On Tue, 05 Mar 2024, Chuck Lever wrote:
> On Tue, Mar 05, 2024 at 08:45:45AM +1100, NeilBrown wrote:
> > On Tue, 05 Mar 2024, Chuck Lever wrote:
> > > 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" ?
> >
> > That same thing that reliably prevented the original mutex_lock from
> > waiting forever.
> > It waits until rp_locked is set to RP_UNLOCKED, which is precisely when
> > we previously called mutex_unlock. But it *also* aborts the wait if
> > rp_locked is set to RP_UNHASHED - so it is now more reliable.
> >
> > Does that answer the question?
>
> Hm. I guess then we are no worse off with wait_var_event().
>
> I'm not as familiar with this logic as perhaps I should be. How long
> does it take for the wake-up to occur, typically?
wait_var_event() is paired with wake_up_var().
The wake up happens when wake_up_var() is called, which in this code is
always immediately after atomic_set() updates the variable.
>
>
> > > > + 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.
> >
> > You need better tools :-)
>
> <shrug> Essentially it is git, importing an mbox. That kind of thing
> is generally the weakest aspect of all these tools. Do you know of
> something more robust?
My tool of choice is "wiggle" - which I wrote.
I just put those 4 patches into an mbox and use "git am < mbox" to apply
to nfsd-next. It hit a problem at this patch - 3/4.
So I ran
wiggle -mrp .git/rebase-apply/patch
which worked without complaint. (you might want --no-backup too).
But you probably want to know that the conflict was that "git am" found
and "wiggle" ignored. So:
git diff | diff -u -I '^@@' -I '^index' .git/rebase-apply/patch -
This shows
+retry:
- status = nfsd4_lookup_stateid(cstate, stateid, typemask, &s, nn);
+ status = nfsd4_lookup_stateid(cstate, stateid,
+ typemask, statusmask, &s, nn);
once you have had a bit of practice reading diffs of diffs you can see
that the patch inserts "retry:" before a line which has changed
between the original and new bases. The difference clearly isn't
important to this patch.
So "git add -u; git am --continue" will accept the wiggle and continue
with the "git am".
Patch 4/4 has one conflict:
struct nfsd_net *nn = net_generic(net, nfsd_net_id);
+ bool need_move_to_close_list;
- dprintk("NFSD: nfsd4_close on file %pd\n",
+ dprintk("NFSD: nfsd4_close on file %pd\n",
cstate->current_fh.fh_dentry);
again - not interesting (white space at end of line changed).
But maybe you would rather avoid that and have submitted base their
patches properly to start with :-)
>
>
> > > In my copy of fs/nfsd/nfs4state.c, nfs4_preprocess_seqid_op() starts
> > > at line 7180, so something is whack-a-doodle.
> >
> > I have been working against master because the old version of the fix
> > was in nfsd-next. I should have rebased when you removed it from
> > nfsd-next. I have now and will repost once you confirm you are
> > comfortable with the answer about waiting above. Would adding a comment
> > there help?
>
> The mechanism is clear from the code, so a new comment, if you add
> one, should spell out what condition(s) mark rp_locked as UNLOCKED.
>
> But I might be missing something that should be obvious, in which
> case no comment is necessary.
>
I've added a comment emphasising that it is much like a mutex, and
pointing to the matching unlocks.
Thanks,
NeilBrown
