Re: [PATCH v7 3/5] blk-mq: Fix races between iterating over requests and freeing requests

Bart Van Assche
2021-04-22T15:51:10Z

On 4/22/21 12:13 AM, Ming Lei wrote:
> On Wed, Apr 21, 2021 at 08:54:30PM -0700, Bart Van Assche wrote:
>> On 4/21/21 8:15 PM, Ming Lei wrote:
>>> On Tue, Apr 20, 2021 at 05:02:33PM -0700, Bart Van Assche wrote:
>>>> +static bool bt_tags_iter(struct sbitmap *bitmap, unsigned int bitnr, void *data)
>>>> +{
>>>> + struct bt_tags_iter_data *iter_data = data;
>>>> + struct blk_mq_tags *tags = iter_data->tags;
>>>> + bool res;
>>>> +
>>>> + if (iter_data->flags & BT_TAG_ITER_MAY_SLEEP) {
>>>> + down_read(&tags->iter_rwsem);
>>>> + res = __bt_tags_iter(bitmap, bitnr, data);
>>>> + up_read(&tags->iter_rwsem);
>>>> + } else {
>>>> + rcu_read_lock();
>>>> + res = __bt_tags_iter(bitmap, bitnr, data);
>>>> + rcu_read_unlock();
>>>> + }
>>>> +
>>>> + return res;
>>>> +}
>>>
>>> Holding one rwsem or rcu read lock won't avoid the issue completely
>>> because request may be completed remotely in iter_data->fn(), such as
>>> nbd_clear_req(), nvme_cancel_request(), complete_all_cmds_iter(),
>>> mtip_no_dev_cleanup(), because blk_mq_complete_request() may complete
>>> request in softirq, remote IPI, even wq, and the request is still
>>> referenced in these contexts after bt_tags_iter() returns.
>>
>> The rwsem and RCU read lock are used to serialize iterating over
>> requests against blk_mq_sched_free_requests() calls. I don't think it
>> matters for this patch from which context requests are freed.
>
> Requests still can be referred in other context after blk_mq_wait_for_tag_iter()
> returns, then follows freeing request pool. And use-after-free exists too, doesn't it?

The request pool should only be freed after it has been guaranteed that
all pending requests have finished and also that no new requests will be
started. This patch series adds two blk_mq_wait_for_tag_iter() calls.
Both calls happen while the queue is frozen so I don't think that the
issue mentioned in your email can happen.

Bart.