linux-kernel - [PATCH tip/core/rcu 05/13] rcu: Always set .need_qs from __rcu_read

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Date:   Mon, 31 Aug 2020 11:11:12 -0700
From:   paulmck@...nel.org
To:     rcu@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, kernel-team@...com, 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@...nel.org>
Subject: [PATCH tip/core/rcu 05/13] rcu: Always set .need_qs from __rcu_read_lock() for strict GPs

From: "Paul E. McKenney" <paulmck@...nel.org>

The ->rcu_read_unlock_special.b.need_qs field in the task_struct
structure indicates that the RCU core needs a quiscent state from the
corresponding task.  The __rcu_read_unlock() function checks this (via
an eventual call to rcu_preempt_deferred_qs_irqrestore()), and if set
reports a quiscent state immediately upon exit from the outermost RCU
read-side critical section.

Currently, this flag is only set when the scheduling-clock interrupt
decides that the current RCU grace period is too old, as in about
one full second too old.  But if the kernel has been built with
CONFIG_RCU_STRICT_GRACE_PERIOD=y, we clearly do not want to wait that
long.  This commit therefore sets the .need_qs field immediately at the
start of the RCU read-side critical section from within __rcu_read_lock()
in order to unconditionally enlist help from __rcu_read_unlock().

But note the additional check for rcu_state.gp_kthread, which prevents
attempts to awaken RCU's grace-period kthread during early boot before
there is a scheduler.  Leaving off this check results in early boot hangs.
So early that there is no console output.  Thus, this additional check
fails until such time as RCU's grace-period kthread has been created,
avoiding these empty-console hangs.

Reported-by Jann Horn <jannh@...gle.com>
Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
---
 kernel/rcu/tree_plugin.h | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 44cf77d..668bbd2 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -376,6 +376,8 @@ void __rcu_read_lock(void)
 	rcu_preempt_read_enter();
 	if (IS_ENABLED(CONFIG_PROVE_LOCKING))
 		WARN_ON_ONCE(rcu_preempt_depth() > RCU_NEST_PMAX);
+	if (IS_ENABLED(CONFIG_RCU_STRICT_GRACE_PERIOD) && rcu_state.gp_kthread)
+		WRITE_ONCE(current->rcu_read_unlock_special.b.need_qs, true);
 	barrier();  /* critical section after entry code. */
 }
 EXPORT_SYMBOL_GPL(__rcu_read_lock);
-- 
2.9.5