| Message ID | 20231208220545.7452-2-frederic@kernel.org (mailing list archive) |
|---|---|
| State | Superseded |
| Headers | show |
| Series | rcu: Fix expedited GP deadlock (and cleanup some nocb stuff) | expand |
On Sat, Dec 9, 2023 at 3:35 AM Frederic Weisbecker <frederic@kernel.org> wrote: > > Currently IRQs are disabled on call_rcu() and then depending on the > context: > > * If the CPU is in nocb mode: > > - If the callback is enqueued in the bypass list, IRQs are re-enabled > implictly by rcu_nocb_try_bypass() > > - If the callback is enqueued in the normal list, IRQs are re-enabled > implicitly by __call_rcu_nocb_wake() > > * If the CPU is NOT in nocb mode, IRQs are reenabled explicitly from call_rcu() > > This makes the code a bit hard to follow, especially as it interleaves > with nocb locking. > > To make the IRQ flags coverage clearer and also in order to prepare for > moving all the nocb enqueue code to its own function, always re-enable > the IRQ flags explicitly from call_rcu(). > > Signed-off-by: Frederic Weisbecker <frederic@kernel.org> > --- > kernel/rcu/tree.c | 9 ++++++--- > kernel/rcu/tree_nocb.h | 20 +++++++++----------- > 2 files changed, 15 insertions(+), 14 deletions(-) > Nit: s/symetric/symmetric/ Reviewed-by: Neeraj Upadhyay (AMD) <neeraj.iitr10@gmail.com> Thanks Neeraj > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > index 52c2fdbc6363..74159c6d3bdf 100644 > --- a/kernel/rcu/tree.c > +++ b/kernel/rcu/tree.c > @@ -2697,8 +2697,10 @@ __call_rcu_common(struct rcu_head *head, rcu_callback_t func, bool lazy_in) > } > > check_cb_ovld(rdp); > - if (rcu_nocb_try_bypass(rdp, head, &was_alldone, flags, lazy)) > + if (rcu_nocb_try_bypass(rdp, head, &was_alldone, flags, lazy)) { > + local_irq_restore(flags); > return; // Enqueued onto ->nocb_bypass, so just leave. > + } > // If no-CBs CPU gets here, rcu_nocb_try_bypass() acquired ->nocb_lock. > rcu_segcblist_enqueue(&rdp->cblist, head); > if (__is_kvfree_rcu_offset((unsigned long)func)) > @@ -2716,8 +2718,8 @@ __call_rcu_common(struct rcu_head *head, rcu_callback_t func, bool lazy_in) > __call_rcu_nocb_wake(rdp, was_alldone, flags); /* unlocks */ > } else { > __call_rcu_core(rdp, head, flags); > - local_irq_restore(flags); > } > + local_irq_restore(flags); > } > > #ifdef CONFIG_RCU_LAZY > @@ -4615,8 +4617,9 @@ void rcutree_migrate_callbacks(int cpu) > __call_rcu_nocb_wake(my_rdp, true, flags); > } else { > rcu_nocb_unlock(my_rdp); /* irqs remain disabled. */ > - raw_spin_unlock_irqrestore_rcu_node(my_rnp, flags); > + raw_spin_unlock_rcu_node(my_rnp); /* irqs remain disabled. */ > } > + local_irq_restore(flags); > if (needwake) > rcu_gp_kthread_wake(); > lockdep_assert_irqs_enabled(); > diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h > index 5598212d1f27..3f70fd0a2db4 100644 > --- a/kernel/rcu/tree_nocb.h > +++ b/kernel/rcu/tree_nocb.h > @@ -532,9 +532,7 @@ static bool rcu_nocb_try_bypass(struct rcu_data *rdp, struct rcu_head *rhp, > // 2. Both of these conditions are met: > // a. The bypass list previously had only lazy CBs, and: > // b. The new CB is non-lazy. > - if (ncbs && (!bypass_is_lazy || lazy)) { > - local_irq_restore(flags); > - } else { > + if (!ncbs || (bypass_is_lazy && !lazy)) { > // No-CBs GP kthread might be indefinitely asleep, if so, wake. > rcu_nocb_lock(rdp); // Rare during call_rcu() flood. > if (!rcu_segcblist_pend_cbs(&rdp->cblist)) { > @@ -544,7 +542,7 @@ static bool rcu_nocb_try_bypass(struct