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

[Sricharan Ramabadhran <quic_srichara@quicinc.com>]

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.

Regards,
  Sricharan