linux-kernel - Re: [PATCH v2 1/2] aio: change exit_aio() to load mm->ioctx_table once and avoid rcu_read

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Message-ID: <20140430152344.GD28959@kvack.org>
Date:	Wed, 30 Apr 2014 11:23:44 -0400
From:	Benjamin LaHaise <bcrl@...ck.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Kent Overstreet <kmo@...erainc.com>,
	Al Viro <viro@...iv.linux.org.uk>, linux-aio@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] aio: change exit_aio() to load mm->ioctx_table once and avoid rcu_read_lock()

Hi Oleg,

On Wed, Apr 30, 2014 at 04:16:16PM +0200, Oleg Nesterov wrote:
> 1. We can read ->ioctx_table only once and we do not read rcu_read_lock()
>    or even rcu_dereference().
> 
>    This mm has no users, nobody else can play with ->ioctx_table. Otherwise
>    the code is buggy anyway, if we need rcu_read_lock() in a loop because
>    ->ioctx_table can be updated then kfree(table) is obviously wrong.
> 
> 2. Update the comment. "exit_mmap(mm) is coming" is the good reason to avoid
>    munmap(), but another reason is that we simply can't do vm_munmap() unless
>    current->mm == mm and this is not true in general, the caller is mmput().
> 
> 3. We do not really need to nullify mm->ioctx_table before return, probably
>    the current code does this to catch the potential problems. But in this
>    case RCU_INIT_POINTER(NULL) looks better.

Looks pretty good.  One minor style comment below.

> Signed-off-by: Oleg Nesterov <oleg@...hat.com>
> ---
>  fs/aio.c |   47 ++++++++++++++++++-----------------------------
>  1 files changed, 18 insertions(+), 29 deletions(-)
> 
> diff --git a/fs/aio.c b/fs/aio.c
> index 79b7e69..3526c2b 100644
> --- a/fs/aio.c
> +++ b/fs/aio.c
> @@ -791,40 +791,29 @@ EXPORT_SYMBOL(wait_on_sync_kiocb);
>   */
>  void exit_aio(struct mm_struct *mm)
>  {
> -	struct kioctx_table *table;
> -	struct kioctx *ctx;
> -	unsigned i = 0;
> -
> -	while (1) {
> -		rcu_read_lock();
> -		table = rcu_dereference(mm->ioctx_table);
> -
> -		do {
> -			if (!table || i >= table->nr) {
> -				rcu_read_unlock();
> -				rcu_assign_pointer(mm->ioctx_table, NULL);
> -				if (table)
> -					kfree(table);
> -				return;
> -			}
> -
> -			ctx = table->table[i++];
> -		} while (!ctx);
> +	struct kioctx_table *table = rcu_dereference_raw(mm->ioctx_table);
> +	int i;
>  
> -		rcu_read_unlock();
> +	if (!table)
> +		return;
>  
> +	for (i = 0; i < table->nr; ++i) {
> +		struct kioctx *ctx = table->table[i];
>  		/*
> -		 * We don't need to bother with munmap() here -
> -		 * exit_mmap(mm) is coming and it'll unmap everything.
> -		 * Since aio_free_ring() uses non-zero ->mmap_size
> -		 * as indicator that it needs to unmap the area,
> -		 * just set it to 0; aio_free_ring() is the only
> -		 * place that uses ->mmap_size, so it's safe.
> +		 * We don't need to bother with munmap() here - exit_mmap(mm)
> +		 * is coming and it'll unmap everything. And we simply can't,
> +		 * this is not necessarily our ->mm.
> +		 * Since kill_ioctx() uses non-zero ->mmap_size as indicator
> +		 * that it needs to unmap the area, just set it to 0.
>  		 */
> -		ctx->mmap_size = 0;
> -
> -		kill_ioctx(mm, ctx, NULL);
> +		if (ctx) {
> +			ctx->mmap_size = 0;
> +			kill_ioctx(mm, ctx, NULL);
> +		}

Rather than indenting and moving the two lines changing mmap_size and the 
kill_ioctx() call, why not just do "if (!ctx) ... continue;"?  That reduces 
the number of lines changed and avoid excessive indentation.

		-ben

>  	}
> +
> +	RCU_INIT_POINTER(mm->ioctx_table, NULL);
> +	kfree(table);
>  }
>  
>  static void put_reqs_available(struct kioctx *ctx, unsigned nr)
> -- 
> 1.5.5.1
> 

-- 
"Thought is the essence of where you are now."
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