Re: [PATCH 1/2] io_uring: add uring_lock as an argument to io_sqe_files_unregister()

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

On 04/02/2021 03:34, Hao Xu wrote:
> 在 2021/2/4 上午12:33, Pavel Begunkov 写道:
>> On 03/02/2021 14:57, Hao Xu wrote:
>>> io_sqe_files_unregister is currently called from several places:
>>>      - syscall io_uring_register (with uring_lock)
>>>      - io_ring_ctx_wait_and_kill() (without uring_lock)
>>>
>>> There is a AA type deadlock in io_sqe_files_unregister(), thus we need
>>> to know if we hold uring_lock in io_sqe_files_unregister() to fix the
>>> issue.
>>
>> It's ugly, just take the lock and kill the patch. There can't be any
>> contention during io_ring_ctx_free anyway.
> Hi Pavel, I don't get it, do you mean this patch isn't needed, and we can just unlock(&uring_lock) before io_run_task_work_sig() and lock(&uring_lock) after it? I knew there won't be contention during io_ring_ctx_free that's why there is no uring_lock in it.
> I tried to just do unlock(&uring_lock) before io_run_task_sig() without if(locked) check, it reports something like "there are unpaired mutex lock/unlock" since we cannot just unlock if it's from io_ring_ctx_free.


The ugly part is @locked. I know that there is already similar stuff
around, but I may go long why and how much I don't like it.

io_ring_ctx_free()
{
    ...
    lock(uring_lock);
    files_unregister();
    unlock(uring_lock);
    ...
}

With this you'll always have the mutex locked in unregister, so
can drop it unconditionally (if that will ever be needed). It's
also cleaner from the synchronisation perspective.

>>
>>>
>>> Signed-off-by: Hao Xu <haoxu@linux.alibaba.com>
>>> ---
>>>   fs/io_uring.c | 10 +++++-----
>>>   1 file changed, 5 insertions(+), 5 deletions(-)
>>>
>>> diff --git a/fs/io_uring.c b/fs/io_uring.c
>>> index 38c6cbe1ab38..efb6d02fea6f 100644
>>> --- a/fs/io_uring.c
>>> +++ b/fs/io_uring.c
>>> @@ -7339,7 +7339,7 @@ static void io_sqe_files_set_node(struct fixed_file_data *file_data,
>>>       percpu_ref_get(&file_data->refs);
>>>   }
>>>   -static int io_sqe_files_unregister(struct io_ring_ctx *ctx)
>>> +static int io_sqe_files_unregister(struct io_ring_ctx *ctx, bool locked)
>>>   {
>>>       struct fixed_file_data *data = ctx->file_data;
>>>       struct fixed_file_ref_node *backup_node, *ref_node = NULL;
>>> @@ -7872,13 +7872,13 @@ static int io_sqe_files_register(struct io_ring_ctx *ctx, void __user *arg,
>>>         ret = io_sqe_files_scm(ctx);
>>>       if (ret) {
>>> -        io_sqe_files_unregister(ctx);
>>> +        io_sqe_files_unregister(ctx, true);
>>>           return ret;
>>>       }
>>>         ref_node = alloc_fixed_file_ref_node(ctx);
>>>       if (!ref_node) {
>>> -        io_sqe_files_unregister(ctx);
>>> +        io_sqe_files_unregister(ctx, true);
>>>           return -ENOMEM;
>>>       }
>>>   @@ -8682,7 +8682,7 @@ static void io_ring_ctx_free(struct io_ring_ctx *ctx)
>>>           css_put(ctx->sqo_blkcg_css);
>>>   #endif
>>>   -    io_sqe_files_unregister(ctx);
>>> +    io_sqe_files_unregister(ctx, false);
>>>       io_eventfd_unregister(ctx);
>>>       io_destroy_buffers(ctx);
>>>       idr_destroy(&ctx->personality_idr);
>>> @@ -10065,7 +10065,7 @@ static int __io_uring_register(struct io_ring_ctx *ctx, unsigned opcode,
>>>           ret = -EINVAL;
>>>           if (arg || nr_args)
>>>               break;
>>> -        ret = io_sqe_files_unregister(ctx);
>>> +        ret = io_sqe_files_unregister(ctx, true);
>>>           break;
>>>       case IORING_REGISTER_FILES_UPDATE:
>>>           ret = io_sqe_files_update(ctx, arg, nr_args);
>>>
>>
> 

-- 
Pavel Begunkov