Re: fcntl(F GETLEASE) semantics??

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

  • Next message: Bodo Eggert: "Re: Need help in understanding x86 syscall" 
    Date:	Thu, 11 Aug 2005 15:22:39 +0200 (MEST)
To: Trond Myklebust <trond.myklebust@fys.uio.no>

    Trond,

    > Von: Trond Myklebust <trond.myklebust@fys.uio.no>
    >
    > to den 11.08.2005 Klokka 14:27 (+0200) skreiv Michael Kerrisk:
    > > 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.
    >
    > Then what do you think that leases are supposed to do, and why?

    As noted already, I don't know much of CIFS and SAMBA.
    But are you saying that it is sensible and consistent that
    "a process can open a file read-write, and can't place a
    read lease, but can place a write lease"?

    > An exclusive (i.e. write) lease should mean that _nothing_ other than
    > your process is accessing the file. A client may cache the file data,
    > metadata and read/write locks because nobody else can change that
    > information, and nobody else holds locks on the file. It may also cache
    > file acl/access information, and hence cache new OPEN calls.
    >
    > A shared (i.e. read) lease means that there are currently no processes
    > that can change the data or metadata (including your own).
                                            ^^^^^^^^^^^^^^^^^

    This is precisely the point of the problem. Stephen
    Rothwell, and Matthew Wilcox seem to be saying that
    the last bit is not the case.

    Cheers,

    Michael 

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