Re: [PATCH v4 06/30] xprtrdma: Don't wake pending tasks until disconnect is done

[Trond Myklebust <trondmy@hammerspace.com>]

On Mon, 2018-12-17 at 14:19 -0500, Chuck Lever wrote:
> > On Dec 17, 2018, at 2:09 PM, Trond Myklebust <
> > trondmy@hammerspace.com> wrote:
> > 
> > On Mon, 2018-12-17 at 14:00 -0500, Chuck Lever wrote:
> > > > On Dec 17, 2018, at 1:55 PM, Trond Myklebust <
> > > > trondmy@hammerspace.com> wrote:
> > > > 
> > > > On Mon, 2018-12-17 at 13:37 -0500, Chuck Lever wrote:
> > > > > > On Dec 17, 2018, at 12:28 PM, Trond Myklebust <
> > > > > > trondmy@hammerspace.com> wrote:
> > > > > > 
> > > > > > On Mon, 2018-12-17 at 11:39 -0500, Chuck Lever wrote:
> > > > > > > Transport disconnect processing does a "wake pending
> > > > > > > tasks"
> > > > > > > at
> > > > > > > various points.
> > > > > > > 
> > > > > > > Suppose an RPC Reply is being processed. The RPC task
> > > > > > > that
> > > > > > > Reply
> > > > > > > goes with is waiting on the pending queue. If a
> > > > > > > disconnect
> > > > > > > wake-
> > > > > > > up
> > > > > > > happens before reply processing is done, that reply, even
> > > > > > > if
> > > > > > > it
> > > > > > > is
> > > > > > > good, is thrown away, and the RPC has to be sent again.
> > > > > > > 
> > > > > > > This window apparently does not exist for socket
> > > > > > > transports
> > > > > > > because
> > > > > > > there is a lock held while a reply is being received
> > > > > > > which
> > > > > > > prevents
> > > > > > > the wake-up call until after reply processing is done.
> > > > > > > 
> > > > > > > To resolve this, all RPC replies being processed on an
> > > > > > > RPC-
> > > > > > > over-
> > > > > > > RDMA
> > > > > > > transport have to complete before pending tasks are
> > > > > > > awoken
> > > > > > > due to
> > > > > > > a
> > > > > > > transport disconnect.
> > > > > > > 
> > > > > > > Callers that already hold the transport write lock may
> > > > > > > invoke
> > > > > > > ->ops->close directly. Others use a generic helper that
> > > > > > > schedules
> > > > > > > a close when the write lock can be taken safely.
> > > > > > > 
> > > > > > > Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
> > > > > > > ---
> > > > > > > include/linux/sunrpc/xprt.h                |    1 +
> > > > > > > net/sunrpc/xprt.c                          |   19
> > > > > > > +++++++++++++++++++
> > > > > > > net/sunrpc/xprtrdma/backchannel.c          |   13
> > > > > > > +++++++--
> > > > > > > ----
> > > > > > > net/sunrpc/xprtrdma/svc_rdma_backchannel.c |    8 +++++
> > > > > > > ---
> > > > > > > net/sunrpc/xprtrdma/transport.c            |   16
> > > > > > > ++++++++++-
> > > > > > > ----
> > > > > > > -
> > > > > > > net/sunrpc/xprtrdma/verbs.c                |    5 ++---
> > > > > > > 6 files changed, 44 insertions(+), 18 deletions(-)
> > > > > > > 
> > > > > > > diff --git a/include/linux/sunrpc/xprt.h
> > > > > > > b/include/linux/sunrpc/xprt.h
> > > > > > > index a4ab4f8..ee94ed0 100644
> > > > > > > --- a/include/linux/sunrpc/xprt.h
> > > > > > > +++ b/include/linux/sunrpc/xprt.h
> > > > > > > @@ -401,6 +401,7 @@ static inline __be32
> > > > > > > *xprt_skip_transport_header(struct rpc_xprt *xprt, __be32
> > > > > > > *
> > > > > > > bool			xprt_request_get_cong(struct
> > > > > > > rpc_xprt
> > > > > > > *xprt,
> > > > > > > struct rpc_rqst *req);
> > > > > > > void			xprt_disconnect_done(struct
> > > > > > > rpc_xprt
> > > > > > > *xprt);
> > > > > > > void			xprt_force_disconnect(struct
> > > > > > > rpc_xprt
> > > > > > > *xprt);
> > > > > > > +void			xprt_disconnect_nowake(struct
> > > > > > > rpc_xprt
> > > > > > > *xprt);
> > > > > > > void			xprt_conditional_disconnect(str
> > > > > > > uct
> > > > > > > rpc_xprt
> > > > > > > *xprt, unsigned int cookie);
> > > > > > > 
> > > > > > > bool			xprt_lock_connect(struct
> > > > > > > rpc_xprt *,
> > > > > > > struct
> > > > > > > rpc_task *, void *);
> > > > > > > diff --git a/net/sunrpc/xprt.c b/net/sunrpc/xprt.c
> > > > > > > index ce92700..afe412e 100644
> > > > > > > --- a/net/sunrpc/xprt.c
> > > > > > > +++ b/net/sunrpc/xprt.c
> > > > > > > @@ -685,6 +685,25 @@ void xprt_force_disconnect(struct
> > > > > > > rpc_xprt
> > > > > > > *xprt)
> > > > > > > }
> > > > > > > EXPORT_SYMBOL_GPL(xprt_force_disconnect);
> > > > > > > 
> > > > > > > +/**
> > > > > > > + * xprt_disconnect_nowake - force a call to xprt->ops-
> > > > > > > >close
> > > > > > > + * @xprt: transport to disconnect
> > > > > > > + *
> > > > > > > + * The caller must ensure that xprt_wake_pending_tasks()
> > > > > > > is
> > > > > > > + * called later.
> > > > > > > + */
> > > > > > > +void xprt_disconnect_nowake(struct rpc_xprt *xprt)
> > > > > > > +{
> > > > > > > +       /* Don't race with the test_bit() in
> > > > > > > xprt_clear_locked()
> > > > > > > */
> > > > > > > +       spin_lock_bh(&xprt->transport_lock);
> > > > > > > +       set_bit(XPRT_CLOSE_WAIT, &xprt->state);
> > > > > > > +       /* Try to schedule an autoclose RPC call */
> > > > > > > +       if (test_and_set_bit(XPRT_LOCKED, &xprt->state)
> > > > > > > == 0)
> > > > > > > +               queue_work(xprtiod_workqueue, &xprt-
> > > > > > > > task_cleanup);
> > > > > > > +       spin_unlock_bh(&xprt->transport_lock);
> > > > > > > +}
> > > > > > > +EXPORT_SYMBOL_GPL(xprt_disconnect_nowake);
> > > > > > > +
> > > > > > 
> > > > > > We shouldn't need both xprt_disconnect_nowake() and
> > > > > > xprt_force_disconnect() to be exported given that you can
> > > > > > build
> > > > > > the
> > > > > > latter from the former + xprt_wake_pending_tasks() (which
> > > > > > is
> > > > > > also
> > > > > > already exported).
> > > > > 
> > > > > Thanks for your review!
> > > > > 
> > > > > I can get rid of xprt_disconnect_nowake. There are some
> > > > > variations,
> > > > > depending on why wake_pending_tasks is protected by xprt-
> > > > > > transport_lock.
> > > > 
> > > > I'm having some second thoughts about the patch that Scott sent
> > > > out
> > > > last week to fix the issue that Dave and he were seeing. I
> > > > think
> > > > that
> > > > what we really need to do to fix his issue is to call
> > > > xprt_disconnect_done() after we've released the TCP socket.
> > > > 
> > > > Given that realisation, I think that we can fix up
> > > > xprt_force_disconnect() to only wake up the task that holds the
> > > > XPRT_LOCKED instead of doing a thundering herd wakeup like we
> > > > do
> > > > today.
> > > > That should (I think) obviate the need for a separate
> > > > xprt_disconnect_nowake().
> > > 
> > > For RPC-over-RDMA, there really is a dangerous race between the
> > > waking
> > > task(s) and work being done by the deferred RPC completion
> > > handler.
> > > IMO
> > > the only safe thing RPC-over-RDMA can do is not wake anything
> > > until
> > > the
> > > deferred queue is well and truly drained.
> > 
> > The deferred RPC completion handler (and hence the close) cannot
> > execute if another task is holding XPRT_LOCKED,
> 
> Just to be certain we are speaking of the same thing,
> rpcrdma_deferred_completion is queued by the Receive handler, and
> can indeed run independently of an rpc_task. It is always running
> outside the purview of XPRT_LOCKED.

