runtime: TestGoroutineProfile often times out on freebsd

[aclements]

TestGoroutineProfile in the runtime tests has been timing out on freebsd-amd64 and freebsd-386 fairly regularly since June 19th.

For example, http://build.golang.org/log/d17f225a550e374216b3b3659083528163b22af9 (2015-07-01T10:37:10-1cbbd7f/freebsd-amd64-gce101) is the most recent and http://build.golang.org/log/60872d743598fe54d315bfcbded6321fb9c18ccc (2015-06-19T05:15:22-a3c0730/freebsd-amd64-gce101) is the first.

It looks like runtime.GoroutineProfile is stuck trying to acquire worldsema, but it's unclear who's holding it (presumably some runtime G that isn't being printed).

Looking through the commit history leading up to the first failure, 90a1996 (runtime: reduce latency by aggressively ending mark phase, @RLH), e72f5f6 (runtime: fix tracing of syscallexit, @dvyukov), cfa3eda (runtime: fix race in scanvalid assertion, @rsc), and 3c60e6e (runtime: fix races in stack scan, @rsc) are the only ones that jump out at me as potentially related, but it may be none of those.

[dvyukov]

I can imagine that that test can be super slow on some hardware/OS.

[aclements]

It's possible this was fixed in the last week, or it's possible we've just been lucky. Here are the most recent 10 occurrences on the dashboard:

2015-07-13T19:29:25-7ef6a9f/freebsd-amd64-gce101
2015-07-15T01:56:14-8b6527b/freebsd-amd64-gce101
2015-07-15T02:53:34-e6a0c21/freebsd-386-gce101
2015-07-15T04:18:39-3585f21/freebsd-amd64-gce101
2015-07-15T04:41:08-af96030/freebsd-amd64-gce101
2015-07-15T05:06:50-238a395/freebsd-amd64-gce101
2015-07-15T05:13:05-47f22ab/freebsd-386-gce101
2015-07-17T22:33:51-3549178/freebsd-386-gce101
2015-07-20T00:50:46-1125cd4/freebsd-amd64-gce101
2015-07-21T14:02:34-ac1935b/freebsd-386-gce101

[rsc]

I'm feeling lucky.