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Date:   Wed, 29 Jan 2020 01:29:29 +0000
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Sascha Hauer <s.hauer@pengutronix.de>
Cc:     Jan Kara <jack@suse.cz>, linux-fsdevel@vger.kernel.org,
        linux-mtd@lists.infradead.org, Jan Kara <jack@suse.com>,
        Richard Weinberger <richard@nod.at>, kernel@pengutronix.de
Subject: Re: [PATCH 1/8] quota: Allow to pass mount path to quotactl
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On Tue, Jan 28, 2020 at 11:06:31AM +0100, Sascha Hauer wrote:
> Hi Jan,

> @@ -810,6 +811,36 @@ static struct super_block *quotactl_block(const char __user *special, int cmd)
>  #endif
>  }
>  
> +static struct super_block *quotactl_path(const char __user *special, int cmd,
> +					 struct path *path)
> +{
> +	struct super_block *sb;
> +	int ret;
> +
> +	ret = user_path_at(AT_FDCWD, special, LOOKUP_FOLLOW | LOOKUP_AUTOMOUNT,
> +			   path);
> +	if (ret)
> +		return ERR_PTR(ret);
> +
> +	sb = path->mnt->mnt_sb;
> +restart:
> +	if (quotactl_cmd_onoff(cmd))
> +		down_write(&sb->s_umount);
> +	else
> +		down_read(&sb->s_umount);
> +
> +	if (quotactl_cmd_write(cmd) && sb->s_writers.frozen != SB_UNFROZEN) {
> +		if (quotactl_cmd_onoff(cmd))
> +			up_write(&sb->s_umount);
> +		else
> +			up_read(&sb->s_umount);
> +		wait_event(sb->s_writers.wait_unfrozen,
> +			   sb->s_writers.frozen == SB_UNFROZEN);
> +		goto restart;
> +	}
> +
> +	return sb;
> +}

This partial duplicate of __get_super_thawed() guts does *not* belong here,
especially not interleaved with quota-specific checks.

> +	if (q_path) {
> +		if (quotactl_cmd_onoff(cmd))
> +			up_write(&sb->s_umount);
> +		else
> +			up_read(&sb->s_umount);
> +
> +		path_put(&sb_path);
> +	} else {
> +		if (!quotactl_cmd_onoff(cmds))
> +			drop_super(sb);
> +		else
> +			drop_super_exclusive(sb);
> +	}

Er...  Why not have the same code that you've used to lock the damn thing
(needs to be moved to fs/super.c) simply get a passive ref to it?  Then
you could do the same thing, q_path or no q_path...