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    SubjectRe: [PATCH] Wake up mandatory locks waiter on chmod (v2)
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    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.
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