Marking as release-blocker for Go 1.18 because the failure rate is markedly higher as of 2021-11-04: this looks like a regression, or at the very least a flaky test exacerbated by 1.18 changes.

(@mknyszek, another one for the “GC running during test” pile, perhaps?)

Oh, I think I see what's wrong here. The test already says that it's unsafe for a GC to run, and calls SetGCPercent(-1). However, I think what's missing here is that one could actively be in progress, so runtime.GC needs to be called to make sure.

I'll try to confirm it.

Change https://golang.org/cl/366534 mentions this issue: runtime: ensure no GC is running in TestParallelRWMutexReaders

I've had trouble reproducing the issue, but I do think my previous theory does indicate a real potential problem. That theory does also fit into the change in minimum heap size triggering the issue.

Sent golang.org/cl/366534. We can try landing it and see if it fixes it, placing this issue in a WaitingForInfo state. I think it's something that the test should be doing anyway.

Actually, I'm more confident this is actually the fix. If I simply start a GC on another goroutine before the test starts, it easily reproduces. I guess it's possible this is some other bug, but it's unlikely at this point. Once I land https://golang.org/cl/366534 I'll close this optimistically.

Change https://golang.org/cl/367874 mentions this issue: misc/cgo/test: further reduce likeliness of hang in Test9400

Update on the root cause here: the fixes for #49680, #49695, and #45867 all assumed that SetGCPercent(-1) doesn't block until the GC's mark phase is done, but it actually does. The cause of those failures is that at the beginning of the sweep phase, the GC does try to preempt every P once, and this may run concurrently with test code that has non-preemptible goroutines that create the potential for a deadlock. In short, the fix was correct, but the reasoning was incorrect.

Change https://golang.org/cl/369815 mentions this issue: runtime: fix comments on the behavior of SetGCPercent