Messages in this thread | | | Date | Thu, 23 Jun 2022 09:14:50 +0200 | From | Michal Hocko <> | Subject | Re: [PATCH] workqueue: Make create_worker() safe against spurious wakeups |
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On Thu 23-06-22 09:00:12, Petr Mladek wrote: > 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@suse.com> > > --- > > 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)) > ;
This is really ugly and I do not really see why this is needed. Should there be such an overloaded system that create_worker cannot finish the initialization before khungtask triggers then we should be notified. I do not see how papering this over would be helpful.
That being said, I really thing that this should be plain wait_for_completion()
With that changed, feel free to add Reviewed-by: Michal Hocko <mhocko@suse.com>
and thanks helping to debug this and prepare the fix. I was not really aware of the spurious wake up scenario. Btw. it would be great to describe that in a more detail as well for future reference.
Thanks! -- Michal Hocko SUSE Labs
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