linux-kernel - [PATCH tip/core/rcu 06/10] rcu: Allow only one expedited GP to run concurrently with wakeups

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Message-Id: <20191210040154.2498-6-paulmck@kernel.org>
Date:   Mon,  9 Dec 2019 20:01:50 -0800
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,
        Neeraj Upadhyay <neeraju@...eaurora.org>,
        "Paul E . McKenney" <paulmck@...nel.org>
Subject: [PATCH tip/core/rcu 06/10] rcu: Allow only one expedited GP to run concurrently with wakeups

From: Neeraj Upadhyay <neeraju@...eaurora.org>

The current expedited RCU grace-period code expects that a task
requesting an expedited grace period cannot awaken until that grace
period has reached the wakeup phase.  However, it is possible for a long
preemption to result in the waiting task never sleeping.  For example,
consider the following sequence of events:

1.	Task A starts an expedited grace period by invoking
	synchronize_rcu_expedited().  It proceeds normally up to the
	wait_event() near the end of that function, and is then preempted
	(or interrupted or whatever).

2.	The expedited grace period completes, and a kworker task starts
	the awaken phase, having incremented the counter and acquired
	the rcu_state structure's .exp_wake_mutex.  This kworker task
	is then preempted or interrupted or whatever.

3.	Task A resumes and enters wait_event(), which notes that the
	expedited grace period has completed, and thus doesn't sleep.

4.	Task B starts an expedited grace period exactly as did Task A,
	complete with the preemption (or whatever delay) just before
	the call to wait_event().

5.	The expedited grace period completes, and another kworker
	task starts the awaken phase, having incremented the counter.
	However, it blocks when attempting to acquire the rcu_state
	structure's .exp_wake_mutex because step 2's kworker task has
	not yet released it.

6.	Steps 4 and 5 repeat, resulting in overflow of the rcu_node
	structure's ->exp_wq[] array.

In theory, this is harmless.  Tasks waiting on the various ->exp_wq[]
array will just be spuriously awakened, but they will just sleep again
on noting that the rcu_state structure's ->expedited_sequence value has
not advanced far enough.

In practice, this wastes CPU time and is an accident waiting to happen.
This commit therefore moves the rcu_exp_gp_seq_end() call that officially
ends the expedited grace period (along with associate tracing) until
after the ->exp_wake_mutex has been acquired.  This prevents Task A from
awakening prematurely, thus preventing more than one expedited grace
period from being in flight during a previous expedited grace period's
wakeup phase.

Fixes: 3b5f668e715b ("rcu: Overlap wakeups with next expedited grace period")
Signed-off-by: Neeraj Upadhyay <neeraju@...eaurora.org>
[ paulmck: Added updated comment. ]
Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
---
 kernel/rcu/tree_exp.h | 11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)

diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
index fa143e4..7a1f093 100644
--- a/kernel/rcu/tree_exp.h
+++ b/kernel/rcu/tree_exp.h
@@ -539,14 +539,13 @@ static void rcu_exp_wait_wake(unsigned long s)
 	struct rcu_node *rnp;
 
 	synchronize_sched_expedited_wait();
-	rcu_exp_gp_seq_end();
-	trace_rcu_exp_grace_period(rcu_state.name, s, TPS("end"));
 
-	/*
-	 * Switch over to wakeup mode, allowing the next GP, but -only- the
-	 * next GP, to proceed.
-	 */
+	// Switch over to wakeup mode, allowing the next GP to proceed.
+	// End the previous grace period only after acquiring the mutex
+	// to ensure that only one GP runs concurrently with wakeups.
 	mutex_lock(&rcu_state.exp_wake_mutex);
+	rcu_exp_gp_seq_end();
+	trace_rcu_exp_grace_period(rcu_state.name, s, TPS("end"));
 
 	rcu_for_each_node_breadth_first(rnp) {
 		if (ULONG_CMP_LT(READ_ONCE(rnp->exp_seq_rq), s)) {
-- 
2.9.5