Re: [PATCH 1/1] nfsd41: renew_client() needs to be called with the client_mutex held

["J. Bruce Fields" <bfields@fieldses.org>]

On Thu, Jun 18, 2009 at 10:39:10AM -0700, Labiaga, Ricardo wrote:
> On 6/18/09 10:31 AM, "J. Bruce Fields" <bfields@fieldses.org> wrote:
> 
> > On Wed, Jun 17, 2009 at 06:50:45PM -0700, Ricardo Labiaga wrote:
> >> renew_client() manipulates the client queue for lease renewal.  Need to
> >> obtain the client_mutex before manipulating it.
> >> 
> >> Signed-off-by: Ricardo Labiaga <Ricardo.Labiaga@netapp.com>
> >> ---
> >>  fs/nfsd/nfs4state.c |    2 ++
> >>  1 files changed, 2 insertions(+), 0 deletions(-)
> >> 
> >> diff --git a/fs/nfsd/nfs4state.c b/fs/nfsd/nfs4state.c
> >> index 4cb5d1d..18258d7 100644
> >> --- a/fs/nfsd/nfs4state.c
> >> +++ b/fs/nfsd/nfs4state.c
> >> @@ -1537,7 +1537,9 @@ replay_cache:
> >> * Hold a session reference until done processing the compound:
> >> * nfsd4_put_session called only if the cstate slot is set.
> >> */
> >> + nfs4_lock_state();
> >> renew_client(session->se_client);
> >> + nfs4_unlock_state();
> >> nfsd4_get_session(session);
> >>  out:
> >> spin_unlock(&sessionid_lock);
> > 
> > We can't take a mutex while handling a spinlock.  (And you should have
> > gotten warnings about this--not sure what kernel hacking config options
> > you may need turned on for that.)
> > 
> 
> Ah, OK.  I didn't know that.

The deadlock:

	Task 1 takes a spinlock.
	Task 1 sleeps on the mutex.
	Task 2 is scheduled in.
	Task 2 requests the same spinlock, spins.

Now Task 2 is spinning waiting for the spinlock, keeping task 1 from
getting the cpu back and releaseing the spinlock.  So, no sleeping under
spinlocks.

> > Possible hack: add a spinlock that for now will just cover the client
> > lru?
> 
> Let me study this some more and see if I can simply drop the sessionid
> spinlock before renewing the state.

OK.

--b.