runtime: apparent deadlocks on Windows builders

[bcmills]

Starting in November, there appears to be a dramatic uptick in the number of test timeouts on the windows-* builders.

Many of these are for tests that normally run nearly instantaneously, such as archive/tar and bufio.

2019-11-22T03:06:22-0e02cfb/windows-amd64-race
2019-11-21T22:20:17-94e9a5e/windows-amd64-race
2019-11-21T19:27:16-f4a8bf1/windows-amd64-longtest
2019-11-21T19:09:24-2434869/windows-amd64-longtest
2019-11-21T19:09:24-2434869/windows-amd64-race
2019-11-21T16:56:47-37715cc/windows-amd64-longtest
2019-11-21T16:56:47-37715cc/windows-amd64-race
2019-11-21T16:01:14-c7e73ef/windows-amd64-race
2019-11-20T20:51:13-9852b4b/windows-amd64-race
2019-11-13T19:15:27-7ad2748/windows-amd64-longtest
2019-11-12T22:09:05-a56d755/windows-amd64-2016
2019-11-12T01:07:15-ec73263/windows-amd64-2012
2019-11-08T17:01:05-a5a6f61/windows-amd64-2012
2019-11-07T19:18:12-1b0b980/windows-amd64-2012
2019-11-07T05:52:34-3eabdd2/windows-amd64-longtest
2019-11-06T09:09:59-0c5d545/windows-amd64-2008
2019-11-06T02:52:51-f71bd51/windows-amd64-2016
2019-11-05T16:31:48-414c1d4/windows-amd64-2016
2019-11-05T14:44:56-e457cc3/windows-amd64-race
2019-11-05T05:19:08-d51f7f3/windows-amd64-longtest
2019-11-05T03:50:54-979d65d/windows-amd64-2016
2019-11-05T00:19:10-6cbd737/windows-amd64-race
2019-11-01T14:48:28-a570fcf/windows-amd64-2012
2019-03-19T08:30:50-451a2eb/windows-amd64-2008
2018-12-05T21:54:54-6454a09/windows-amd64-race

CC @ianlancetaylor @aclements @alexbrainman @zx2c4

[bcmills]

I suspect that the same root cause is responsible for apparent deadlocks in the os/exec test.
2019-11-22T04:26:34-95be9b7/windows-amd64-2008
2019-11-21T22:20:17-94e9a5e/windows-amd64-2012

[ianlancetaylor]

2019-11-22T14:32:02-ea18a1c/windows-amd64-race
2019-11-22T15:32:23-c02f3b8/windows-amd64-longtest
2019-11-22T15:32:23-c02f3b8/windows-amd64-race
2019-11-22T19:01:42-c931f1b/windows-amd64-longtest
2019-11-22T19:01:42-c931f1b/windows-amd64-race
2019-11-22T19:03:26-28314cf/windows-amd64-race
2019-11-22T19:03:50-941ac9c/windows-amd64-race
2019-11-22T19:17:28-8324aca/windows-amd64-race
2019-11-22T21:09:43-9f3c2b6/windows-amd64-race
2019-11-22T22:24:38-980f0c0/windows-amd64-race
2019-11-22T23:23:13-6ba1bf3/windows-amd64-race
2019-11-23T00:06:41-f29e53d/windows-amd64-race
2019-11-23T11:00:41-6f7b96f/windows-amd64-longtest
2019-11-23T11:00:41-6f7b96f/windows-amd64-race
2019-11-24T15:06:38-b81dd1c/windows-amd64-longtest
2019-11-24T15:06:38-b81dd1c/windows-amd64-race
2019-11-25T15:08:18-01f15b6/windows-amd64-race
2019-11-25T16:26:15-476395c/windows-amd64-race
2019-11-25T16:43:27-e16f64c/windows-amd64-2016
2019-11-25T16:43:27-e16f64c/windows-amd64-race
2019-11-25T19:02:50-7adafc9/windows-amd64-2016
2019-11-25T21:15:11-5de3b97/windows-amd64-race
2019-11-25T23:37:03-87805c9/windows-amd64-race
2019-11-26T16:55:36-67f0f83/windows-amd64-race
2019-11-26T20:21:04-4a378d7/windows-amd64-2008
2019-11-27T01:29:00-c3f1492/windows-amd64-race
2019-11-27T01:30:19-b2482e4/windows-amd64-2008
2019-11-27T01:30:32-0f25102/windows-amd64-2008
2019-11-27T15:06:55-4e3d580/windows-amd64-race
2019-11-27T18:42:33-f1c8e2c/windows-amd64-race
2019-12-02T16:11:24-3edd1d8/windows-amd64-race
2019-12-02T16:45:37-bf3ee57/windows-amd64-race
2019-12-02T19:05:01-e4c01f0/windows-amd64-race
2019-12-02T20:12:54-a18608a/windows-amd64-2012
2019-12-02T20:12:54-a18608a/windows-amd64-longtest

[ianlancetaylor]

These failures are all either on the race builder or for tests with the race detector enabled. Seems like something makes deadlocks possible with the race detector.

The findflakes programs reports that this might be due to https://golang.org/issue/204340 "runtime: support preemption on windows/{386,amd64}".

CC @aclements @cherrymui

[bcmills]

Seems like something makes deadlocks possible with the race detector.

That could be related to #18717 — does the runtime assume that its preemption signals are eventually delivered?

[aclements]

does the runtime assume that its preemption signals are eventually delivered?

It does not. It will always try both a cooperative and a non-cooperative preemption and take whichever one it can get. If the signal goes missing, it will wait for a cooperative preemption just like before.