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    SubjectRe: nfs: infinite loop in fcntl(F_SETLKW)
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    On Thu, 2008-04-10 at 17:07 -0400, Trond Myklebust wrote:
    > On Thu, 2008-04-10 at 17:02 -0400, Trond Myklebust wrote:
    > > 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(). :-(
    >
    > Duh... Sorry, EAGAIN is indeed the correct return value for fcntl() when
    > the lock attempt failed. I should have reread the manpage/posix spec
    > before replying.

    OK. So the correct fix here should really be applied to fcntl_setlk().
    There is absolutely no reason why we should be looping at all if the
    filesystem has a ->lock() method.

    In fact, this looping behaviour was introduced recently in commit
    7723ec9777d9832849b76475b1a21a2872a40d20. Marc, Bruce, why was this
    done, and how are filesystems now supposed to behave?



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