No. I was thinking of the xprt->task_cleanup. It can't execute, and
complete your close until it holds XPRT_LOCKED.

> 
> 
> > so we do need to wake up that task (and only that one).
> > 
> > Note that in the new code, the only reason why a task would be
> > holding
> > XPRT_LOCKED while sleeping is because
> > 
> >   1. It is waiting for a connection attempt to complete following a
> > call
> >      to xprt_connect().
> >   2. It is waiting for a write_space event following an attempt to
> >      transmit.
> 
> xprt_rdma_close can sleep in rpcrdma_ep_disconnect:
> 
>  -> ib_drain_{qp,sq,rq} can all sleep waiting for the last FLUSH
> 
>  -> drain_workqueue, added in this patch, can sleep waiting for the
>     deferred RPC completion workqueue to drain
> 
> 
> > > As you observed when we last spoke, socket transports are already
> > > protected from this race by the socket lock.... RPC-over-RDMA is
> > > going
> > > to have to be more careful.
> > > 
> > > 
> > > > A patch is forthcoming later today. I'll make sure you are Cced
> > > > so
> > > > you
> > > > can comment.
> > > > 
> > > > -- 
> > > > Trond Myklebust
> > > > Linux NFS client maintainer, Hammerspace
> > > > trond.myklebust@hammerspace.com
> > > 
> > > --
> > > Chuck Lever
> > > 
> > > 
> > > 
> > -- 
> > Trond Myklebust
> > CTO, Hammerspace Inc
> > 4300 El Camino Real, Suite 105
> > Los Altos, CA 94022
> > www.hammer.space
> 
> --
> Chuck Lever
> 
> 
> 
-- 
Trond Myklebust
CTO, Hammerspace Inc
4300 El Camino Real, Suite 105
Los Altos, CA 94022
www.hammer.space