Re: [tip:locking/core] jump_label/lockdep: Assert we hold the hotplug lock for _cpuslocked() operations

[Peter Zijlstra]

On Mon, Sep 10, 2018 at 03:09:29AM -0700, tip-bot for Peter Zijlstra wrote:
> Commit-ID:  cb538267ea1e9e025ec692577c9ae75797261889
> Gitweb:     https://git.kernel.org/tip/cb538267ea1e9e025ec692577c9ae75797261889
> Author:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> AuthorDate: Tue, 31 Jul 2018 14:35:32 +0200
> Committer:  Ingo Molnar <mingo@xxxxxxxxxx>
> CommitDate: Mon, 10 Sep 2018 10:16:38 +0200
> 
> jump_label/lockdep: Assert we hold the hotplug lock for _cpuslocked() operations
> 
> Weirdly we seem to have forgotten this...

This of course instantly triggered a false positive :/

---
Subject: lockdep,cpu/hotplug: Annotate AP thread

Anybody trying to assert the cpu_hotplug_lock is held
(lockdep_assert_cpus_held()) from AP callbacks will fail, because the
lock is held by the BP.

Stick in an explicit annotation in cpuhp_thread_fun() to make this work.

Fixes: cb538267ea1e ("jump_label/lockdep: Assert we hold the hotplug lock for _cpuslocked() operations")
Reported-by: Ingo Molnar <mingo@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
 kernel/cpu.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/kernel/cpu.c b/kernel/cpu.c
index 0097acec1c71..08c168b159da 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -616,6 +616,12 @@ static void cpuhp_thread_fun(unsigned int cpu)
 	 */
 	smp_mb();
 
+	/*
+	 * The BP holds the hotplug lock, but we're now running on the AP,
+	 * ensure that anybody asserting the lock is held, will actually find
+	 * it so.
+	 */
+	rwsem_acquire(&cpu_hotplug_lock.rw_sem.dep_map, 0, 0, _THIS_IP_);
 	cpuhp_lock_acquire(bringup);
 
 	if (st->single) {
@@ -661,6 +667,7 @@ static void cpuhp_thread_fun(unsigned int cpu)
 	}
 
 	cpuhp_lock_release(bringup);
+	rwsem_release(&cpu_hotplug_lock.rw_sem.dep_map, 1, _THIS_IP_);
 
 	if (!st->should_run)
 		complete_ap_thread(st, bringup);