linux-kernel - Re: [PATCH 7/9 updated] vfs: protect remounting superblock read-only

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Message-ID: <20101023214235.GT19804@ZenIV.linux.org.uk>
Date:	Sat, 23 Oct 2010 22:42:35 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Miklos Szeredi <miklos@...redi.hu>
Cc:	dave@...ux.vnet.ibm.com, akpm@...ux-foundation.org,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 7/9 updated] vfs: protect remounting superblock read-only

On Sat, Oct 23, 2010 at 09:35:16PM +0200, Miklos Szeredi wrote:

> > Another thing:
> > 	"If clone_mnt() happens while mnt_make_readonly() is running, the
> > 	cloned mount might have MNT_WRITE_HOLD flag set, which results in
> > 	mnt_want_write() spinning forever on this mount."
> > actually means
> > 	"neither clone_mnt() nor remounts should ever be done without
> > namespace_sem held exclusive; if that ever happens, we have a serious
> > bug that can lead to any number of bad things happening".
> > 
> > Do you actually see such places?  If so, that's what needs fixing.
> 
> do_remount() takes s_umount, but not namespace_sem.

Duh...  Right, ignore that part; we really don't want to do anything
blocking beyond simple allocations under namespace_sem (e.g. everything
that gets unmounted is collected to be dropped after namespace_sem
is released).

Applied.
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