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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Sat, 21 Nov 2020 10:07:16 -0800
Message-ID: <CAHk-=wgyRpBW_NOCKpJ1rZGD9jVOX80EWqKwwZxFeief2Khotg@mail.gmail.com>
Subject: Re: [GIT PULL] io_uring fixes for 5.10-rc
To:     Jens Axboe <axboe@kernel.dk>
Cc:     Al Viro <viro@zeniv.linux.org.uk>,
        io-uring <io-uring@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
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On Fri, Nov 20, 2020 at 7:00 PM Jens Axboe <axboe@kernel.dk> wrote:
>
> Actually, I think we can do even better. How about just having
> do_filp_open() exit after LOOKUP_RCU fails, if LOOKUP_RCU was already
> set in the lookup flags? Then we don't need to change much else, and
> most of it falls out naturally.

So I was thinking doing the RCU lookup unconditionally, and then doing
the nn-RCU lookup if that fails afterwards.

But your patch looks good to me.

Except for the issue you noticed.

> Except it seems that should work, except LOOKUP_RCU does not guarantee
> that we're not going to do IO:

Well, almost nothing guarantees lack of IO, since allocations etc can
still block, but..

> [   20.463195]  schedule+0x5f/0xd0
> [   20.463444]  io_schedule+0x45/0x70
> [   20.463712]  bit_wait_io+0x11/0x50
> [   20.463981]  __wait_on_bit+0x2c/0x90
> [   20.464264]  out_of_line_wait_on_bit+0x86/0x90
> [   20.464611]  ? var_wake_function+0x30/0x30
> [   20.464932]  __ext4_find_entry+0x2b5/0x410
> [   20.465254]  ? d_alloc_parallel+0x241/0x4e0
> [   20.465581]  ext4_lookup+0x51/0x1b0
> [   20.465855]  ? __d_lookup+0x77/0x120
> [   20.466136]  path_openat+0x4e8/0xe40
> [   20.466417]  do_filp_open+0x79/0x100

Hmm. Is this perhaps an O_CREAT case? I think we only do the dcache
lookups under RCU, not the final path component creation.

And there are probably lots of other situations where we finish with
LOOKUP_RCU (with unlazy_walk()), and then continue.

Example: look at "may_lookup()" - if inode_permission() says "I can't
do this without blocking" the logic actually just tries to validate
the current state (that "unlazy_walk()" thing), and then continue
without RCU.

It obviously hasn't been about lockless semantics, it's been about
really being lockless. So LOOKUP_RCU has been a "try to do this
locklessly" rather than "you cannot take any locks".

I guess we would have to add a LOOKUP_NOBLOCK thing to actually then
say "if the RCU lookup fails, return -EAGAIN".

That's probably not a huge undertaking, but yeah, I didn't think of
it. I think this is a "we need to have Al tell us if it's reasonable".

                Linus