Re: [PATCH 2/2] io_uring: don't hold uring_lock when calling io_run_task_work*

[Pavel Begunkov <asml.silence@gmail.com>]

On 04/02/2021 11:17, Pavel Begunkov wrote:
> On 04/02/2021 03:25, Hao Xu wrote:
>> 在 2021/2/4 上午12:45, Pavel Begunkov 写道:
>>> On 03/02/2021 16:35, Pavel Begunkov wrote:
>>>> On 03/02/2021 14:57, Hao Xu wrote:
>>>>> This is caused by calling io_run_task_work_sig() to do work under
>>>>> uring_lock while the caller io_sqe_files_unregister() already held
>>>>> uring_lock.
>>>>> we need to check if uring_lock is held by us when doing unlock around
>>>>> io_run_task_work_sig() since there are code paths down to that place
>>>>> without uring_lock held.
>>>>
>>>> 1. we don't want to allow parallel io_sqe_files_unregister()s
>>>> happening, it's synchronised by uring_lock atm. Otherwise it's
>>>> buggy.
>> Here "since there are code paths down to that place without uring_lock held" I mean code path of io_ring_ctx_free().
> 
> I guess it's to the 1/2, but let me outline the problem again:
> if you have two tasks userspace threads sharing a ring, then they
> can both and in parallel call syscall:files_unregeister. That's
> a potential double percpu_ref_kill(&data->refs), or even worse.
> 
> Same for 2, but racing for the table and refs.

There is a couple of thoughts for this:

1. I don't like waiting without holding the lock in general, because
someone can submit more reqs in-between and so indefinitely postponing
the files_unregister.
2. I wouldn't want to add checks for that in submission path.

So, a solution I think about is to wait under the lock, If we need to
run task_works -- briefly drop the lock, run task_works and then do
all unregister all over again. Keep an eye on refs, e.g. probably
need to resurrect it.

Because we current task is busy nobody submits new requests on
its behalf, and so there can't be infinite number of in-task_work
reqs, and eventually it will just go wait/sleep forever (if not
signalled) under the mutex, so we can a kind of upper bound on
time.

> 
>>>
>>> This one should be simple, alike to
>>>
>>> if (percpu_refs_is_dying())
>>>     return error; // fail *files_unregister();
>>>
>>>>
>>>> 2. probably same with unregister and submit.
>>>>
>>>>>
>>>>> Reported-by: Abaci <abaci@linux.alibaba.com>
>>>>> Fixes: 1ffc54220c44 ("io_uring: fix io_sqe_files_unregister() hangs")
>>>>> Signed-off-by: Hao Xu <haoxu@linux.alibaba.com>
>>>>> ---
>>>>>   fs/io_uring.c | 19 +++++++++++++------
>>>>>   1 file changed, 13 insertions(+), 6 deletions(-)
>>>>>
>>>>> diff --git a/fs/io_uring.c b/fs/io_uring.c
>>>>> index efb6d02fea6f..b093977713ee 100644
>>>>> --- a/fs/io_uring.c
>>>>> +++ b/fs/io_uring.c
>>>>> @@ -7362,18 +7362,25 @@ static int io_sqe_files_unregister(struct io_ring_ctx *ctx, bool locked)
>>>>>         /* wait for all refs nodes to complete */
>>>>>       flush_delayed_work(&ctx->file_put_work);
>>>>> +    if (locked)
>>>>> +        mutex_unlock(&ctx->uring_lock);
>>>>>       do {
>>>>>           ret = wait_for_completion_interruptible(&data->done);
>>>>>           if (!ret)
>>>>>               break;
>>>>>           ret = io_run_task_work_sig();
>>>>> -        if (ret < 0) {
>>>>> -            percpu_ref_resurrect(&data->refs);
>>>>> -            reinit_completion(&data->done);
>>>>> -            io_sqe_files_set_node(data, backup_node);
>>>>> -            return ret;
>>>>> -        }
>>>>> +        if (ret < 0)
>>>>> +            break;
>>>>>       } while (1);
>>>>> +    if (locked)
>>>>> +        mutex_lock(&ctx->uring_lock);
>>>>> +
>>>>> +    if (ret < 0) {
>>>>> +        percpu_ref_resurrect(&data->refs);
>>>>> +        reinit_completion(&data->done);
>>>>> +        io_sqe_files_set_node(data, backup_node);
>>>>> +        return ret;
>>>>> +    }
>>>>>         __io_sqe_files_unregister(ctx);
>>>>>       nr_tables = DIV_ROUND_UP(ctx->nr_user_files, IORING_MAX_FILES_TABLE);
>>>>>
>>>>
>>>
>>
> 

-- 
Pavel Begunkov