runtime: runtime.GC can return without finishing sweep #45315
Description
runtime.TestGCTestIsReachable has been flaky since I added it. The failures look like:
##### GOMAXPROCS=2 runtime -cpu=1,2,4 -quick
runtime: object 0 was not swept
fatal error: IsReachable failed
goroutine 176934 [running]:
runtime.throw(0x688d74, 0x12)
/workdir/go/src/runtime/panic.go:1126 +0x74 fp=0xc000276628 sp=0xc0002765f8 pc=0x43e334
runtime.gcTestIsReachable(0xc00108e200, 0x10, 0x10, 0x4)
/workdir/go/src/runtime/mgc.go:2397 +0x2ad fp=0xc000276690 sp=0xc000276628 pc=0x423a0d
runtime.GCTestIsReachable(...)
/workdir/go/src/runtime/export_test.go:1256
runtime_test.TestGCTestIsReachable(0xc00090d380)
/workdir/go/src/runtime/gc_test.go:249 +0x25c fp=0xc000276780 sp=0xc000276690 pc=0x58a8fc
testing.tRunner(0xc00090d380, 0x69c980)
/workdir/go/src/testing/testing.go:1235 +0x102 fp=0xc0002767d0 sp=0xc000276780 pc=0x4eee02
runtime.goexit()
/workdir/go/src/runtime/asm_amd64.s:1454 +0x1 fp=0xc0002767d8 sp=0xc0002767d0 pc=0x47b861
created by testing.(*T).Run
/workdir/go/src/testing/testing.go:1280 +0x3ab
Some examples:
https://build.golang.org/log/6664ed5c503173b3f29eb85cb093baa61fb04838
https://build.golang.org/log/5608852a242f93bb2f36b6a56bebdefda3797e82
https://build.golang.org/log/231e6d68ccc2b858f8b002b45596356d8beb856c
I'm sure this has something to do with GC scheduling.
I haven't had any luck reproducing this locally. I'm now trying on the gomotes.
gopool create -setup 'gomote push $VM && gomote run $VM go/src/make.bash && gomote run $VM go/bin/go test -c -o ../src/runtime/runtime.test runtime' linux-amd64-sid
stress2 -p 5 gopool run 'gomote run $VM go/src/runtime/runtime.test -test.short -test.cpu=1,2,4'