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


-Doug

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