linux-kernel - Re: [patch] fs: aio fix rcu lookup

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Date:	Thu, 20 Jan 2011 16:16:30 -0500
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Jan Kara <jack@...e.cz>
Cc:	Nick Piggin <npiggin@...il.com>, paulmck@...ux.vnet.ibm.com,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [patch] fs: aio fix rcu lookup

Jan Kara <jack@...e.cz> writes:

> So in the end, I've realized I don't need synchronize_rcu() at all and
> in fact everything is OK even without call_rcu() if I base my fix on top
> of your patch.
>
> Attached is your patch with added comment I proposed and also a patch
> fixing the second race. Better?

[snip]

> From 6d5375d55b5d88e8ceda739052566e033be620c2 Mon Sep 17 00:00:00 2001
> From: Jan Kara <jack@...e.cz>
> Date: Wed, 19 Jan 2011 00:37:48 +0100
> Subject: [PATCH 2/2] fs: Fix race between io_destroy() and io_submit() in AIO
>
> A race can occur when io_submit() races with io_destroy():
>
>  CPU1						CPU2
> io_submit()
>   do_io_submit()
>     ...
>     ctx = lookup_ioctx(ctx_id);
> 						io_destroy()
>     Now do_io_submit() holds the last reference to ctx.
>     ...
>     queue new AIO
>     put_ioctx(ctx) - frees ctx with active AIOs

[snip]

> We solve this issue by checking whether ctx is being destroyed
> in AIO submission path after adding new AIO to ctx. Then we
> are guaranteed that either io_destroy() waits for new AIO or
> we see that ctx is being destroyed and bail out.
>
> Signed-off-by: Jan Kara <jack@...e.cz>
> ---
>  fs/aio.c |   15 +++++++++++++++
>  1 files changed, 15 insertions(+), 0 deletions(-)
>
> diff --git a/fs/aio.c b/fs/aio.c
> index b4dd668..0244c04 100644
> --- a/fs/aio.c
> +++ b/fs/aio.c
> @@ -1642,6 +1642,21 @@ static int io_submit_one(struct kioctx *ctx, struct iocb __user *user_iocb,
>  		goto out_put_req;
>  
>  	spin_lock_irq(&ctx->ctx_lock);
> +	/*
> +	 * We could have raced with io_destroy() and are currently holding a
> +	 * reference to ctx which should be destroyed. We cannot submit IO
> +	 * since ctx gets freed as soon as io_submit() puts its reference.
> +	 * The check here is reliable since io_destroy() sets ctx->dead before
> +	 * waiting for outstanding IO. Thus if we don't see ctx->dead set here,
> +	 * io_destroy() waits for our IO to finish.
> +	 * The check is inside ctx->ctx_lock to avoid extra memory barrier
> +	 * in this fast path...
> +	 */
> +	if (ctx->dead) {
> +		spin_unlock_irq(&ctx->ctx_lock);
> +		ret = -EINVAL;
> +		goto out_put_req;
> +	}
>  	aio_run_iocb(req);
>  	if (!list_empty(&ctx->run_list)) {
>  		/* drain the run list */

OK, that's clever.  Thanks for looking into this, Jan!

You can put my:

Reviewed-by: Jeff Moyer <jmoyer@...hat.com>

on both patches.

Cheers,
Jeff
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