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

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Message-ID: <20160320012610.GX17997@ZenIV.linux.org.uk>
Date:	Sun, 20 Mar 2016 01:26:10 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
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 06:20:24PM -0700, Linus Torvalds wrote:
> On Mon, Mar 14, 2016 at 10:17 AM, Benjamin LaHaise <bcrl@...ck.org> wrote:
> >
> > I had some time last week to make an aio openat do what it can in
> > submit context.  The results are an improvement: when openat is handled
> > in submit context it completes in about half the time it takes compared
> > to the round trip via the work queue, and it's not terribly much code
> > either.
> 
> This looks good to me, and I do suspect that any of these aio paths
> should strive to have a synchronous vs threaded model. I think that
> makes the whole thing much more interesting from a performance
> standpoint.

Umm...  You do realize that LOOKUP_RCU in flags does *NOT* guarantee that
it won't block, right?  At the very least one would need to refuse to
fall back on non-RCU mode without a full restart.  Furthermore, vfs_open()
itself can easily block.

So this new LOOKUP flag makes no sense, and it's in the just about _the_
worst place possible for adding special cases with ill-defined semantics -
do_last() is already far too convoluted and needs untangling, not adding
half-assed kludges.