linux-kernel - Re: [PATCH] race condition fixing in sysfs_create

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Date:	Fri, 26 Jul 2013 17:59:00 +0800
From:	Dennis Chen <xschen@...oft.com.cn>
To:	linux-kernel@...r.kernel.org, gregkh@...uxfoundation.org
CC:	Tejun Heo <tj@...nel.org>, xiyou.wangcong@...il.com,
	Dennis Chen <xschen@...oft.com.cn>
Subject: Re: [PATCH] race condition fixing in sysfs_create_dir

On 07/26/2013 05:49 PM, Dennis Chen wrote:

> The patch is trying its best to avoid creating a dir under a parent dir which is removing from
> the system:
>      PATH0 (create a dir under 'PARENT/...')         PATH1 (remove the 'PARENT/...')
>           sysfs_create_dir() {                         sysfs_remove_dir() {
>           ...                                          ...
>           if (kobj->parent)                            spin_lock(&sysfs_assoc_lock);
>          parent_sd = kobj->parent->sd;  <----- kobj->sd = NULL;
>       else                                         spin_unlock(&sysfs_assoc_lock);
>          parent_sd = &sysfs_root;
> Suppose PATH1 enter the critical section first, then PATH0 begin to execute before kobj->sd
> has been reset to NULL, possibly PATH0 will get a non-NULL parent_sd since lack of the
> sysfs_assoc_lock protection in PATH0. In this case, PATH0 think it has a valid parent_sd which
> can be freed by PATH1 in the followed, refer to the comments in the patch. Maybe we need
> to figure out a perfect solution to solve the race condition, although the codes in question are
> in slow path...
> Signed-off-by: Dennis Chen <xschen@...oft.com.cn>
> Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> Cc: Tejun Heo <teheo@...e.de>

should be Tejun Heo <tj@...nel.org>

> Cc: Wang Cong <xiyou.wangcong@...il.com>
> ---
> fs/sysfs/dir.c |   11 ++++++++++-
> 1 file changed, 10 insertions(+), 1 deletion(-)
> diff --git a/fs/sysfs/dir.c b/fs/sysfs/dir.c
> index e068e74..114073d 100644
> --- a/fs/sysfs/dir.c
> +++ b/fs/sysfs/dir.c
> @@ -746,13 +746,22 @@ int sysfs_create_dir(struct kobject * kobj)
>          BUG_ON(!kobj);
> +       spin_lock(&sysfs_assoc_lock);
>      if (kobj->parent)
>          parent_sd = kobj->parent->sd;
>      else
>          parent_sd = &sysfs_root;
> -       if (!parent_sd)
> +       if (!parent_sd) {
> +               spin_unlock(&sysfs_assoc_lock);
>                  return -ENOENT;
> +       }
> +       spin_unlock(&sysfs_assoc_lock);
> +       /* TODO: although the sysfs is in a slow path, but in the operation
> +       * followed, we still have a window to let the sysfs_remove_dir to
> +       * free the memory space pointered by parent_sd till we inc its ref
> +       * count in __sysfs_add_one()
> +       */
>          if (sysfs_ns_type(parent_sd))
>                  ns = kobj->ktype->namespace(kobj);

Re CC Tejun whose email addess <@suse.de> is obsolete :)

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