On Aug 1, 2011, at 6:35 PM, Trond Myklebust wrote:
> On Mon, 2011-08-01 at 17:10 -0400, Trond Myklebust wrote:
>> Looking at the callback code, I see that if tbl->highest_used_slotid !=
>> 0, then we BUG() while holding the backchannel's tbl->slot_tbl_lock
>> spinlock. That seems a likely candidate for the above hang.
>>
>> Andy, how we are guaranteed that tbl->highest_used_slotid won't take
>> values other than 0, and why do we commit suicide when it does? As far
>> as I can see, there is no guarantee that we call nfs4_cb_take_slot() in
>> nfs4_callback_compound(), however we appear to unconditionally call
>> nfs4_cb_free_slot() provided there is a session.
>>
>> The other strangeness would be the fact that there is nothing enforcing
>> the NFS4_SESSION_DRAINING flag. If the session is draining, then the
>> back-channel simply ignores that and goes ahead with processing the
>> callback. Is this to avoid deadlocks with the server returning
>> NFS4ERR_BACK_CHAN_BUSY when the client does a DESTROY_SESSION?
When NFS4_SESSION_DRAINING is set, the backchannel is drained first - and waits for current processing to complete signaled by the highest_used_slotid == -1. Any other backchannel requests that occur under the NFS4_SESSION_DRAINING flag are processed - but just the CB_SEQUENCE operation which returns NFS4ERR_DELAY. It does indeed prevent the BACK_CHAN_BUSY deadlock.
>
> How about something like the following?
Looks good. Nice catch. One change below and a comment inline
-->Andy
>
> 8<-------------------------------------------------------------------------------
> From c0c499b0ca9d0af8cbdc29c40effba38475461d9 Mon Sep 17 00:00:00 2001
> From: Trond Myklebust <Trond.Myklebust@xxxxxxxxxx>
> Date: Mon, 1 Aug 2011 18:31:12 -0400
> Subject: [PATCH] NFSv4.1: Fix the callback 'highest_used_slotid' behaviour
>
> Currently, there is no guarantee that we will call nfs4_cb_take_slot() even
> though nfs4_callback_compound() will consistently call
> nfs4_cb_free_slot() provided the cb_process_state has set the 'clp' field.
> The result is that we can trigger the BUG_ON() upon the next call to
> nfs4_cb_take_slot().
> This patch fixes the above problem by using the slot id that was taken in
> the CB_SEQUENCE operation as a flag for whether or not we need to call
> nfs4_cb_free_slot().
> It also fixes an atomicity problem: we need to set tbl->highest_used_slotid
> atomically with the check for NFS4_SESSION_DRAINING, otherwise we end up
> racing with the various tests in nfs4_begin_drain_session().
But the code below doesn't do this - it locks the backchannel slot table to check the flag, but does not set the highest_used_slot atomically with this check.
>
> Signed-off-by: Trond Myklebust <Trond.Myklebust@xxxxxxxxxx>
> ---
> fs/nfs/callback.h | 2 +-
> fs/nfs/callback_proc.c | 18 +++++++++++++-----
> fs/nfs/callback_xdr.c | 24 +++++++-----------------
> 3 files changed, 21 insertions(+), 23 deletions(-)
>
> diff --git a/fs/nfs/callback.h b/fs/nfs/callback.h
> index b257383..07df5f1 100644
> --- a/fs/nfs/callback.h
> +++ b/fs/nfs/callback.h
> @@ -38,6 +38,7 @@ enum nfs4_callback_opnum {
> struct cb_process_state {
> __be32 drc_status;
> struct nfs_client *clp;
> + int slotid;
> };
>
> struct cb_compound_hdr_arg {
> @@ -166,7 +167,6 @@ extern unsigned nfs4_callback_layoutrecall(
> void *dummy, struct cb_process_state *cps);
>
> extern void nfs4_check_drain_bc_complete(struct nfs4_session *ses);
> -extern void nfs4_cb_take_slot(struct nfs_client *clp);
>
> struct cb_devicenotifyitem {
> uint32_t cbd_notify_type;
> diff --git a/fs/nfs/callback_proc.c b/fs/nfs/callback_proc.c
> index 74780f9..1bd2c81 100644
> --- a/fs/nfs/callback_proc.c
> +++ b/fs/nfs/callback_proc.c
> @@ -348,7 +348,7 @@ validate_seqid(struct nfs4_slot_table *tbl, struct cb_sequenceargs * args)
> /* Normal */
> if (likely(args->csa_sequenceid == slot->seq_nr + 1)) {
> slot->seq_nr++;
> - return htonl(NFS4_OK);
> + goto out_ok;
> }
>
> /* Replay */
