[PATCH stable 5.15/5.10 0/2] rcu-tasks: Idle tasks on offline CPUs are in quiescent states

[PATCH stable 5.15/5.10 0/2] rcu-tasks: Idle tasks on offline CPUs are in quiescent states

[Krister Johansen]

Paul, Neeraj, and Stable Team:
I've run into a case with rcu_tasks_postscan where the warning introduced as
part of 46aa886c4("rcu-tasks: Fix IPI failure handling in
trc_wait_for_one_reader") is getting triggered when trc_wait_for_one_reader
sends an IPI to a CPU that is offline.  This is occurring on a platform that has
hotplug slots available but not populated.  I don't believe the bug is caused by
this change, but I do think that Paul's commit that confines the postscan
operation to just the active CPUs would help prevent this from happening.

Would the RCU maintainers be amenable to having this patch backported to the
5.10 and 5.15 branches as well?  I've attached cherry-picks of the relevant
commits to minimize the additional work needed.

Thanks,

-K

Paul E. McKenney (1):
  rcu-tasks: Idle tasks on offline CPUs are in quiescent states

 kernel/rcu/tasks.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

-- 
2.25.1

[PATCH 5.15.y 1/2] rcu-tasks: Idle tasks on offline CPUs are in quiescent states

[Krister Johansen]

From: Paul E. McKenney <paulmck@kernel.org>

commit 5c9a9ca44fda41c5e82f50efced5297a9c19760d upstream

Any idle task corresponding to an offline CPU is in an RCU Tasks Trace
quiescent state.  This commit causes rcu_tasks_trace_postscan() to ignore
idle tasks for offline CPUs, which it can do safely due to CPU-hotplug
operations being disabled.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Neeraj Upadhyay <quic_neeraju@quicinc.com>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Andrii Nakryiko <andrii@kernel.org>
Cc: Martin KaFai Lau <kafai@fb.com>
Cc: KP Singh <kpsingh@kernel.org>
Signed-off-by: Krister Johansen <kjlx@templeofstupid.com>
---
 kernel/rcu/tasks.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index 5528c172570b..8648685e7dfa 100644
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -1090,7 +1090,7 @@ static void rcu_tasks_trace_postscan(struct list_head *hop)
 {
 	int cpu;
 
-	for_each_possible_cpu(cpu)
+	for_each_online_cpu(cpu)
 		rcu_tasks_trace_pertask(idle_task(cpu), hop);
 
 	// Re-enable CPU hotplug now that the tasklist scan has completed.
-- 
2.25.1

[PATCH 5.10.y 2/2] rcu-tasks: Idle tasks on offline CPUs are in quiescent states

[Krister Johansen]

From: Paul E. McKenney <paulmck@kernel.org>

commit 5c9a9ca44fda41c5e82f50efced5297a9c19760d upstream

Any idle task corresponding to an offline CPU is in an RCU Tasks Trace
quiescent state.  This commit causes rcu_tasks_trace_postscan() to ignore
idle tasks for offline CPUs, which it can do safely due to CPU-hotplug
operations being disabled.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Neeraj Upadhyay <quic_neeraju@quicinc.com>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Andrii Nakryiko <andrii@kernel.org>
Cc: Martin KaFai Lau <kafai@fb.com>
Cc: KP Singh <kpsingh@kernel.org>
Signed-off-by: Krister Johansen <kjlx@templeofstupid.com>
---
 kernel/rcu/tasks.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index bede3a4f108e..ea45a2d53a99 100644
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -1007,7 +1007,7 @@ static void rcu_tasks_trace_postscan(struct list_head *hop)
 {
 	int cpu;
 
-	for_each_possible_cpu(cpu)
+	for_each_online_cpu(cpu)
 		rcu_tasks_trace_pertask(idle_task(cpu), hop);
 
 	// Re-enable CPU hotplug now that the tasklist scan has completed.
-- 
2.25.1