Re: CVS commit: src/sys/kern

["Kurt J. Lidl" <lidl%pix.net@localhost>]

On Thu, Sep 25, 2008 at 03:06:59PM +0000, Andrew Doran wrote:
> > If anyone can give me an idea what actually has to be verified, I can (since
> > I have to) put considerable time into it.
> 
> It would mean checking all uses of MNT_RDONLY, and thinking about what
> happens if the flag changes while an operation is in-flight, and afterwards
> if dirty buffers/pages have been generated. There might be barriers to
> prevent flushing those after the flag is set. I don't know.

Several years ago, I had a conversation with Kirk McKusick about
this issue. (OK, it had to do with a similar problem under BSD/OS, but
the central issue is the same)  

The softupdates code, along with the snapshot facilities for FFS/UFS
address the issue.  If one has done enough buffer flushing and I/O
to be able to finish a snapshot, the filesystem is at a point where
you could safely move from RW to RO state too.

-Kurt