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From:   Lai Jiangshan <laijs@linux.alibaba.com>
To:     linux-kernel@vger.kernel.org
Cc:     Lai Jiangshan <laijs@linux.alibaba.com>,
        Peter Zijlstra <peterz@infradead.org>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Josh Triplett <josh@joshtriplett.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Lai Jiangshan <jiangshanlai@gmail.com>,
        Joel Fernandes <joel@joelfernandes.org>, rcu@vger.kernel.org
Subject: [PATCH V2 1/7] rcu: use preempt_count to test whether scheduler locks is held
Date:   Sat,  2 Nov 2019 12:45:53 +0000
Message-Id: <20191102124559.1135-2-laijs@linux.alibaba.com>
In-Reply-To: <20191102124559.1135-1-laijs@linux.alibaba.com>
References: <20191102124559.1135-1-laijs@linux.alibaba.com>
Reply-To: <20191101162109.GN20975@paulmck-ThinkPad-P72>
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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@infradead.org>
Signed-off-by: Lai Jiangshan <laijs@linux.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