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

[Petr Mladek]

On Wed 2022-06-22 16:08:53, Petr Mladek wrote:
> 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
> @@ -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))

I have realized that this is wrong. I used _interruptible() variant
because we do not know how long it would need to sleep. And long
sleeping in TASK_UNINTERRUPTIBLE might trigger hung task warning.

But kthread_bind_mask() requires that the kthread will be in
TASK_UNINTERRUPTIBLE state.

Note that even the freshly created kthread is sleeping in
TASK_UNTERRUPTIBLE, see kthread() in kernel/kthread.c. But
it does not trigger the hung task warning because there is
a special check for this case, see check_hung_task():

	/*
	 * When a freshly created task is scheduled once, changes its state to
	 * TASK_UNINTERRUPTIBLE without having ever been switched out once, it
	 * musn't be checked.
	 */
	if (unlikely(!switch_count))
		return;

The following seems to work well:

	while (!wait_for_completion_timeout(&worker->ready_to_start, 5 * HZ))
		;

That said, I am not super happy with this solution. IMHO, it is a flaw
in the kthread() API. But I do not see a reasonable way how to fix
this except for adding a special API how to wake up the freshly
created kthread for the first time. It would require updating all
users. I am not sure how this problem is common and if it is worth it.

And I used completion API to avoid tricky code using low level scheduling
API.

> +		;
> +	pool = worker->pool;
> +
>  woke_up:
>  	raw_spin_lock_irq(&pool->lock);
>  

I am going to wait with v2 until I get some feedback for this
approach.

Best Regards,
Petr