linux-kernel - Re: aio openat Re: [PATCH 07/13] aio: enabled thread based async fsync

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Date:	Sat, 19 Mar 2016 19:03:25 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Al Viro <viro@...iv.linux.org.uk>
Cc:	Benjamin LaHaise <bcrl@...ck.org>,
	Dave Chinner <david@...morbit.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Linux API <linux-api@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-aio@...ck.org, linux-mm <linux-mm@...ck.org>
Subject: Re: aio openat Re: [PATCH 07/13] aio: enabled thread based async fsync

On Sat, Mar 19, 2016 at 6:55 PM, Al Viro <viro@...iv.linux.org.uk> wrote:
>
> What would make unlazy_walk() fail?  And if it succeeds, you are not
> in RCU mode anymore *without* restarting from scratch...

I don't see your point.

You don't want to be in RCU mode any more. You want to either succeed
or fail with ECHILD/ESTALE. Then, in the failure case, you go to the
thread.

What I meant by restarting was the restart that do_filp_open() does,
and there it just restarts with "op->lookup_flags", which has
RCU_LOOKUP still set, so it would just try to do the RCU lookup again.

But I actually notice now that Ben actually disabled that restart if
LOOKUP_RCU was set, so that ends up not even happening.

Anyway, I'm not saying it's polished and pretty. I think the changes
to do_filp_open() are a bit silly, and the code should just use
path_openat() directly. Possibly using a new helper (ie perhaps just
introduce a "rcu_filp_openat()" thing). But from a design perspective,
I think this all looks fine.

              Linus