Ever since preemption was introduced to linux kernel,
irq disabled spinlocks are always held with preemption
disabled. One of the reason is that sometimes we need
to use spin_unlock() which will do preempt_enable()
to unlock the irq disabled spinlock with keeping irq
disabled. So preempt_count can be used to test whether
scheduler locks is possible held.
CC: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Signed-off-by: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>
---
kernel/rcu/tree_plugin.h | 8 ++++++--
1 file changed, 6 insertions(+), 2 deletions(-)
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 0982e9886103..aba5896d67e3 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -603,10 +603,14 @@ static void rcu_read_unlock_special(struct task_struct *t)
tick_nohz_full_cpu(rdp->cpu);
// Need to defer quiescent state until everything is enabled.
if (irqs_were_disabled && use_softirq &&
- (in_interrupt() ||
- (exp && !t->rcu_read_unlock_special.b.deferred_qs))) {
+ (in_interrupt() || (exp && !preempt_bh_were_disabled))) {
// Using softirq, safe to awaken, and we get
// no help from enabling irqs, unlike bh/preempt.
+ // in_interrupt(): raise_softirq_irqoff() is
+ // guaranteed not to not do wakeup
+ // !preempt_bh_were_disabled: scheduler locks cannot
+ // be held, since spinlocks are always held with
+ // preempt_disable(), so the wakeup will be safe.
raise_softirq_irqoff(RCU_SOFTIRQ);
} else {
// Enabling BH or preempt does reschedule, so...
--
2.20.1
