Re: [PATCH] move file->f_op->open out of mutex (misc_mtx) in misc_open

[One Thousand Gnomes <gnomes@lxorguk.ukuu.org.uk>]

On Tue,  6 Jan 2015 08:34:07 +0800
xiaomin1 <xiaoming.wang@intel.com> wrote:

> misc_mtx was used to mutex misc_list.
> But file->f_op->open may be blocked by downloading
> firmware in some devices' open.
> So move file->f_op->open out of mutex (misc_mtx)
> in misc_open to avoid this block.
> It also make code more efficiency.
> 
> Signed-off-by: Zhang Dongxing <dongxing.zhang@intel.com>
> Signed-off-by: xiaoming wang <xiaoming.wang@intel.com>
> ---
>  drivers/char/misc.c |    3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/char/misc.c b/drivers/char/misc.c
> index ffa97d2..289c59e 100644
> --- a/drivers/char/misc.c
> +++ b/drivers/char/misc.c
> @@ -144,7 +144,8 @@ static int misc_open(struct inode * inode, struct file * file)
>  	replace_fops(file, new_fops);
>  	if (file->f_op->open) {
>  		file->private_data = c;
> -		err = file->f_op->open(inode,file);
> +		mutex_unlock(&misc_mtx);
> +		return file->f_op->open(inode,file);
>  	}

This changes all the users of misc device open from singlethreaded to
multi-threaded. Having read through the users I can find this seems safe
and they all seem to have the right checks in place and to assume they
are multi-threaded.

Reviewed-by: Alan Cox <alan@linux.intel.com>