Messages in this thread | | | From | Joel Fernandes <> | Subject | Re: [PATCH 1/2] rcu: Fix missing nocb gp wake on rcu_barrier() | Date | Tue, 11 Oct 2022 03:33:37 -0400 |
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> On Oct 11, 2022, at 3:25 AM, Paul E. McKenney <paulmck@kernel.org> wrote: > > On Tue, Oct 11, 2022 at 02:01:23AM +0000, Joel Fernandes wrote: >>> 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. > > Joel, could you please incorporate this into your series? My internet > access is likely to be a bit iffy over the next few days. Likely no > problem for email and the occasional test-systme access, but best not > to take it for granted. ;-)
Sure, I’ll do that. Thanks.
Fingers crossed on the internet ;-)
Thanks,
- Joel
> > Thanx, Paul > >> 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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