Re: fcntl(F GETLEASE) semantics??

From: Michael Kerrisk (mtk-lkml_at_gmx.net)
Date: 08/11/05

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    Date:	Thu, 11 Aug 2005 14:27:24 +0200 (MEST)
To: Trond Myklebust <trond.myklebust@fys.uio.no>

    > Von: Trond Myklebust <trond.myklebust@fys.uio.no>
    > to den 11.08.2005 Klokka 10:14 (+0200) skreiv Michael Kerrisk:
    >
    > > No. The behavior in Linux recently, and arbitrarily (IMO) changed:
    >
    > The change was NOT arbitrary.

    Okay -- I'm puzzled. Two of the people that I understand to have
    had a strong interest in file leases seemed to think the change
    shouldn't have occurred:

    Stephen Rothwell:
    http://marc.theaimsgroup.com/?l=linux-kernel&m=111512619520116&w=2

        Thanks for the testing. My expectation is that it shouldn't
        matter how the current process opened the file for either
        type of lease. However, you are right (IMHO) that the current
        process should *not* be counted as a writer in the case of
        trying to obtain a F_RDLCK lease.

    And Stephen suggested a one line patch to fix the problem

    Matthew Wilcox also wrote:
    http://marc.theaimsgroup.com/?l=linux-kernel&m=111512898520775&w=2

        On Tue, May 03, 2005 at 09:55:42AM -0400, William A.(Andy)
        Adamson wrote:
    > i believe the current implementation is correct. opening a
    > file for write means that you can not have a read lease,
    > caller included.

        Why not? Certainly, others will not be able to take out a
        read lease, so there's very little point to only having a
        read lease, but I don't see why we should deny it.

    (By the way, I wrote the fcntl.2 manpage text for file
    leases -- because there was no existing documentation, or
    specification of the desired behavior; the text was based
    on my experiments, and some email discussions with Stephen
    Rothwell.)

    And I pointed out that the existing behaviour (which is
    still current in 2.6.13-rc4) is inconsistent:

    http://marc.theaimsgroup.com/?l=linux-kernel&m=111511455406623&w=2

        Some further testing showed the following (both open()
        and fcntl(F_SETLEASE) from same process):

         open() | lease requested
          flag | F_RDLCK | F_WRLCK
        ---------+----------+----------
        O_RDONLY | okay | okay
        O_WRONLY | EAGAIN | okay
        O_RDWR | EAGAIN | okay

    In other words, a process can open a file read-write, and
    can't place a read lease, but can place a write lease!
    That does not seem to make any sense to me.

    > It was deliberate and for the reasons
    > stated.

    Can you elaborate -- which reasons are you referring to?

    > The whole point of leases is to support CIFS oplocks for Samba and NFSv4
    > delegations in the kernel.

    Yep, I understand that much.

    > Both have the same specific expected
    > behaviour.
    > The original deviates from that expected behaviour by allowing you to
    > get a shared lease when in a condition that does not allow actual
    > sharing.

    And I should add -- I know little about SAMBA or CIFS. But from
    what I've seen (the quoted messages above), the change seems to
    have been accidental, and inconsistent.

    Cheers,

    Michael 

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