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SubjectRe: [PATCH v8 04/10] drivers: qcom: rpmh: add RPMH helper functions
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Hi,

On 5/15/2018 11:52 PM, Doug Anderson wrote:
> Hi,
>
> On Tue, May 15, 2018 at 10:47 AM, Lina Iyer <ilina@codeaurora.org> wrote:
>> On Fri, May 11 2018 at 14:17 -0600, Doug Anderson wrote:
>>>
>>> Hi,
>>>
>>> On Wed, May 9, 2018 at 10:01 AM, Lina Iyer <ilina@codeaurora.org> wrote:
>>>>
>>>> +int rpmh_write(const struct device *dev, enum rpmh_state state,
>>>> + const struct tcs_cmd *cmd, u32 n)
>>>> +{
>>>> + DECLARE_COMPLETION_ONSTACK(compl);
>>>> + DEFINE_RPMH_MSG_ONSTACK(dev, state, &compl, rpm_msg);
>>>> + int ret;
>>>> +
>>>> + if (!cmd || !n || n > MAX_RPMH_PAYLOAD)
>>>> + return -EINVAL;
>>>> +
>>>> + memcpy(rpm_msg.cmd, cmd, n * sizeof(*cmd));
>>>> + rpm_msg.msg.num_cmds = n;
>>>> +
>>>> + ret = __rpmh_write(dev, state, &rpm_msg);
>>>> + if (ret)
>>>> + return ret;
>>>> +
>>>> + ret = wait_for_completion_timeout(&compl, RPMH_TIMEOUT_MS);
>>>
>>>
>>> IMO it's almost never a good idea to use wait_for_completion_timeout()
>>> together with a completion that's declared on the stack. If you
>>> somehow insist that this is a good idea then I need to see incredibly
>>> clear and obvious code/comments that say why it's impossible that the
>>> process might somehow try to signal the completion _after_
>>> RPMH_TIMEOUT_MS has expired.
>>>
>>> Specifically if the timeout happens but the process could still signal
>>> a completion later then they will access random data on the stack of a
>>> function that has already returned. This causes ridiculously
>>> difficult-to-debug crashes.
>>>
>>>
>>> NOTE: You've got timeout set to 10 seconds here. Is that really even
>>> useful? IMO just call wait_for_completion() without a timeout. It's
>>> much better to have a nice clean hang than a random stack corruption.
>>>
>>>
>> The 10 sec timeout will guarantee that we will not get a response at all
>> anymore for the request. Usually requests can be considered failed if
>> there is no response in a few tens of microseconds. 10 sec is just an
>> arbitarily large number.
>>
>> The reason we use timeout is that once the timeout happens, we know we
>> have failed, we could trigger a watchdog or crash the system. This is
>> very important for our productization in debugging RPMH failures. A
>> hang would not always trigger a watchdog and the failure would be silent
>> and possibly fatal but hard to debug.
>
> If you intend the system to crash when this timeout happens then IMHO
> add a BUG_ON. Then I won't worry about something coming around later
> and clobbering the stack.
>
> -Doug
>

Sure. Will add BUG_ON in next patch.

Thanks,
Raju.

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