From: Al Viro <viro@ZenIV.linux.org.uk>
To: Aditya Kali <adityakali@google.com>
Cc: linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
anatol@google.com
Subject: Re: [RFC] vfs: avoid sb->s_umount lock while changing bind-mount flags
Date: Tue, 17 Sep 2013 03:40:40 +0100 [thread overview]
Message-ID: <20130917024040.GH13318@ZenIV.linux.org.uk> (raw)
In-Reply-To: <1379353350-11320-1-git-send-email-adityakali@google.com>
On Mon, Sep 16, 2013 at 10:42:30AM -0700, Aditya Kali wrote:
> During remount of a bind mount (mount -o remount,bind,ro,... /mnt/mntpt),
> we currently take down_write(&sb->s_umount). This causes the remount
> operation to get blocked behind writes occuring on device (possibly
> mounted somewhere else). We have observed that simply trying to change
> the bind-mount from read-write to read-only can take several seconds
> becuase writeback is in progress. Looking at the code it seems to me that
> we need s_umount lock only around the do_remount_sb() call.
> vfsmount_lock seems enough to protect the flag change on the mount.
> So this patch fixes the locking so that changing of flags can happen
> outside the down_write(&sb->s_umount).
What's to prevent mount -o remount,ro /mnt and mount -o remount,rw,nodev /mnt
racing and ending up with that sucker rw and without nodev? As for
lock_mount... nope - we carefully do *not* hold namespace_sem over any kind
of fs operations. Anything getting stuck while holding it will have
really nasty consequences.
So ->s_umount here is inelegant, but alternatives sucks worse...
next prev parent reply other threads:[~2013-09-17 2:40 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-09-16 17:42 [RFC] vfs: avoid sb->s_umount lock while changing bind-mount flags Aditya Kali
2013-09-17 2:40 ` Al Viro [this message]
2013-09-19 20:13 ` Aditya Kali
2013-09-30 17:54 ` Aditya Kali
2013-09-30 18:13 ` Aditya Kali
2013-09-30 20:03 ` Al Viro
2013-09-30 21:44 ` Aditya Kali
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