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Subject: Re: [GIT PULL] io_uring fixes for 5.10-rc
To:     Linus Torvalds <torvalds@linux-foundation.org>,
        Al Viro <viro@zeniv.linux.org.uk>
Cc:     io-uring <io-uring@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
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 <CAHk-=wimYoUtY4ygMNknkKZHqgYBZbkU4Koo5cE6ar8XjHkzGg@mail.gmail.com>
From:   Jens Axboe <axboe@kernel.dk>
Message-ID: <ad8db5d0-2fac-90b6-b9e4-746a52b8ac57@kernel.dk>
Date:   Fri, 20 Nov 2020 19:41:30 -0700
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On 11/20/20 5:23 PM, Linus Torvalds wrote:
> On Fri, Nov 20, 2020 at 1:36 PM Jens Axboe <axboe@kernel.dk> wrote:
>>
>> I don't disagree with you on that. I've been a bit gun shy on touching
>> the VFS side of things, but this one isn't too bad. I hacked up a patch
>> that allows io_uring to do LOOKUP_RCU and a quick test seems to indicate
>> it's fine. On top of that, we just propagate the error if we do fail and
>> get rid of that odd retry loop.
> 
> Ok, this looks better to me (but is obviously not 5.10 material).
> 
> That said, I think I'd prefer to keep 'struct nameidata' internal to
> just fs/namei.c, and maybe we can just expert that
> 
>         struct nameidata nd;
> 
>         set_nameidata(&nd, req->open.dfd, req->open.filename);
>         file = path_openat(&nd, &op, op.lookup_flags | LOOKUP_RCU);
>         restore_nameidata();
>         return filp == ERR_PTR(-ECHILD) ? -EAGAIN : filp;
> 
> as a helper from namei.c instead? Call it "do_filp_open_rcu()" or something?

Yes, that's probably a better idea. I'll move in that direction.

> That "force_nonblock" test seems a bit off, though. Why is that RCU
> case only done when "!force_nonblock"? It would seem that if
> force_nonblock is set, you want to do this too?

Taking a second look at it, it's inverted. So if force_nonblock == true,
we want to do just the RCU lookup.

But I think the bit that you're missing is that the other case will do
the normal lookup, which does an RCU lookup first. It looks needs to
look like this:

if (force_nonblock)
	file = do_filp_open_rcu();
else
	file = do_filp_open();

-- 
Jens Axboe