Re: [PATCH v8 04/10] drivers: qcom: rpmh: add RPMH helper functions

[Doug Anderson]

Hi,

On Tue, May 15, 2018 at 10:47 AM, Lina Iyer <ilina@xxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxx> 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