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Subject[PATCH tip/core/rcu 09/40] rcu: Make rcu_all_qs() do deferred NOCB wakeups
Date
If a CONFIG_RCU_NOCB_CPUS kernel invokes call_rcu() with interrupts
disabled, wakeups must be deferred in order to avoid self-deadlock in
the cases where the disabled interrupts are due to scheduler locks being
held. In this case, a flag is set and is checked on entry to extended
quiescent states (usermode, idle), on exit from the RCU_SOFTIRQ handler,
when the CPU in question goes offline, and on a subsequent invocation
of call_rcu(). However, a given CPU could avoid all of those states
for a considerable length of time.

This commit therefore allows an invocation of rcu_all_qs() to do the
wakeup. It also makes the wakeup function clear the deferred-wakeup flag.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
kernel/rcu/tree.c | 6 ++++++
kernel/rcu/tree_plugin.h | 3 ++-
2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index e8843bf0f5e1..eaa2b6d7abe2 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -522,6 +522,7 @@ EXPORT_SYMBOL_GPL(rcu_note_context_switch);
void rcu_all_qs(void)
{
unsigned long flags;
+ struct rcu_state *rsp;

barrier(); /* Avoid RCU read-side critical sections leaking down. */
if (unlikely(raw_cpu_read(rcu_sched_qs_mask))) {
@@ -544,6 +545,11 @@ void rcu_all_qs(void)
rcu_sched_qs();
preempt_enable();
}
+ for_each_rcu_flavor(rsp) {
+ preempt_disable();
+ do_nocb_deferred_wakeup(this_cpu_ptr(rsp->rda));
+ preempt_enable();
+ }
this_cpu_inc(rcu_qs_ctr);
barrier(); /* Avoid RCU read-side critical sections leaking up. */
}
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 5a149091d262..0e2114e8076b 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -2201,7 +2201,8 @@ static void do_nocb_deferred_wakeup(struct rcu_data *rdp)
{
int ndw;

- if (!rcu_nocb_need_deferred_wakeup(rdp))
+ if (!rcu_nocb_need_deferred_wakeup(rdp) ||
+ unlikely(rcu_scheduler_active != RCU_SCHEDULER_RUNNING))
return;
ndw = READ_ONCE(rdp->nocb_defer_wakeup);
WRITE_ONCE(rdp->nocb_defer_wakeup, RCU_NOGP_WAKE_NOT);
--
2.5.2
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