linux-kernel - [PATCH RFC v2 tip/core/rcu 4/4] rcu: Allow rcu_read_unlock_special() to raise_softirq() if in

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Message-Id: <20190402143842.16068-4-paulmck@linux.ibm.com>
Date:   Tue,  2 Apr 2019 07:38:42 -0700
From:   "Paul E. McKenney" <paulmck@...ux.ibm.com>
To:     rcu@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, mingo@...nel.org,
        jiangshanlai@...il.com, dipankar@...ibm.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
        rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
        fweisbec@...il.com, oleg@...hat.com, joel@...lfernandes.org,
        "Paul E. McKenney" <paulmck@...ux.ibm.com>
Subject: [PATCH RFC v2 tip/core/rcu 4/4] rcu: Allow rcu_read_unlock_special() to raise_softirq() if in_irq()

When running in an interrupt handler, raise_softirq() and
raise_softirq_irqoff() have extremely low overhead: They simply set a
bit in a per-CPU mask, which is checked upon exit from that interrupt
handler.  Therefore, if rcu_read_unlock_special() is invoked within an
interrupt handler and RCU_SOFTIRQ is in use, this commit make use of
raise_softirq_irqoff() even if there is no expedited grace period in
flight and even if this is not a nohz_full CPU.

Signed-off-by: Paul E. McKenney <paulmck@...ux.ibm.com>
---
 kernel/rcu/tree_plugin.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index d15cdab6aeb4..e1005f5e8094 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -615,7 +615,7 @@ static void rcu_read_unlock_special(struct task_struct *t)
 		      (rdp->grpmask & rnp->expmask) ||
 		      tick_nohz_full_cpu(rdp->cpu);
 		// Need to defer quiescent state until everything is enabled.
-		if (exp && irqs_were_disabled && use_softirq &&
+		if ((exp || in_irq()) && irqs_were_disabled && use_softirq &&
 		    (in_irq() || !t->rcu_read_unlock_special.b.deferred_qs)) {
 			// Using softirq, safe to awaken, and we get
 			// no help from enabling irqs, unlike bh/preempt.
-- 
2.17.1