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

[Hao Xu <haoxu@linux.alibaba.com>]

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

>>
>>
>>> 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);
>>>
>>