rcu_data *rdp, struct rcu_head *rhp, > } else { > trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, > TPS("FirstBQnoWake")); > - rcu_nocb_unlock_irqrestore(rdp, flags); > + rcu_nocb_unlock(rdp); > } > } > return true; // Callback already enqueued. > @@ -570,7 +568,7 @@ static void __call_rcu_nocb_wake(struct rcu_data *rdp, bool was_alldone, > // If we are being polled or there is no kthread, just leave. > t = READ_ONCE(rdp->nocb_gp_kthread); > if (rcu_nocb_poll || !t) { > - rcu_nocb_unlock_irqrestore(rdp, flags); > + rcu_nocb_unlock(rdp); > trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, > TPS("WakeNotPoll")); > return; > @@ -583,17 +581,17 @@ static void __call_rcu_nocb_wake(struct rcu_data *rdp, bool was_alldone, > rdp->qlen_last_fqs_check = len; > // Only lazy CBs in bypass list > if (lazy_len && bypass_len == lazy_len) { > - rcu_nocb_unlock_irqrestore(rdp, flags); > + rcu_nocb_unlock(rdp); > wake_nocb_gp_defer(rdp, RCU_NOCB_WAKE_LAZY, > TPS("WakeLazy")); > } else if (!irqs_disabled_flags(flags)) { > /* ... if queue was empty ... */ > - rcu_nocb_unlock_irqrestore(rdp, flags); > + rcu_nocb_unlock(rdp); > wake_nocb_gp(rdp, false); > trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, > TPS("WakeEmpty")); > } else { > - rcu_nocb_unlock_irqrestore(rdp, flags); > + rcu_nocb_unlock(rdp); > wake_nocb_gp_defer(rdp, RCU_NOCB_WAKE, > TPS("WakeEmptyIsDeferred")); > } > @@ -611,15 +609,15 @@ static void __call_rcu_nocb_wake(struct rcu_data *rdp, bool was_alldone, > if ((rdp->nocb_cb_sleep || > !rcu_segcblist_ready_cbs(&rdp->cblist)) && > !timer_pending(&rdp->nocb_timer)) { > - rcu_nocb_unlock_irqrestore(rdp, flags); > + rcu_nocb_unlock(rdp); > wake_nocb_gp_defer(rdp, RCU_NOCB_WAKE_FORCE, > TPS("WakeOvfIsDeferred")); > } else { > - rcu_nocb_unlock_irqrestore(rdp, flags); > + rcu_nocb_unlock(rdp); > trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("WakeNot")); > } > } else { > - rcu_nocb_unlock_irqrestore(rdp, flags); > + rcu_nocb_unlock(rdp); > trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("WakeNot")); > } > } > -- > 2.42.1 > >
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index 52c2fdbc6363..74159c6d3bdf 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -2697,8 +2697,10 @@ __call_rcu_common(struct rcu_head *head, rcu_callback_t func, bool lazy_in) } check_cb_ovld(rdp); - if (rcu_nocb_try_bypass(rdp, head, &was_alldone, flags, lazy)) + if (rcu_nocb_try_bypass(rdp, head, &was_alldone, flags, lazy)) { + local_irq_restore(flags); return; // Enqueued onto ->nocb_bypass, so just leave. + } // If no-CBs CPU gets here, rcu_nocb_try_bypass() acquired ->nocb_lock. rcu_segcblist_enqueue(&rdp->cblist, head); if (__is_kvfree_rcu_offset((unsigned long)func)) @@ -2716,8 +2718,8 @@ __call_rcu_common(struct rcu_head *head, rcu_callback_t func, bool lazy_in) __call_rcu_nocb_wake(rdp, was_alldone, flags); /* unlocks */ } else { __call_rcu_core(rdp, head, flags); - local_irq_restore(flags); } + local_irq_restore(flags); } #ifdef CONFIG_RCU_LAZY @@ -4615,8 +4617,9 @@ void rcutree_migrate_callbacks(int cpu) __call_rcu_nocb_wake(my_rdp, true, flags); } else { rcu_nocb_unlock(my_rdp); /* irqs remain disabled. */ - raw_spin_unlock_irqrestore_rcu_node(my_rnp, flags); + raw_spin_unlock_rcu_node(my_rnp); /* irqs remain disabled. */ } + local_irq_restore(flags); if (needwake) rcu_gp_kthread_wake(); lockdep_assert_irqs_enabled(); diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h index 