Why does the bitcoin mining code have the same if statement twice?

From the v0.9.3 miner.cpp source (https://github.com/bitcoin/bitcoin/blob/v0.9.3/src/miner.cpp#L598-L604):

if (GetTimeMillis() - nHPSTimerStart > 4000)
{
    static CCriticalSection cs;
    {
        LOCK(cs);
        if (GetTimeMillis() - nHPSTimerStart > 4000)
        {
        ....

Why is the same if statement "if (GetTimeMillis() - nHPSTimerStart > 4000)" executed twice? GetTimeMillis() can only go up, so it seems like if the first one is true, then the second statement can only ever evaluate to true as well, and is useless. I'm assuming this is not entirely correct, though, and am also assuming it has something to do with the CCriticalSelection, but would like to understand the nature of it better.

Thanks!

morsecoder

Posted 2014-12-04T21:41:11.430

Reputation: 12 624

I agree, I think that's a copy-paste bug. Maybe you should ask #bitcoin-dev to double-check. – Nick ODell – 2014-12-04T22:01:47.817

1Can any other thread modify nHPSTimerStart? – Greg Hewgill – 2014-12-05T00:31:44.843

Yes, I think so. There could be quite a few threads working on the mining. – morsecoder – 2014-12-05T00:45:46.497

1I have used something similar before. The first if checks if we should bother to do the potentially expensive lock. If yes, then we lock and now that nHPSTimerStart is guaranteed not to change we can safely execute the if again. – Floris – 2014-12-05T13:13:06.233

Thanks, Floris, that seems reasonable. If you post as an answer, I could mark it as such. – morsecoder – 2014-12-11T00:17:22.880

Why does the bitcoin mining code have the same if statement twice?

Answers