[Devel] Re: [PATCH] Wake up mandatory locks waiter on chmod (v2)

[J. Bruce Fields]

On Tue, Sep 18, 2007 at 12:14:55PM -0400, Trond Myklebust wrote:
> Note also that strictly speaking, we're not even compliant with the
> System V behaviour on read() and write(). See:
> 
>   http://www.unix.org.ua/orelly/networking_2ndEd/nfs/ch11_01.htm
> and
>   http://docs.sun.com/app/docs/doc/801-6736/6i13fom0a?l=en&a=view&q=mandatory+lock
> 
> According to these docs, we should be wrapping each and every read() and
> write() syscall with a mandatory lock. The fact that we're not, and yet
> still not seeing any complaints just goes to show how few people are
> actually using and relying on this...

So currently there's nothing to prevent this:

				- write passes locks_mandatory_area() checks
	- get mandatory lock
	- read old data
				- write updates file data
	- read new data

You can see the data change even while you hold a mandatory lock that
should exclude writes.

Similarly you might think that an application could prevent anyone from
seeing the intermediate state of a file while it performs a series of
writes under an exclusive mandatory lock, but actually there's nothing
to stop a read in progress from racing with acquisition of the lock.

Unless I'm missing something, that makes our mandatory lock
implementation pretty pointless.  I wish we could either fix it or just
ditch it, but I suppose either option would be unpopular.

--b.