Re: [PATCH 1/2] io_uring: fix tw list mess-up by adding tw while it's already in tw list

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

On 9/26/21 10:48 AM, Hao Xu wrote:
> 在 2021/9/15 下午6:48, Hao Xu 写道:
>> 在 2021/9/15 下午5:44, Pavel Begunkov 写道:
>>> On 9/12/21 5:23 PM, Hao Xu wrote:
>>>> For multishot mode, there may be cases like:
>>>> io_poll_task_func()
>>>> -> add_wait_queue()
>>>>                              async_wake()
>>>>                              ->io_req_task_work_add()
>>>>                              this one mess up the running task_work list
>>>>                              since req->io_task_work.node is in use.
>>>>
>>>> similar situation for req->io_task_work.fallback_node.
>>>> Fix it by set node->next = NULL before we run the tw, so that when we
>>>> add req back to the wait queue in middle of tw running, we can safely
>>>> re-add it to the tw list.
>>>
>>> It may get screwed before we get to "node->next = NULL;",
>>>
>>> -> async_wake()
>>>    -> io_req_task_work_add()
>>> -> async_wake()
>>>    -> io_req_task_work_add()
>>> tctx_task_work()
>> True, this may happen if there is second poll wait entry.
>> This pacth is for single wait entry only..
>> I'm thinking about the second poll entry issue, would be in a separate
>> patch.
> hmm, reviewed this email again and now I think I got what you were
> saying, do you mean the second async_wake() triggered before we removed
> the wait entry in the first async_wake(), like
> 
> async_wake
>                           async_wake
> ->del wait entry

Looks we had different problems in mind, let's move the conversation to
the new thread with resent patches



>>>> Fixes: 7cbf1722d5fc ("io_uring: provide FIFO ordering for task_work")
>>>> Signed-off-by: Hao Xu <haoxu@linux.alibaba.com>
>>>> ---
>>>>
>>>>   fs/io_uring.c | 11 ++++++++---
>>>>   1 file changed, 8 insertions(+), 3 deletions(-)
>>>>
>>>> diff --git a/fs/io_uring.c b/fs/io_uring.c
>>>> index 30d959416eba..c16f6be3d46b 100644
>>>> --- a/fs/io_uring.c
>>>> +++ b/fs/io_uring.c
>>>> @@ -1216,13 +1216,17 @@ static void io_fallback_req_func(struct work_struct *work)
>>>>       struct io_ring_ctx *ctx = container_of(work, struct io_ring_ctx,
>>>>                           fallback_work.work);
>>>>       struct llist_node *node = llist_del_all(&ctx->fallback_llist);
>>>> -    struct io_kiocb *req, *tmp;
>>>> +    struct io_kiocb *req;
>>>>       bool locked = false;
>>>>       percpu_ref_get(&ctx->refs);
>>>> -    llist_for_each_entry_safe(req, tmp, node, io_task_work.fallback_node)
>>>> +    req = llist_entry(node, struct io_kiocb, io_task_work.fallback_node);
>>>> +    while (member_address_is_nonnull(req, io_task_work.fallback_node)) {
>>>> +        node = req->io_task_work.fallback_node.next;
>>>> +        req->io_task_work.fallback_node.next = NULL;
>>>>           req->io_task_work.func(req, &locked);
>>>> -
>>>> +        req = llist_entry(node, struct io_kiocb, io_task_work.fallback_node);
>>>> +    }
>>>>       if (locked) {
>>>>           if (ctx->submit_state.compl_nr)
>>>>               io_submit_flush_completions(ctx);
>>>> @@ -2126,6 +2130,7 @@ static void tctx_task_work(struct callback_head *cb)
>>>>                   locked = mutex_trylock(&ctx->uring_lock);
>>>>                   percpu_ref_get(&ctx->refs);
>>>>               }
>>>> +            node->next = NULL;
>>>>               req->io_task_work.func(req, &locked);
>>>>               node = next;
>>>>           } while (node);
>>>>
>>>
> 

-- 
Pavel Begunkov