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From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     rcu@vger.kernel.org
Cc:     linux-kernel@vger.kernel.org, kernel-team@fb.com,
        rostedt@goodmis.org, "Paul E. McKenney" <paulmck@kernel.org>,
        Matthew Wilcox <willy@infradead.org>,
        Zhouyi Zhou <zhouzhouyi@gmail.com>
Subject: [PATCH rcu 04/32] rcu-tasks: Drive synchronous grace periods from calling task
Date:   Mon, 20 Jun 2022 15:53:43 -0700
Message-Id: <20220620225411.3842519-4-paulmck@kernel.org>
In-Reply-To: <20220620225402.GA3842369@paulmck-ThinkPad-P17-Gen-1>
References: <20220620225402.GA3842369@paulmck-ThinkPad-P17-Gen-1>
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This commit causes synchronous grace periods to be driven from the task
invoking synchronize_rcu_*(), allowing these functions to be invoked from
the mid-boot dead zone extending from when the scheduler was initialized
to to point that the various RCU tasks grace-period kthreads are spawned.
This change will allow the self-tests to run in a consistent manner.

Reported-by: Matthew Wilcox <willy@infradead.org>
Reported-by: Zhouyi Zhou <zhouzhouyi@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
 kernel/rcu/tasks.h | 24 ++++++++++++++++--------
 1 file changed, 16 insertions(+), 8 deletions(-)

diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index ad993c4ed924f..bd9f2e24f5c73 100644
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -495,17 +495,21 @@ static void rcu_tasks_invoke_cbs_wq(struct work_struct *wp)
 }
 
 // Wait for one grace period.
-static void rcu_tasks_one_gp(struct rcu_tasks *rtp)
+static void rcu_tasks_one_gp(struct rcu_tasks *rtp, bool midboot)
 {
 	int needgpcb;
 
 	mutex_lock(&rtp->tasks_gp_mutex);
-	set_tasks_gp_state(rtp, RTGS_WAIT_CBS);
 
 	// If there were none, wait a bit and start over.
-	rcuwait_wait_event(&rtp->cbs_wait,
-			   (needgpcb = rcu_tasks_need_gpcb(rtp)),
-			   TASK_IDLE);
+	if (unlikely(midboot)) {
+		needgpcb = 0x2;
+	} else {
+		set_tasks_gp_state(rtp, RTGS_WAIT_CBS);
+		rcuwait_wait_event(&rtp->cbs_wait,
+				   (needgpcb = rcu_tasks_need_gpcb(rtp)),
+				   TASK_IDLE);
+	}
 
 	if (needgpcb & 0x2) {
 		// Wait for one grace period.
@@ -540,7 +544,7 @@ static int __noreturn rcu_tasks_kthread(void *arg)
 	for (;;) {
 		// Wait for one grace period and invoke any callbacks
 		// that are ready.
-		rcu_tasks_one_gp(rtp);
+		rcu_tasks_one_gp(rtp, false);
 
 		// Paranoid sleep to keep this from entering a tight loop.
 		schedule_timeout_idle(rtp->gp_sleep);
@@ -554,8 +558,12 @@ static void synchronize_rcu_tasks_generic(struct rcu_tasks *rtp)
 	RCU_LOCKDEP_WARN(rcu_scheduler_active == RCU_SCHEDULER_INACTIVE,
 			 "synchronize_rcu_tasks called too soon");
 
-	/* Wait for the grace period. */
-	wait_rcu_gp(rtp->call_func);
+	// If the grace-period kthread is running, use it.
+	if (READ_ONCE(rtp->kthread_ptr)) {
+		wait_rcu_gp(rtp->call_func);
+		return;
+	}
+	rcu_tasks_one_gp(rtp, true);
 }
 
 /* Spawn RCU-tasks grace-period kthread. */
-- 
2.31.1.189.g2e36527f23