[PATCH] workqueue: Make create_worker() safe against spurious wakeups

[Petr Mladek]

A system crashed with the following BUG() report:

  [115147.050484] BUG: kernel NULL pointer dereference, address: 0000000000000000
  [115147.050488] #PF: supervisor write access in kernel mode
  [115147.050489] #PF: error_code(0x0002) - not-present page
  [115147.050490] PGD 0 P4D 0
  [115147.050494] Oops: 0002 [#1] PREEMPT_RT SMP NOPTI
  [115147.050498] CPU: 1 PID: 16213 Comm: kthreadd Kdump: loaded Tainted: G           O   X    5.3.18-2-rt #1 SLE15-SP2 (unreleased)
  [115147.050510] RIP: 0010:_raw_spin_lock_irq+0x14/0x30
  [115147.050513] Code: 89 c6 e8 5f 7a 9b ff 66 90 c3 66 66 2e 0f 1f 84 00 00 00 00 00 90 0f 1f 44 00 00 fa 65 ff 05 fb 53 6c 55 31 c0 ba 01 00 00 00 <f0> 0f b1 17 75 01 c3 89 c6 e8 2e 7a 9b ff 66 90 c3 90 90 90 90 90
  [115147.050514] RSP: 0018:ffffb0f68822fed8 EFLAGS: 00010046
  [115147.050515] RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
  [115147.050516] RDX: 0000000000000001 RSI: 0000000000000002 RDI: 0000000000000000
  [115147.050517] RBP: ffff9ca73af40a40 R08: 0000000000000001 R09: 0000000000027340
  [115147.050519] R10: ffffb0f68822fe70 R11: 00000000000000a9 R12: ffffb0f688067dc0
  [115147.050520] R13: ffff9ca77e9a8000 R14: ffff9ca7634ca780 R15: ffff9ca7634ca780
  [115147.050521] FS:  0000000000000000(0000) GS:ffff9ca77fb00000(0000) knlGS:0000000000000000
  [115147.050523] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
  [115147.050524] CR2: 00000000000000b8 CR3: 000000004472e000 CR4: 00000000003406e0
  [115147.050524] Call Trace:
  [115147.050533]  worker_thread+0xb4/0x3c0
  [115147.050538]  ? process_one_work+0x4a0/0x4a0
  [115147.050540]  kthread+0x152/0x170
  [115147.050542]  ? kthread_park+0xa0/0xa0
  [115147.050544]  ret_from_fork+0x35/0x40

Further debugging shown that the worker thread was woken
before worker_attach_to_pool() finished in create_worker().

Any kthread is supposed to stay in TASK_UNINTERRUPTIBLE sleep
until it is explicitly woken. But a spurious wakeup might
break this expectation.

As a result, worker_thread() might read worker->pool before
it was set in worker create_worker() by worker_attach_to_pool().
Also manage_workers() might want to create yet another worker
before worker->pool->nr_workers is updated. It is a kind off
a chicken & egg problem.

Synchronize these operations using a completion API.

Note that worker->pool might be then read without wq_pool_attach_mutex.
Normal worker always belongs to the same pool.

Also note that rescuer_thread() does not need this because all
needed values are set before the kthreads is created. It is
connected with a particular workqueue. It is attached to different
pools when needed.

Signed-off-by: Petr Mladek <pmladek@xxxxxxxx>
---
 kernel/workqueue.c          | 13 ++++++++++++-
 kernel/workqueue_internal.h |  1 +
 2 files changed, 13 insertions(+), 1 deletion(-)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 1ea50f6be843..9586b0797145 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -1939,6 +1939,7 @@ static struct worker *create_worker(struct worker_pool *pool)
 		goto fail;
 
 	worker->id = id;
+	init_completion(&worker->ready_to_start);
 
 	if (pool->cpu >= 0)
 		snprintf(id_buf, sizeof(id_buf), "%d:%d%s", pool->cpu, id,
@@ -1961,6 +1962,7 @@ static struct worker *create_worker(struct worker_pool *pool)
 	raw_spin_lock_irq(&pool->lock);
 	worker->pool->nr_workers++;
 	worker_enter_idle(worker);
+	complete(&worker->ready_to_start);
 	wake_up_process(worker->task);
 	raw_spin_unlock_irq(&pool->lock);
 
@@ -2378,10 +2380,19 @@ static void set_pf_worker(bool val)
 static int worker_thread(void *__worker)
 {
 	struct worker *worker = __worker;
-	struct worker_pool *pool = worker->pool;
+	struct worker_pool *pool;
 
 	/* tell the scheduler that this is a workqueue worker */
 	set_pf_worker(true);
+
+	/*
+	 * Wait until the worker is ready to get started. It must be attached
+	 * to a pool first. This is needed because of spurious wakeups.
+	 */
+	while (wait_for_completion_interruptible(&worker->ready_to_start))
+		;
+	pool = worker->pool;
+
 woke_up:
 	raw_spin_lock_irq(&pool->lock);
 
diff --git a/kernel/workqueue_internal.h b/kernel/workqueue_internal.h
index e00b1204a8e9..ffca2783c8bf 100644
--- a/kernel/workqueue_internal.h
+++ b/kernel/workqueue_internal.h
@@ -41,6 +41,7 @@ struct worker {
 						/* L: for rescuers */
 	struct list_head	node;		/* A: anchored at pool->workers */
 						/* A: runs through worker->node */
+	struct completion	ready_to_start;	/* None: handle spurious wakeup */
 
 	unsigned long		last_active;	/* L: last active timestamp */
 	unsigned int		flags;		/* X: flags */
-- 
2.35.3