blob: 34e1d8bb1c05bd9feb0e3ee0cbcd7b4043900761 [file] [log] [blame]
From c379e9ecb19788687fd93a8dcb812e0414a82c29 Mon Sep 17 00:00:00 2001
From: Neeraj Upadhyay <>
Date: Tue, 19 Nov 2019 11:50:52 -0800
Subject: [PATCH] rcu: Allow only one expedited GP to run concurrently with
commit 4bc6b745e5cbefed92c48071e28a5f41246d0470 upstream.
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 <>
[ paulmck: Added updated comment. ]
Signed-off-by: Paul E. McKenney <>
Signed-off-by: Paul Gortmaker <>
diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
index eb0eeb7616bb..91357562f168 100644
--- a/kernel/rcu/tree_exp.h
+++ b/kernel/rcu/tree_exp.h
@@ -536,14 +536,13 @@ static void rcu_exp_wait_wake(unsigned long s)
struct rcu_node *rnp;
- rcu_exp_gp_seq_end();
- trace_rcu_exp_grace_period(, 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.
+ rcu_exp_gp_seq_end();
+ trace_rcu_exp_grace_period(, s, TPS("end"));
rcu_for_each_node_breadth_first(rnp) {
if (ULONG_CMP_LT(READ_ONCE(rnp->exp_seq_rq), s)) {