[RFC] workqueue: Fix irq inversion deadlock in manage_workers()

[Boqun Feng <boqun.feng@gmail.com>]

Josef reported a HARDIRQ-safe -> HARDIRQ-unsafe lock order detected by
lockdep:

| [ 1270.472259] WARNING: HARDIRQ-safe -> HARDIRQ-unsafe lock order detected
| [ 1270.472783] 4.14.0-rc1-xfstests-12888-g76833e8 #110 Not tainted
| [ 1270.473240] -----------------------------------------------------
| [ 1270.473710] kworker/u5:2/5157 [HC0[0]:SC0[0]:HE0:SE1] is trying to acquire:
| [ 1270.474239]  (&(&lock->wait_lock)->rlock){+.+.}, at: [<ffffffff8da253d2>] __mutex_unlock_slowpath+0xa2/0x280
| [ 1270.474994]
| [ 1270.474994] and this task is already holding:
| [ 1270.475440]  (&pool->lock/1){-.-.}, at: [<ffffffff8d2992f6>] worker_thread+0x366/0x3c0
| [ 1270.476046] which would create a new lock dependency:
| [ 1270.476436]  (&pool->lock/1){-.-.} -> (&(&lock->wait_lock)->rlock){+.+.}
| [ 1270.476949]
| [ 1270.476949] but this new dependency connects a HARDIRQ-irq-safe lock:
| [ 1270.477553]  (&pool->lock/1){-.-.}
...
| [ 1270.488900] to a HARDIRQ-irq-unsafe lock:
| [ 1270.489327]  (&(&lock->wait_lock)->rlock){+.+.}
...
| [ 1270.494735]  Possible interrupt unsafe locking scenario:
| [ 1270.494735]
| [ 1270.495250]        CPU0                    CPU1
| [ 1270.495600]        ----                    ----
| [ 1270.495947]   lock(&(&lock->wait_lock)->rlock);
| [ 1270.496295]                                local_irq_disable();
| [ 1270.496753]                                lock(&pool->lock/1);
| [ 1270.497205]                                lock(&(&lock->wait_lock)->rlock);
| [ 1270.497744]   <Interrupt>
| [ 1270.497948]     lock(&pool->lock/1);

, which will cause a irq inversion deadlock if the above lock scenario
happens.

The root cause of this safe -> unsafe lock order is the
mutex_unlock(pool::manager_arb) in manage_workers() with pool::lock
held. An obvious fix is dropping the pool::lock before mutex_unlock()
and re-grabing afterwards, which however will introduce a race condition
between worker_thread() and put_unbound_pool():

put_unbound_pool() will grab both pool::manager_arb and pool::lock to
set all current IDLE workers to DIE, and may wait on the
pool::detach_completion for the last worker to detach from the pool.

And when manage_workers() is called, the caller worker_thread is in
non-ILDE state, so if the worker dropped both pool::{manager_arb, lock}
and got delayed for a while long enough for a put_unbound_pool(), the
put_unbound_pool() would not switch that worker to DIE. As a result, the
worker will not detach from the pool as it's not DIE and the
put_unbound_pool() will not proceed as it's waiting for the last worker
to detach, therefore deadlock.

To overcome this, put the worker back to IDLE state before it drops
pool::lock in manage_workers(), and make the worker check again whether
it's DIE after it re-grabs the pool::lock. In this way, we fix the
potential deadlock reported by lockdep without introducing another.

Reported-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
---
 kernel/workqueue.c | 35 ++++++++++++++++++++++++++++++++++-
 1 file changed, 34 insertions(+), 1 deletion(-)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 64d0edf428f8..2ea7b04cc48b 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -1997,7 +1997,40 @@ static bool manage_workers(struct worker *worker)
 	maybe_create_worker(pool);
 
 	pool->manager = NULL;
+
+	/*
+	 * Put the manager back to ->idle_list, this allows us to drop the
+	 * pool->lock safely without racing with put_unbound_pool()
+	 *
+	 * 						<in "manager worker" thread>
+	 * 						worker_thread():
+	 * 						  spin_lock_irq(&pool->lock);
+	 * 						  worker_leave_idle();
+	 * 						  manage_workers(): // return true
+	 * 						    mutex_trylock(&pool->manager_arb);
+	 *						    <without entering idle here>
+	 * 						    spin_unlock_irq(&pool->lock);
+	 * 						    mutex_unlock(&pool->manager_arb);
+	 *
+	 * 	put_unbound_pool():
+	 * 	  mutex_lock(&pool->manager_arb);
+	 * 	  spin_lock_irq(&pool->lock);
+	 * 	  <set ILDE worker to DIE>
+	 * 	  <the manager worker is not set to be DIE, because it's not IDLE>
+	 * 	  ...
+	 * 	  wait_for_completion(&pool->detach_completion);
+	 * 	  <no one will complete() because pool->workers is not empty>
+	 *
+	 * 	  					  spin_lock_irq(&pool->lock);
+	 * 	  					  <pool->worklist is empty, go to sleep>
+	 *
+	 * No one is going to wake up the manager worker, even so, it won't
+	 * complete(->detach_completion), since it's not a DIE worker.
+	 */
+	worker_enter_idle(worker);
+	spin_unlock_irq(&pool->lock);
 	mutex_unlock(&pool->manager_arb);
+	spin_lock_irq(&pool->lock);
 	return true;
 }
 
@@ -2202,6 +2235,7 @@ static int worker_thread(void *__worker)
 woke_up:
 	spin_lock_irq(&pool->lock);
 
+recheck:
 	/* am I supposed to die? */
 	if (unlikely(worker->flags & WORKER_DIE)) {
 		spin_unlock_irq(&pool->lock);
@@ -2216,7 +2250,6 @@ static int worker_thread(void *__worker)
 	}
 
 	worker_leave_idle(worker);
-recheck:
 	/* no more worker necessary? */
 	if (!need_more_worker(pool))
 		goto sleep;
-- 
2.14.1