oct 18, 2019 at 4:01 PM Jens Axboe <axboe@...nel.dk> wrote:
>>>> On 10/17/19 8:41 PM, Jann Horn wrote:
>>>>> On Fri, Oct 18, 2019 at 4:01 AM Jens Axboe <axboe@...nel.dk> wrote:
>>>>>> This is in preparation for adding opcodes that need to modify files
>>>>>> in a process file table, either adding new ones or closing old ones.
>>> [...]
>>>> Updated patch1:
>>>>
>>>> http://git.kernel.dk/cgit/linux-block/commit/?h=for-5.5/io_uring-test&id=df6caac708dae8ee9a74c9016e479b02ad78d436
>>>
>>> I don't understand what you're doing with old_files in there. In the
>>> "s->files && !old_files" branch, "current->files = s->files" happens
>>> without holding task_lock(), but current->files and s->files are also
>>> the same already at that point anyway. And what's the intent behind
>>> assigning stuff to old_files inside the loop? Isn't that going to
>>> cause the workqueue to keep a modified current->files beyond the
>>> runtime of the work?
>>
>> I simply forgot to remove the old block, it should only have this one:
>>
>> if (s->files && s->files != cur_files) {
>>          task_lock(current);
>>          current->files = s->files;
>>          task_unlock(current);
>>          if (cur_files)
>>                  put_files_struct(cur_files);
>>          cur_files = s->files;
>> }
> 
> Don't you still need a put_files_struct() in the case where "s->files
> == cur_files"?

I want to hold on to the files for as long as I can, to avoid unnecessary
shuffling of it. But I take it your worry here is that we'll be calling
something that manipulates ->files? Nothing should do that, unless
s->files is set. We didn't hide the workqueue ->files[] before this
change either.

-- 
Jens Axboe