> @@ -367,11 +367,14 @@ validate_seqid(struct nfs4_slot_table *tbl, struct cb_sequenceargs * args)
> /* Wraparound */
> if (args->csa_sequenceid == 1 && (slot->seq_nr + 1) == 0) {
> slot->seq_nr = 1;
> - return htonl(NFS4_OK);
> + goto out_ok;
> }
>
> /* Misordered request */
> return htonl(NFS4ERR_SEQ_MISORDERED);
> +out_ok:
> + tbl->highest_used_slotid = args->csa_slotid;
> + return htonl(NFS4_OK);
> }
>
> /*
> @@ -433,26 +436,32 @@ __be32 nfs4_callback_sequence(struct cb_sequenceargs *args,
> struct cb_sequenceres *res,
> struct cb_process_state *cps)
> {
> + struct nfs4_slot_table *tbl;
> struct nfs_client *clp;
> int i;
> __be32 status = htonl(NFS4ERR_BADSESSION);
>
> - cps->clp = NULL;
> -
> clp = nfs4_find_client_sessionid(args->csa_addr, &args->csa_sessionid);
> if (clp == NULL)
> goto out;
>
> + tbl = &clp->cl_session->bc_slot_table;
> +
> + spin_lock(&tbl->slot_tbl_lock);
> /* state manager is resetting the session */
> if (test_bit(NFS4_SESSION_DRAINING, &clp->cl_session->session_state)) {
> status = NFS4ERR_DELAY;
status = htonl(NFS4ERR_DELAY);
> + spin_unlock(&tbl->slot_tbl_lock);
> goto out;
> }
>
> status = validate_seqid(&clp->cl_session->bc_slot_table, args);
> + spin_unlock(&tbl->slot_tbl_lock);
> if (status)
> goto out;
>
> + cps->slotid = args->csa_slotid;
> +
> /*
> * Check for pending referring calls. If a match is found, a
> * related callback was received before the response to the original
> @@ -469,7 +478,6 @@ __be32 nfs4_callback_sequence(struct cb_sequenceargs *args,
> res->csr_slotid = args->csa_slotid;
> res->csr_highestslotid = NFS41_BC_MAX_CALLBACKS - 1;
> res->csr_target_highestslotid = NFS41_BC_MAX_CALLBACKS - 1;
> - nfs4_cb_take_slot(clp);
>
> out:
> cps->clp = clp; /* put in nfs4_callback_compound */
> diff --git a/fs/nfs/callback_xdr.c b/fs/nfs/callback_xdr.c
> index c6c86a7..918ad64 100644
> --- a/fs/nfs/callback_xdr.c
> +++ b/fs/nfs/callback_xdr.c
> @@ -754,26 +754,15 @@ static void nfs4_callback_free_slot(struct nfs4_session *session)
> * Let the state manager know callback processing done.
> * A single slot, so highest used slotid is either 0 or -1
> */
> - tbl->highest_used_slotid--;
> + tbl->highest_used_slotid = -1;
> nfs4_check_drain_bc_complete(session);
> spin_unlock(&tbl->slot_tbl_lock);
> }
>
> -static void nfs4_cb_free_slot(struct nfs_client *clp)
> +static void nfs4_cb_free_slot(struct cb_process_state *cps)
> {
> - if (clp && clp->cl_session)
> - nfs4_callback_free_slot(clp->cl_session);
> -}
> -
> -/* A single slot, so highest used slotid is either 0 or -1 */
> -void nfs4_cb_take_slot(struct nfs_client *clp)
> -{
> - struct nfs4_slot_table *tbl = &clp->cl_session->bc_slot_table;
> -
> - spin_lock(&tbl->slot_tbl_lock);
> - tbl->highest_used_slotid++;
> - BUG_ON(tbl->highest_used_slotid != 0);
> - spin_unlock(&tbl->slot_tbl_lock);
> + if (cps->slotid != -1)
> + nfs4_callback_free_slot(cps->clp->cl_session);
> }
>
> #else /* CONFIG_NFS_V4_1 */
> @@ -784,7 +773,7 @@ preprocess_nfs41_op(int nop, unsigned int op_nr, struct callback_op **op)
> return htonl(NFS4ERR_MINOR_VERS_MISMATCH);
> }
>
> -static void nfs4_cb_free_slot(struct nfs_client *clp)
> +static void nfs4_cb_free_slot(struct cb_process_state *cps)
> {
> }
> #endif /* CONFIG_NFS_V4_1 */
> @@ -866,6 +855,7 @@ static __be32 nfs4_callback_compound(struct svc_rqst *rqstp, void *argp, void *r
> struct cb_process_state cps = {
> .drc_status = 0,
> .clp = NULL,
> + .slotid = -1,
> };
> unsigned int nops = 0;
>
> @@ -906,7 +896,7 @@ static __be32 nfs4_callback_compound(struct svc_rqst *rqstp, void *argp, void *r
>
> *hdr_res.status = status;
> *hdr_res.nops = htonl(nops);
> - nfs4_cb_free_slot(cps.clp);
> + nfs4_cb_free_slot(&cps);
> nfs_put_client(cps.clp);
> dprintk("%s: done, status = %u\n", __func__, ntohl(status));
> return rpc_success;
> --
> 1.7.6
>
>
> --
> Trond Myklebust
> Linux NFS client maintainer
>
> NetApp
> Trond.Myklebust@xxxxxxxxxx
> www.netapp.com
>
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