linux-kernel - [PATCH V2 1/7] rcu: use preempt_count to test whether scheduler locks is held

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Date:   Sat,  2 Nov 2019 12:45:53 +0000
From:   Lai Jiangshan <laijs@...ux.alibaba.com>
To:     linux-kernel@...r.kernel.org
Cc:     Lai Jiangshan <laijs@...ux.alibaba.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Josh Triplett <josh@...htriplett.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Joel Fernandes <joel@...lfernandes.org>, rcu@...r.kernel.org
Subject: [PATCH V2 1/7] rcu: use preempt_count to test whether scheduler locks is held

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@...radead.org>
Signed-off-by: Lai Jiangshan <laijs@...ux.alibaba.com>
---
 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