Re: [PATCH] null_blk: fix poll request timeout handling

[Chengming Zhou]

On 2023/8/15 16:31, Ming Lei wrote:
> On Tue, Aug 15, 2023 at 02:04:42PM +0800, chengming.zhou@xxxxxxxxx wrote:
>> From: Chengming Zhou <zhouchengming@xxxxxxxxxxxxx>
>>
>> When doing io_uring benchmark on /dev/nullb0, it's easy to crash the
>> kernel if poll requests timeout triggered, as reported by David. [1]
> 
> Just be curious, how is the timeout triggered when running
> "./fio/t/io_uring -r20 /dev/nullb0"?

I tried "./fio/t/io_uring -r20 /dev/nullb0" multiple times, sometimes
program exit ok, sometimes it dump many timeout messages and kernel BUG.

I just used David's "./fio/t/io_uring -n4 /dev/nullb0", run a while
then ctrl-C, the program will always dump many timeout messages.

It seems that program exit is not clean, leave some requests in driver
haven't been polled & completed? I don't know.

> 
> David mentioned that the issue is triggered in 6.5-rc1, maybe one
> regression?
> 

I just tested using v6.4.9, found the same timeout and kernel BUG
using "./fio/t/io_uring -n4 /dev/nullb0".

>>
>> BUG: kernel NULL pointer dereference, address: 0000000000000008
>> Workqueue: kblockd blk_mq_timeout_work
>> RIP: 0010:null_timeout_rq+0x4e/0x91
>> Call Trace:
>>  ? __die_body+0x1a/0x5c
>>  ? page_fault_oops+0x6f/0x9c
>>  ? kernelmode_fixup_or_oops+0xc6/0xd6
>>  ? __bad_area_nosemaphore+0x44/0x1eb
>>  ? exc_page_fault+0xe2/0xf4
>>  ? asm_exc_page_fault+0x22/0x30
>>  ? null_timeout_rq+0x4e/0x91
>>  blk_mq_handle_expired+0x31/0x4b
>>  bt_iter+0x68/0x84
>>  ? bt_tags_iter+0x81/0x81
>>  __sbitmap_for_each_set.constprop.0+0xb0/0xf2
>>  ? __blk_mq_complete_request_remote+0xf/0xf
>>  bt_for_each+0x46/0x64
>>  ? __blk_mq_complete_request_remote+0xf/0xf
>>  ? percpu_ref_get_many+0xc/0x2a
>>  blk_mq_queue_tag_busy_iter+0x14d/0x18e
>>  blk_mq_timeout_work+0x95/0x127
>>  process_one_work+0x185/0x263
>>  worker_thread+0x1b5/0x227
>>  ? rescuer_thread+0x287/0x287
>>  kthread+0xfa/0x102
>>  ? kthread_complete_and_exit+0x1b/0x1b
>>  ret_from_fork+0x22/0x30
>>
>> This is indeed a race problem between null_timeout_rq() and null_poll().
>>
>> null_poll()				null_timeout_rq()
>>   spin_lock(&nq->poll_lock)
>>   list_splice_init(&nq->poll_list, &list)
>>   spin_unlock(&nq->poll_lock)
>>
>>   while (!list_empty(&list))
>>     req = list_first_entry()
>>     list_del_init()
>>     ...
>>     blk_mq_add_to_batch()
>>     // req->rq_next = NULL
>> 					spin_lock(&nq->poll_lock)
>>
>> 					// rq->queuelist->next == NULL
>> 					list_del_init(&rq->queuelist)
>>
>> 					spin_unlock(&nq->poll_lock)
>>
>> What's worse is that we don't call blk_mq_complete_request_remote()
>> before blk_mq_add_to_batch(), so these completed requests have wrong
>> rq->state == MQ_RQ_IN_FLIGHT. We can easily check this using bpftrace:
>>
>> ```
>> bpftrace -e 'kretfunc:null_blk:null_poll {
>>   $iob=(struct io_comp_batch *)args->iob;
>>   @[$iob->req_list->state]=count();
>> }'
>>
>> @[1]: 51708
>> ```
>>
>> Fix these problems by setting requests state to MQ_RQ_COMPLETE under
>> nq->poll_lock protection, in which null_timeout_rq() can safely detect
>> this race and early return.
>>
>> [1] https://lore.kernel.org/all/3893581.1691785261@xxxxxxxxxxxxxxxxxxxxxx/
>>
>> Fixes: 0a593fbbc245 ("null_blk: poll queue support")
>> Reported-by: David Howells <dhowells@xxxxxxxxxx>
>> Signed-off-by: Chengming Zhou <zhouchengming@xxxxxxxxxxxxx>
>> ---
>>  drivers/block/null_blk/main.c | 12 ++++++++++--
>>  1 file changed, 10 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/block/null_blk/main.c b/drivers/block/null_blk/main.c
>> index 864013019d6b..968090935eb2 100644
>> --- a/drivers/block/null_blk/main.c
>> +++ b/drivers/block/null_blk/main.c
>> @@ -1643,9 +1643,12 @@ static int null_poll(struct blk_mq_hw_ctx *hctx, struct io_comp_batch *iob)
>>  	struct nullb_queue *nq = hctx->driver_data;
>>  	LIST_HEAD(list);
>>  	int nr = 0;
>> +	struct request *rq;
>>  
>>  	spin_lock(&nq->poll_lock);
>>  	list_splice_init(&nq->poll_list, &list);
>> +	list_for_each_entry(rq, &list, queuelist)
>> +		blk_mq_set_request_complete(rq);
>>  	spin_unlock(&nq->poll_lock);
>>  
>>  	while (!list_empty(&list)) {
>> @@ -1671,16 +1674,21 @@ static enum blk_eh_timer_return null_timeout_rq(struct request *rq)
>>  	struct blk_mq_hw_ctx *hctx = rq->mq_hctx;
>>  	struct nullb_cmd *cmd = blk_mq_rq_to_pdu(rq);
>>  
>> -	pr_info("rq %p timed out\n", rq);
>> -
>>  	if (hctx->type == HCTX_TYPE_POLL) {
>>  		struct nullb_queue *nq = hctx->driver_data;
>>  
>>  		spin_lock(&nq->poll_lock);
>> +		/* The request may have completed meanwhile. */
>> +		if (blk_mq_request_completed(rq)) {
>> +			spin_unlock(&nq->poll_lock);
>> +			return BLK_EH_DONE;
>> +		}
>>  		list_del_init(&rq->queuelist);
>>  		spin_unlock(&nq->poll_lock);
>>  	}
> 
> I think null_process_cmd() is needed for un-completed request.
> 

The end of function will set BLK_STS_TIMEOUT error and complete request
using blk_mq_complete_request(), not sure if null_process_cmd() is
needed in this error case?

Thanks.