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

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