Re: [PATCH] soc: qcom: qmi: Signal the txn completion after releasing the mutex

[Sricharan Ramabadhran <quic_srichara@quicinc.com>]

On 8/2/2023 5:11 AM, Chris Lew wrote:
> 
> 
> On 8/1/2023 4:13 AM, Sricharan Ramabadhran wrote:
>> Hi,
>>
>> On 8/1/2023 6:06 AM, Chris Lew wrote:
>>>
>>>
>>> On 7/31/2023 8:19 AM, Pavan Kondeti wrote:
>>>> On Mon, Jul 31, 2023 at 06:37:55PM +0530, Praveenkumar I wrote:
>>>>> txn is in #1 stack
>>>>>
>>>>> Worker #1                                       Worker #2
>>>>> ********                    *********
>>>>>
>>>>> qmi_txn_wait(txn)                               qmi_handle_message
>>>>>     |                                                  |
>>>>>     |                                                  |
>>>>>   wait_for_complete(txn->complete)                    ....
>>>>>     |                                             
>>>>> mutex_lock(txn->lock)
>>>>>     |                                                  |
>>>>>   mutex_lock(txn->lock)                                |
>>>>>     .....                                         complete(txn->lock)
>>>>>     | mutex_unlock(txn->lock)
>>>>>     |
>>>>>   mutex_unlock(txn->lock)
>>>>>
>>>>> In this case above, while #2 is doing the mutex_unlock(txn->lock),
>>>>> in between releasing lock and doing other lock related wakeup, #2 gets
>>>>> scheduled out. As a result #1, acquires the lock, unlocks, also
>>>>> frees the txn also (where the lock resides)
>>>>>
>>>>> Now #2, gets scheduled again and tries to do the rest of the lock
>>>>> related wakeup, but lock itself is invalid because txn itself is gone.
>>>>>
>>>>> Fixing this, by doing the mutex_unlock(txn->lock) first and then
>>>>> complete(txn->lock) in #2
>>>>>
>>>>> Fixes: 3830d0771ef6 ("soc: qcom: Introduce QMI helpers")
>>>>> Cc: stable@vger.kernel.org
>>>>> Signed-off-by: Sricharan Ramabadhran <quic_srichara@quicinc.com>
>>>>> Signed-off-by: Praveenkumar I <quic_ipkumar@quicinc.com>
>>>>> ---
>>>>>   drivers/soc/qcom/qmi_interface.c | 3 ++-
>>>>>   1 file changed, 2 insertions(+), 1 deletion(-)
>>>>>
>>>>> diff --git a/drivers/soc/qcom/qmi_interface.c 
>>>>> b/drivers/soc/qcom/qmi_interface.c
>>>>> index 78d7361fdcf2..92e29db97359 100644
>>>>> --- a/drivers/soc/qcom/qmi_interface.c
>>>>> +++ b/drivers/soc/qcom/qmi_interface.c
>>>>> @@ -505,12 +505,13 @@ static void qmi_handle_message(struct 
>>>>> qmi_handle *qmi,
>>>>>                   pr_err("failed to decode incoming message\n");
>>>>>               txn->result = ret;
>>>>> -            complete(&txn->completion);
>>>>>           } else  {
>>>>>               qmi_invoke_handler(qmi, sq, txn, buf, len);
>>>>>           }
>>>>>           mutex_unlock(&txn->lock);
>>>>> +        if (txn->dest && txn->ei)
>>>>> +            complete(&txn->completion);
>>>>>       } else {
>>>>>           /* Create a txn based on the txn_id of the incoming 
>>>>> message */
>>>>>           memset(&tmp_txn, 0, sizeof(tmp_txn));
>>>>
>>>> What happens in a remote scenario where the waiter gets timed out at 
>>>> the
>>>> very same time you are releasing the mutex but before calling
>>>> complete()? The caller might end up freeing txn structure and it 
>>>> results
>>>> in the same issue you are currently facing.
>>>>
>>>> Thanks,
>>>> Pavan
>>>
>>> I think downstream we had various attempts of moving the signal 
>>> around trying to avoid this, but hit scenarios like the one Pavan 
>>> described.
>>>
>>> We eventually settled on removing the txn->lock and treating the 
>>> qmi->txn_lock as a big lock. This remedied the issue where the 
>>> txn->lock goes out of scope since qmi->txn_lock is tied to the qmi 
>>> handle.
>>>
>>
>>   ok agree. Using qmi->txn_lock looks a better approach.
>>   That said, this race between mutex lock/unlock looks odd though.
>>   If i remember we saw the issue only with CONFIG_DEBUG_LOCK_ALLOC.
>>   Was that the same case for you guys as well ?
>>
>>   Otherwise, ideally handling all members of the object inside lock
>>   should be the right solution (ie moving the wait_for_complete(txn)
>>   inside the mutex_lock in qmi_txn_wait. That should take care of the
>>   scenario that Pavan described too.
>>
> 
> No, we saw the issue even without CONFIG_DEBUG_LOCK_ALLOC. The 
> callstacks always ended up showing that the mutex could be acquired 
> before mutex_unlock() completely finished.
> 
> It didn't seem wise to poke at the mutex implementation so we went with 
> the txn_lock.

  ok, that's strange. That effectively means, mutex_lock/unlock are not
  working/protecting the critical section ? Then qmi->txn_lock also would
  result in a similar issue ? I guess, in this case, during issue, txn
  (which holds the lock) was going out of context, while still the txn
  was in used in other thread. That effectively shows up a mutex issue
  maybe. While the downstream change to use qmi->txn_lock would fix the
  mutex issue, will have to check if the txn object itself is protected
  correctly.

Regards,
  Sricharan