5598212d1f27..3f70fd0a2db4 100644 --- a/kernel/rcu/tree_nocb.h +++ b/kernel/rcu/tree_nocb.h @@ -532,9 +532,7 @@ static bool rcu_nocb_try_bypass(struct rcu_data *rdp, struct rcu_head *rhp, // 2. Both of these conditions are met: // a. The bypass list previously had only lazy CBs, and: // b. The new CB is non-lazy. - if (ncbs && (!bypass_is_lazy || lazy)) { - local_irq_restore(flags); - } else { + if (!ncbs || (bypass_is_lazy && !lazy)) { // No-CBs GP kthread might be indefinitely asleep, if so, wake. rcu_nocb_lock(rdp); // Rare during call_rcu() flood. if (!rcu_segcblist_pend_cbs(&rdp->cblist)) { @@ -544,7 +542,7 @@ static bool rcu_nocb_try_bypass(struct rcu_data *rdp, struct rcu_head *rhp, } else { trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("FirstBQnoWake")); - rcu_nocb_unlock_irqrestore(rdp, flags); + rcu_nocb_unlock(rdp); } } return true; // Callback already enqueued. @@ -570,7 +568,7 @@ static void __call_rcu_nocb_wake(struct rcu_data *rdp, bool was_alldone, // If we are being polled or there is no kthread, just leave. t = READ_ONCE(rdp->nocb_gp_kthread); if (rcu_nocb_poll || !t) { - rcu_nocb_unlock_irqrestore(rdp, flags); + rcu_nocb_unlock(rdp); trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("WakeNotPoll")); return; @@ -583,17 +581,17 @@ static void __call_rcu_nocb_wake(struct rcu_data *rdp, bool was_alldone, rdp->qlen_last_fqs_check = len; // Only lazy CBs in bypass list if (lazy_len && bypass_len == lazy_len) { - rcu_nocb_unlock_irqrestore(rdp, flags); + rcu_nocb_unlock(rdp); wake_nocb_gp_defer(rdp, RCU_NOCB_WAKE_LAZY, TPS("WakeLazy")); } else if (!irqs_disabled_flags(flags)) { /* ... if queue was empty ... */ - rcu_nocb_unlock_irqrestore(rdp, flags); + rcu_nocb_unlock(rdp); wake_nocb_gp(rdp, false); trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("WakeEmpty")); } else { - rcu_nocb_unlock_irqrestore(rdp, flags); + rcu_nocb_unlock(rdp); wake_nocb_gp_defer(rdp, RCU_NOCB_WAKE, TPS("WakeEmptyIsDeferred")); } @@ -611,15 +609,15 @@ static void __call_rcu_nocb_wake(struct rcu_data *rdp, bool was_alldone, if ((rdp->nocb_cb_sleep || !rcu_segcblist_ready_cbs(&rdp->cblist)) && !timer_pending(&rdp->nocb_timer)) { - rcu_nocb_unlock_irqrestore(rdp, flags); + rcu_nocb_unlock(rdp); wake_nocb_gp_defer(rdp, RCU_NOCB_WAKE_FORCE, TPS("WakeOvfIsDeferred")); } else { - rcu_nocb_unlock_irqrestore(rdp, flags); + rcu_nocb_unlock(rdp); trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("WakeNot")); } } else { - rcu_nocb_unlock_irqrestore(rdp, flags); + rcu_nocb_unlock(rdp); trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("WakeNot")); } }
Currently IRQs are disabled on call_rcu() and then depending on the context: * If the CPU is in nocb mode: - If the callback is enqueued in the bypass list, IRQs are re-enabled implictly by rcu_nocb_try_bypass() - If the callback is enqueued in the normal list, IRQs are re-enabled implicitly by __call_rcu_nocb_wake() * If the CPU is NOT in nocb mode, IRQs are reenabled explicitly from call_rcu() This makes the code a bit hard to follow, especially as it interleaves with nocb locking. To make the IRQ flags coverage clearer and also in order to prepare for moving all the nocb enqueue code to its own function, always re-enable the IRQ flags explicitly from call_rcu(). Signed-off-by: Frederic Weisbecker <frederic@kernel.org> --- kernel/rcu/tree.c | 9 ++++++--- kernel/rcu/tree_nocb.h | 20 +++++++++----------- 2 files changed, 15 insertions(+), 14 deletions(-)