Re: [PATCH] io_uring: leave clean req to be done in flush overflow

[Joseph Qi <jiangqi903@gmail.com>]

On 1/20/21 8:35 PM, Pavel Begunkov wrote:
> On 20/01/2021 08:11, Joseph Qi wrote:
>> Abaci reported the following BUG:
>>
>> [   27.629441] BUG: sleeping function called from invalid context at fs/file.c:402
>> [   27.631317] in_atomic(): 1, irqs_disabled(): 1, non_block: 0, pid: 1012, name: io_wqe_worker-0
>> [   27.633220] 1 lock held by io_wqe_worker-0/1012:
>> [   27.634286]  #0: ffff888105e26c98 (&ctx->completion_lock){....}-{2:2}, at: __io_req_complete.part.102+0x30/0x70
>> [   27.636487] irq event stamp: 66658
>> [   27.637302] hardirqs last  enabled at (66657): [<ffffffff8144ba02>] kmem_cache_free+0x1f2/0x3b0
>> [   27.639211] hardirqs last disabled at (66658): [<ffffffff82003a77>] _raw_spin_lock_irqsave+0x17/0x50
>> [   27.641196] softirqs last  enabled at (64686): [<ffffffff824003c5>] __do_softirq+0x3c5/0x5aa
>> [   27.643062] softirqs last disabled at (64681): [<ffffffff8220108f>] asm_call_irq_on_stack+0xf/0x20
>> [   27.645029] CPU: 1 PID: 1012 Comm: io_wqe_worker-0 Not tainted 5.11.0-rc4+ #68
>> [   27.646651] Hardware name: Alibaba Cloud Alibaba Cloud ECS, BIOS rel-1.7.5-0-ge51488c-20140602_164612-nilsson.home.kraxel.org 04/01/2014
>> [   27.649249] Call Trace:
>> [   27.649874]  dump_stack+0xac/0xe3
>> [   27.650666]  ___might_sleep+0x284/0x2c0
>> [   27.651566]  put_files_struct+0xb8/0x120
>> [   27.652481]  __io_clean_op+0x10c/0x2a0
>> [   27.653362]  __io_cqring_fill_event+0x2c1/0x350
>> [   27.654399]  __io_req_complete.part.102+0x41/0x70
>> [   27.655464]  io_openat2+0x151/0x300
>> [   27.656297]  io_issue_sqe+0x6c/0x14e0
>> [   27.657170]  ? lock_acquire+0x31a/0x440
>> [   27.658068]  ? io_worker_handle_work+0x24e/0x8a0
>> [   27.659119]  ? find_held_lock+0x28/0xb0
>> [   27.660026]  ? io_wq_submit_work+0x7f/0x240
>> [   27.660991]  io_wq_submit_work+0x7f/0x240
>> [   27.661915]  ? trace_hardirqs_on+0x46/0x110
>> [   27.662890]  io_worker_handle_work+0x501/0x8a0
>> [   27.663917]  ? io_wqe_worker+0x135/0x520
>> [   27.664836]  io_wqe_worker+0x158/0x520
>> [   27.665719]  ? __kthread_parkme+0x96/0xc0
>> [   27.666663]  ? io_worker_handle_work+0x8a0/0x8a0
>> [   27.667726]  kthread+0x134/0x180
>> [   27.668506]  ? kthread_create_worker_on_cpu+0x90/0x90
>> [   27.669641]  ret_from_fork+0x1f/0x30
>>
>> It blames we call cond_resched() with completion_lock when clean
>> request. In fact we will do it during flush overflow and it seems we
>> have no reason to do it before. So just remove io_clean_op() in
>> __io_cqring_fill_event() to fix this BUG.
> 
> Nope, it would be broken. You may override, e.g. iov pointer
> that is dynamically allocated, and the function makes sure all
> those are deleted and freed. Most probably there will be problems
> on flush side as well.
> 
> Looks like the problem is that we do spin_lock_irqsave() in
> __io_req_complete() and then just spin_lock() for put_files_struct().
> Jens, is it a real problem?
> 
From the code, it is because it might sleep in close_files():

...
if (file) {
	filp_close(file, files);
	cond_resched();
}


Thanks,
Joseph

> At least for 5.12 there is a cleanup as below, moving drop_files()
> into io_req_clean_work/io_free_req(), which is out of locks. Depends
> on that don't-cancel-by-files patch, but I guess can be for 5.11