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

[Chris Lew <quic_clew@quicinc.com>]

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.