linux-kernel - Re: [PATCH] r/o bind mounts: add vfsmount and superblock writer counts

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Date:	Tue, 9 Jan 2007 10:53:05 +0000
From:	Christoph Hellwig <hch@...radead.org>
To:	Dave Hansen <haveblue@...ibm.com>
Cc:	viro@....linux.org.uk, hch@...radead.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] r/o bind mounts: add vfsmount and superblock writer counts

On Mon, Jan 08, 2007 at 10:34:45AM -0800, Dave Hansen wrote:
> Removing it had a few side-effects.  First of all, it made me move
> all of the operations on the counts of writers to underneath the
> spinlock that was already there.  I guess this could be a cause
> for potential contention because there _are_ locks in the common
> code paths now.  But, I do agree with Christoph that it would be
> awfully hard to get it contended.
> 
> The other side-effect is that we can't have the bit in mnt_flags
> to be a shortcut to the superblock's writeable state since we
> don't have a way to go find the mounts and that bit when a fs
> changes writeable state.  This causes a potential cache miss
> when we have to check the superblock directly during the
> relatively common __mnt_is_readonly() function.

Why? We _only_ need to check the vfsmount flag.  vfsmount can
become r/w if the superblock is marked r/o which means the
underlying (block/network/etc) device is fundamentally not writeable.

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