linux-kernel - Re: [PATCH 16/33] aio: use cancellation list lazily

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Message-ID: <20130402023609.GE30122@thunk.org>
Date:	Mon, 1 Apr 2013 22:36:09 -0400
From:	Theodore Ts'o <tytso@....edu>
To:	Kent Overstreet <koverstreet@...gle.com>
Cc:	linux-kernel@...r.kernel.org, linux-aio@...ck.org,
	akpm@...ux-foundation.org, Zach Brown <zab@...hat.com>,
	Felipe Balbi <balbi@...com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Mark Fasheh <mfasheh@...e.com>,
	Joel Becker <jlbec@...lplan.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Jens Axboe <axboe@...nel.dk>,
	Asai Thambi S P <asamymuthupa@...ron.com>,
	Selvan Mani <smani@...ron.com>,
	Sam Bradshaw <sbradshaw@...ron.com>,
	Jeff Moyer <jmoyer@...hat.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Benjamin LaHaise <bcrl@...ck.org>
Subject: Re: [PATCH 16/33] aio: use cancellation list lazily

On Thu, Mar 21, 2013 at 09:35:37AM -0700, Kent Overstreet wrote:
> Cancelling kiocbs requires adding them to a per kioctx linked list, which
> is one of the few things we need to take the kioctx lock for in the fast
> path.  But most kiocbs can't be cancelled - so if we just do this lazily,
> we can avoid quite a bit of locking overhead.
> 
> While we're at it, instead of using a flag bit switch to using ki_cancel
> itself to indicate that a kiocb has been cancelled/completed.  This lets
> us get rid of ki_flags entirely.


Reviewed-by: "Theodore Ts'o" <tytso@....edu>

One nit....

> + * And since most things don't implement kiocb cancellation and we'd really like
> + * kiocb completion to be lockless when possible, we use ki_cancel to
> + * synchronize cancellation and completion - we only set it to KIOCB_CANCELLED
> + * with xchg() or cmpxchg(), see batch_complete_aio() and kiocb_cancel().

It's not batch_complete_aio() until later in the patch series.... as
of this commit, it's still aio_complete()

						- Ted
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