On Mon, Mar 04, 2024 at 03:40:21PM +1100, NeilBrown wrote:
> move_to_close_lru() waits for sc_count to become zero while holding
> rp_mutex. This can deadlock if another thread holds a reference and is
> waiting for rp_mutex.
>
> By the time we get to move_to_close_lru() the openowner is unhashed and
> cannot be found any more. So code waiting for the mutex can safely
> retry the lookup if move_to_close_lru() has started.
>
> So change rp_mutex to an atomic_t with three states:
>
> RP_UNLOCK - state is still hashed, not locked for reply
> RP_LOCKED - state is still hashed, is locked for reply
> RP_UNHASHED - state is not hashed, no code can get a lock.
>
> Use wait_var_event() to wait for either a lock, or for the owner to be
> unhashed. In the latter case, retry the lookup.
>
> Signed-off-by: NeilBrown <neilb@xxxxxxx>
> ---
> fs/nfsd/nfs4state.c | 38 +++++++++++++++++++++++++++++++-------
> fs/nfsd/state.h | 2 +-
> 2 files changed, 32 insertions(+), 8 deletions(-)
>
> diff --git a/fs/nfsd/nfs4state.c b/fs/nfsd/nfs4state.c
> index 690d0e697320..47e879d5d68a 100644
> --- a/fs/nfsd/nfs4state.c
> +++ b/fs/nfsd/nfs4state.c
> @@ -4430,21 +4430,32 @@ nfsd4_init_leases_net(struct nfsd_net *nn)
> atomic_set(&nn->nfsd_courtesy_clients, 0);
> }
>
> +enum rp_lock {
> + RP_UNLOCKED,
> + RP_LOCKED,
> + RP_UNHASHED,
> +};
> +
> static void init_nfs4_replay(struct nfs4_replay *rp)
> {
> rp->rp_status = nfserr_serverfault;
> rp->rp_buflen = 0;
> rp->rp_buf = rp->rp_ibuf;
> - mutex_init(&rp->rp_mutex);
> + atomic_set(&rp->rp_locked, RP_UNLOCKED);
> }
>
> -static void nfsd4_cstate_assign_replay(struct nfsd4_compound_state *cstate,
> - struct nfs4_stateowner *so)
> +static int nfsd4_cstate_assign_replay(struct nfsd4_compound_state *cstate,
> + struct nfs4_stateowner *so)
> {
> if (!nfsd4_has_session(cstate)) {
> - mutex_lock(&so->so_replay.rp_mutex);
> + wait_var_event(&so->so_replay.rp_locked,
> + atomic_cmpxchg(&so->so_replay.rp_locked,
> + RP_UNLOCKED, RP_LOCKED) != RP_LOCKED);
What reliably prevents this from being a "wait forever" ?
> + if (atomic_read(&so->so_replay.rp_locked) == RP_UNHASHED)
> + return -EAGAIN;
> cstate->replay_owner = nfs4_get_stateowner(so);
> }
> + return 0;
> }
>
> void nfsd4_cstate_clear_replay(struct nfsd4_compound_state *cstate)
> @@ -4453,7 +4464,8 @@ void nfsd4_cstate_clear_replay(struct nfsd4_compound_state *cstate)
>
> if (so != NULL) {
> cstate->replay_owner = NULL;
> - mutex_unlock(&so->so_replay.rp_mutex);
> + atomic_set(&so->so_replay.rp_locked, RP_UNLOCKED);
> + wake_up_var(&so->so_replay.rp_locked);
> nfs4_put_stateowner(so);
> }
> }
> @@ -4691,7 +4703,11 @@ move_to_close_lru(struct nfs4_ol_stateid *s, struct net *net)
> * Wait for the refcount to drop to 2. Since it has been unhashed,
> * there should be no danger of the refcount going back up again at
> * this point.
> + * Some threads with a reference might be waiting for rp_locked,
> + * so tell them to stop waiting.
> */
> + atomic_set(&oo->oo_owner.so_replay.rp_locked, RP_UNHASHED);
> + wake_up_var(&oo->oo_owner.so_replay.rp_locked);
> wait_event(close_wq, refcount_read(&s->st_stid.sc_count) == 2);
>
> release_all_access(s);
> @@ -5064,11 +5080,15 @@ nfsd4_process_open1(struct nfsd4_compound_state *cstate,
> clp = cstate->clp;
>
> strhashval = ownerstr_hashval(&open->op_owner);
> +retry:
> oo = find_or_alloc_open_stateowner(strhashval, open, cstate);
> open->op_openowner = oo;
> if (!oo)
> return nfserr_jukebox;
> - nfsd4_cstate_assign_replay(cstate, &oo->oo_owner);
> + if (nfsd4_cstate_assign_replay(cstate, &oo->oo_owner) == -EAGAIN) {
> + nfs4_put_stateowner(&oo->oo_owner);
> + goto retry;
> + }
> status = nfsd4_check_seqid(cstate, &oo->oo_owner, open->op_seqid);
> if (status)
> return status;
> @@ -6828,11 +6848,15 @@ nfs4_preprocess_seqid_op(struct nfsd4_compound_state *cstate, u32 seqid,
> trace_nfsd_preprocess(seqid, stateid);
>
> *stpp = NULL;
> +retry:
> status = nfsd4_lookup_stateid(cstate, stateid, typemask, &s, nn);
> if (status)
> return status;
> stp = openlockstateid(s);
> - nfsd4_cstate_assign_replay(cstate, stp->st_stateowner);
> + if (nfsd4_cstate_assign_replay(cstate, stp->st_stateowner) == -EAGAIN) {
> + nfs4_put_stateowner(stp->st_stateowner);
> + goto retry;
> + }
>
> status = nfs4_seqid_op_checks(cstate, stateid, seqid, stp);
> if (!status)
My tool chain reports that this hunk won't apply to nfsd-next.
In my copy of fs/nfsd/nfs4state.c, nfs4_preprocess_seqid_op() starts
at line 7180, so something is whack-a-doodle.
> diff --git a/fs/nfsd/state.h b/fs/nfsd/state.h
> index 41bdc913fa71..6a3becd86112 100644
> --- a/fs/nfsd/state.h
> +++ b/fs/nfsd/state.h
> @@ -446,7 +446,7 @@ struct nfs4_replay {
> unsigned int rp_buflen;
> char *rp_buf;
> struct knfsd_fh rp_openfh;
> - struct mutex rp_mutex;
> + atomic_t rp_locked;
> char rp_ibuf[NFSD4_REPLAY_ISIZE];
> };
>
> --
> 2.43.0
>
--
Chuck Lever
