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Date:   Fri, 11 Dec 2020 17:29:31 +0000
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Jens Axboe <axboe@kernel.dk>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>
Subject: Re: [PATCH 1/2] fs: add support for LOOKUP_NONBLOCK
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On Fri, Dec 11, 2020 at 09:21:20AM -0800, Linus Torvalds wrote:
> On Fri, Dec 11, 2020 at 7:57 AM Jens Axboe <axboe@kernel.dk> wrote:
> >
> > On 12/10/20 7:35 PM, Al Viro wrote:
> > > _IF_ for some theoretical exercise you want to do "lookup without dropping
> > > out of RCU", just add a flag that has unlazy_walk() fail.  With -ECHILD.
> > > Strip it away in complete_walk() and have path_init() with that flag
> > > and without LOOKUP_RCU fail with -EAGAIN.  All there is to it.
> >
> > Thanks Al, that makes for an easier implementation. I like that suggestion,
> > boils it down to just three hunks (see below).
> 
> Ooh. Yes, very nice.

Except for the wrong order in path_init() - the check should go _before_
        if (!*s)
                flags &= ~LOOKUP_RCU;
for obvious reasons.

Again, that part is trivial - what to do with do_open()/open_last_lookups()
is where the nastiness will be.  Basically, it makes sure we bail out in
cases when lookup itself would've blocked, but it does *not* bail out
when equally heavy (and unlikely) blocking sources hit past the complete_walk().
Which makes it rather useless for the caller, unless we get logics added to
that part as well.  And _that_ I want to see before we commit to anything.