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    SubjectRe: nfs: infinite loop in fcntl(F_SETLKW)
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    On Thu, 2008-04-10 at 21:51 +0200, Miklos Szeredi wrote:
    > Another infinite loop, this one involving both client and server.
    >
    > Basically what happens is that on the server nlm_fopen() calls
    > nfsd_open() which returns -EACCES, to which nlm_fopen() returns
    > NLM_LCK_DENIED.
    >
    > On the client this will turn into a -EAGAIN (nlm_stat_to_errno()),
    > which in will cause fcntl_setlk() to retry forever.
    >
    > I _think_ the solution is to turn NLM_LCK_DENIED into ENOLCK for
    > blocking locks, as NLM_LCK_BLOCKED is for the contended case. For
    > testing the lock leave NLM_LCK_DENIED as EAGAIN. That still could be
    > misleading, but at least there's no infinite loop in that case.
    >
    > I've minimally tested this patch to verify that it cures the lockup,
    > and that simple blocking locks keep working.
    >
    > Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
    > ---
    > fs/lockd/clntproc.c | 3 +++
    > 1 file changed, 3 insertions(+)
    >
    > Index: linux/fs/lockd/clntproc.c
    > ===================================================================
    > --- linux.orig/fs/lockd/clntproc.c 2008-04-02 13:34:57.000000000 +0200
    > +++ linux/fs/lockd/clntproc.c 2008-04-10 21:23:46.000000000 +0200
    > @@ -536,6 +536,9 @@ again:
    > up_read(&host->h_rwsem);
    > }
    > status = nlm_stat_to_errno(resp->status);
    > + /* Don't return EAGAIN, as that would make fcntl_setlk() loop */
    > + if (status == -EAGAIN)
    > + status = -ENOLCK;
    > out_unblock:
    > nlmclnt_finish_block(block);
    > /* Cancel the blocked request if it is still pending */


    Wait. There is something really weird going on here.

    According to the spec, LCK_DENIED means 'the request failed' (i.e.
    ENOLCK is definitely correct)

    OTOH, LCK_DENIED_NOLOCKS and LCK_DENIED_GRACE_PERIOD are both temporary
    failures, the first because the server had a resource problem, and the
    second because the server rebooted and is in the grace period (i.e.
    EAGAIN would appear to be more appropriate). See

    http://www.opengroup.org/onlinepubs/9629799/chap10.htm#tagcjh_11_02_02_02

    AFAICS, the correct thing to do is to fix nlm_stat_to_errno() by
    swapping the return values for NLM_LCK_DENIED and
    NLM_LCK_DENIED_NOLOCKS/NLM_LCK_DENIED_GRACE_PERIOD.

    The problem is that there appears to be a similar confusion on the Linux
    server side in nlmsvc_lock(). :-(

    --
    Trond Myklebust
    Linux NFS client maintainer

    NetApp
    Trond.Myklebust@netapp.com
    www.netapp.com


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