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

Pavel Begunkov
2021-02-04T11:21:38Z

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.

>>
>> 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