Messages in this thread | | | Date | Tue, 11 Oct 2022 02:01:23 +0000 | From | Joel Fernandes <> | Subject | Re: [PATCH 1/2] rcu: Fix missing nocb gp wake on rcu_barrier() |
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On Tue, Oct 11, 2022 at 12:39:55AM +0200, Frederic Weisbecker wrote: > Upon entraining a callback to a NOCB CPU, no further wake up is > issued on the corresponding nocb_gp kthread. As a result, the callback > and all the subsequent ones on that CPU may be ignored, at least until > an RCU_NOCB_WAKE_FORCE timer is ever armed or another NOCB CPU belonging > to the same group enqueues a callback on an empty queue. > > Here is a possible bad scenario: > > 1) CPU 0 is NOCB unlike all other CPUs. > 2) CPU 0 queues a callback > 2) The grace period related to that callback elapses > 3) The callback is moved to the done list (but is not invoked yet), > there are no more pending callbacks for CPU 0 > 4) CPU 1 calls rcu_barrier() and sends an IPI to CPU 0 > 5) CPU 0 entrains the callback but doesn't wake up nocb_gp > 6) CPU 1 blocks forever, unless CPU 0 ever queues enough further > callbacks to arm an RCU_NOCB_WAKE_FORCE timer. > > Make sure the necessary wake up is produced whenever necessary. > > Reported-by: Joel Fernandes (Google) <joel@joelfernandes.org> > Fixes: 5d6742b37727 ("rcu/nocb: Use rcu_segcblist for no-CBs CPUs") > Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Acked-by: Joel Fernandes (Google) <joel@joelfernandes.org>
And if Paul is taking this, I'll rebase and drop this patch from the lazy series.
thanks,
- Joel
> --- > kernel/rcu/tree.c | 6 ++++++ > kernel/rcu/tree.h | 1 + > kernel/rcu/tree_nocb.h | 5 +++++ > 3 files changed, 12 insertions(+) > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > index 96d678c9cfb6..025f59f6f97f 100644 > --- a/kernel/rcu/tree.c > +++ b/kernel/rcu/tree.c > @@ -3914,6 +3914,8 @@ static void rcu_barrier_entrain(struct rcu_data *rdp) > { > unsigned long gseq = READ_ONCE(rcu_state.barrier_sequence); > unsigned long lseq = READ_ONCE(rdp->barrier_seq_snap); > + bool wake_nocb = false; > + bool was_alldone = false; > > lockdep_assert_held(&rcu_state.barrier_lock); > if (rcu_seq_state(lseq) || !rcu_seq_state(gseq) || rcu_seq_ctr(lseq) != rcu_seq_ctr(gseq)) > @@ -3922,6 +3924,7 @@ static void rcu_barrier_entrain(struct rcu_data *rdp) > rdp->barrier_head.func = rcu_barrier_callback; > debug_rcu_head_queue(&rdp->barrier_head); > rcu_nocb_lock(rdp); > + was_alldone = rcu_rdp_is_offloaded(rdp) && !rcu_segcblist_pend_cbs(&rdp->cblist); > WARN_ON_ONCE(!rcu_nocb_flush_bypass(rdp, NULL, jiffies)); > if (rcu_segcblist_entrain(&rdp->cblist, &rdp->barrier_head)) { > atomic_inc(&rcu_state.barrier_cpu_count); > @@ -3929,7 +3932,10 @@ static void rcu_barrier_entrain(struct rcu_data *rdp) > debug_rcu_head_unqueue(&rdp->barrier_head); > rcu_barrier_trace(TPS("IRQNQ"), -1, rcu_state.barrier_sequence); > } > + wake_nocb = was_alldone && rcu_segcblist_pend_cbs(&rdp->cblist); > rcu_nocb_unlock(rdp); > + if (wake_nocb) > + wake_nocb_gp(rdp, false); > smp_store_release(&rdp->barrier_seq_snap, gseq); > } > > diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h > index d4a97e40ea9c..925dd98f8b23 100644 > --- a/kernel/rcu/tree.h > +++ b/kernel/rcu/tree.h > @@ -439,6 +439,7 @@ static void zero_cpu_stall_ticks(struct rcu_data *rdp); > static struct swait_queue_head *rcu_nocb_gp_get(struct rcu_node *rnp); > static void rcu_nocb_gp_cleanup(struct swait_queue_head *sq); > static void rcu_init_one_nocb(struct rcu_node *rnp); > +static bool wake_nocb_gp(struct rcu_data *rdp, bool force); > static bool rcu_nocb_flush_bypass(struct rcu_data *rdp, struct rcu_head *rhp, > unsigned long j); > static bool rcu_nocb_try_bypass(struct rcu_data *rdp, struct rcu_head *rhp, > diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h > index f77a6d7e1356..094fd454b6c3 100644 > --- a/kernel/rcu/tree_nocb.h > +++ b/kernel/rcu/tree_nocb.h > @@ -1558,6 +1558,11 @@ static void rcu_init_one_nocb(struct rcu_node *rnp) > { > } > > +static bool wake_nocb_gp(struct rcu_data *rdp, bool force) > +{ > + return false; > +} > + > static bool rcu_nocb_flush_bypass(struct rcu_data *rdp, struct rcu_head *rhp, > unsigned long j) > { > -- > 2.25.1 >
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