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

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Message-ID: <x494o9bbkqo.fsf@segfault.boston.devel.redhat.com>
Date:	Fri, 14 Jan 2011 09:52:47 -0500
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Nick Piggin <npiggin@...il.com>
Cc:	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

Nick Piggin <npiggin@...il.com> writes:

> Hi,
>
> While hunting down a bug in NFS's AIO, I believe I found this
> buggy code...
>
> fs: aio fix rcu ioctx lookup
>
> aio-dio-invalidate-failure GPFs in aio_put_req from io_submit.
>
> lookup_ioctx doesn't implement the rcu lookup pattern properly.
> rcu_read_lock does not prevent refcount going to zero, so we
> might take a refcount on a zero count ioctx.

So, does this patch fix the problem?  You didn't actually say....

> Signed-off-by: Nick Piggin <npiggin@...nel.dk>
>
> Index: linux-2.6/fs/aio.c
> ===================================================================
> --- linux-2.6.orig/fs/aio.c	2011-01-14 00:29:00.000000000 +1100
> +++ linux-2.6/fs/aio.c	2011-01-14 11:31:47.000000000 +1100
> @@ -239,15 +239,23 @@ static void __put_ioctx(struct kioctx *c
>  	call_rcu(&ctx->rcu_head, ctx_rcu_free);
>  }
>  
> -#define get_ioctx(kioctx) do {						\
> -	BUG_ON(atomic_read(&(kioctx)->users) <= 0);			\
> -	atomic_inc(&(kioctx)->users);					\
> -} while (0)
> -#define put_ioctx(kioctx) do {						\
> -	BUG_ON(atomic_read(&(kioctx)->users) <= 0);			\
> -	if (unlikely(atomic_dec_and_test(&(kioctx)->users))) 		\
> -		__put_ioctx(kioctx);					\
> -} while (0)
> +static inline void get_ioctx(struct kioctx *kioctx)
> +{
> +	BUG_ON(atomic_read(&kioctx->users) <= 0);
> +	atomic_inc(&kioctx->users);
> +}
> +
> +static inline int try_get_ioctx(struct kioctx *kioctx)
> +{
> +	return atomic_inc_not_zero(&kioctx->users);
> +}
> +
> +static inline void put_ioctx(struct kioctx *kioctx)
> +{
> +	BUG_ON(atomic_read(&kioctx->users) <= 0);
> +	if (unlikely(atomic_dec_and_test(&kioctx->users)))
> +		__put_ioctx(kioctx);
> +}

Why did you switch from macros?  Personal preference?  Can you at least
mention it in the changelog?

>  
>  /* ioctx_alloc
>   *	Allocates and initializes an ioctx.  Returns an ERR_PTR if it failed.
> @@ -601,8 +609,7 @@ static struct kioctx *lookup_ioctx(unsig
>  	rcu_read_lock();
>  
>  	hlist_for_each_entry_rcu(ctx, n, &mm->ioctx_list, list) {
> -		if (ctx->user_id == ctx_id && !ctx->dead) {
> -			get_ioctx(ctx);
> +		if (ctx->user_id == ctx_id && !ctx->dead && try_get_ioctx(ctx)){
>  			ret = ctx;
>  			break;
>  		}
--
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