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Message-ID: <73f25c8f-6193-6001-d3ff-b7fd060cce83@quicinc.com>
Date: Tue, 1 Aug 2023 16:41:16 -0700
From: Chris Lew <quic_clew@...cinc.com>
To: Sricharan Ramabadhran <quic_srichara@...cinc.com>,
Pavan Kondeti <quic_pkondeti@...cinc.com>,
Praveenkumar I <quic_ipkumar@...cinc.com>
CC: <agross@...nel.org>, <andersson@...nel.org>,
<konrad.dybcio@...aro.org>, <linux-arm-msm@...r.kernel.org>,
<linux-kernel@...r.kernel.org>, <quic_varada@...cinc.com>
Subject: Re: [PATCH] soc: qcom: qmi: Signal the txn completion after releasing
the mutex
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@...r.kernel.org
>>>> Signed-off-by: Sricharan Ramabadhran <quic_srichara@...cinc.com>
>>>> Signed-off-by: Praveenkumar I <quic_ipkumar@...cinc.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.
> Regards,
> Sricharan
>
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