[RFC PATCH 2/2] workqueue: Fix unbind_workers() VS wq_worker_sleeping() race

[Frederic Weisbecker <frederic@kernel.org>]

At CPU-hotplug time, unbind_workers() may preempt a worker while it is
going to sleep. In that case the following scenario can happen:

    unbind_workers()                     wq_worker_sleeping()
    --------------                      -------------------
                                      if (worker->flags & WORKER_NOT_RUNNING)
                                          return;
                                      //PREEMPTED by unbind_workers
    worker->flags |= WORKER_UNBOUND;
    [...]
    atomic_set(&pool->nr_running, 0);
    //resume to worker
                                       atomic_dec_and_test(&pool->nr_running);

After unbind_worker() resets pool->nr_running, the value is expected to
remain 0 until the pool ever gets rebound in case cpu_up() is called on
the target CPU in the future. But here the race leaves pool->nr_running
with a value of -1, triggering the following warning when the worker goes
idle:

        WARNING: CPU: 3 PID: 34 at kernel/workqueue.c:1823 worker_enter_idle+0x95/0xc0
        Modules linked in:
        CPU: 3 PID: 34 Comm: kworker/3:0 Not tainted 5.16.0-rc1+ #34
        Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.12.0-59-gc9ba527-rebuilt.opensuse.org 04/01/2014
        Workqueue:  0x0 (rcu_par_gp)
        RIP: 0010:worker_enter_idle+0x95/0xc0
        Code: 04 85 f8 ff ff ff 39 c1 7f 09 48 8b 43 50 48 85 c0 74 1b 83 e2 04 75 99 8b 43 34 39 43 30 75 91 8b 83 00 03 00 00 85 c0 74 87 <0f> 0b 5b c3 48 8b 35 70 f1 37 01 48 8d 7b 48 48 81 c6 e0 93  0
        RSP: 0000:ffff9b7680277ed0 EFLAGS: 00010086
        RAX: 00000000ffffffff RBX: ffff93465eae9c00 RCX: 0000000000000000
        RDX: 0000000000000000 RSI: ffff9346418a0000 RDI: ffff934641057140
        RBP: ffff934641057170 R08: 0000000000000001 R09: ffff9346418a0080
        R10: ffff9b768027fdf0 R11: 0000000000002400 R12: ffff93465eae9c20
        R13: ffff93465eae9c20 R14: ffff93465eae9c70 R15: ffff934641057140
        FS:  0000000000000000(0000) GS:ffff93465eac0000(0000) knlGS:0000000000000000
        CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
        CR2: 0000000000000000 CR3: 000000001cc0c000 CR4: 00000000000006e0
        DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
        DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
        Call Trace:
          <TASK>
          worker_thread+0x89/0x3d0
          ? process_one_work+0x400/0x400
          kthread+0x162/0x190
          ? set_kthread_struct+0x40/0x40
          ret_from_fork+0x22/0x30
          </TASK>

Also due to this incorrect "nr_running == -1", all sorts of hazards can
happen, starting with queued works being ignored because no workers are
awaken at insert_work() time.

Fix this with checking again the worker flags while pool->lock is locked.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Cc: Paul E. McKenney <paulmck@kernel.org>
---
 kernel/workqueue.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 5094573e8b45..5557d19ea81c 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -912,6 +912,16 @@ void wq_worker_sleeping(struct task_struct *task)
 	worker->sleeping = 1;
 	raw_spin_lock_irq(&pool->lock);
 
+	/*
+	 * Recheck in case unbind_workers() preempted us. We don't
+	 * want to decrement nr_running after the worker is unbound
+	 * and nr_running has been reset.
+	 */
+	if (worker->flags & WORKER_NOT_RUNNING) {
+		raw_spin_unlock_irq(&pool->lock);
+		return;
+	}
+
 	/*
 	 * The counterpart of the following dec_and_test, implied mb,
 	 * worklist not empty test sequence is in insert_work().
-- 
2.25.1