runtime: "fatal error: schedule: holding locks" on various builders after CL 200577

[bcmills]

There are multiple schedule: holding locks failures in the builders as of CL 200577.

linux-arm64-packet is failing consistently (https://build.golang.org/log/40361c2d5d7a8cd5df1a49db496c265e4ed07c32):

fatal error: schedule: holding locks
fatal error: runtime·lock: lock count
fatal error: runtime·unlock: lock count
fatal error: runtime·unlock: lock count
[…]

freebsd-386-11_2 shows a flake with the same message on what seems to be a different codepath (https://build.golang.org/log/52cd107e4b520e625c9dc0030c172cb91be0e2a9):

go test proxy running at GOPROXY=http://127.0.0.1:52392/mod
go proxy: no archive rsc.io v1.5.2: file does not exist
go proxy: no archive rsc.io v1.0.0: file does not exist
go proxy: no archive rsc.io v1.1.0: file does not exist
go proxy: no archive rsc.io v1.0.0: file does not exist
go proxy: no archive rsc.io v1.0.0: file does not exist
go proxy: no archive rsc.io v1.0.0: file does not exist
go proxy: no archive rsc.io v1.5.1: file does not exist
fatal error: schedule: holding locks

runtime stack:
runtime.throw(0x862e93d, 0x17)
	/tmp/workdir/go/src/runtime/panic.go:1106 +0x64
runtime.schedule()
	/tmp/workdir/go/src/runtime/proc.go:2478 +0x475
runtime.goyield_m(0x39b29ea0)
	/tmp/workdir/go/src/runtime/proc.go:2772 +0x70
runtime.mcall(0x80a05f8)
	/tmp/workdir/go/src/runtime/asm_386.s:344 +0x43

CC @ianlancetaylor @CAFxX @dvyukov @cherrymui

[bcmills]

solaris-amd64-oraclerel is also failing consistently (https://build.golang.org/log/2c7a708c1218710a40539b526e8d6984b90edb23):

fatal error: schedule: holding locks

runtime stack:
runtime.throw(0xa2c81e, 0x17)
	/tmp/workdir-host-solaris-oracle-amd64-oraclerel/go/src/runtime/panic.go:1106 +0x72
runtime.schedule()
	/tmp/workdir-host-solaris-oracle-amd64-oraclerel/go/src/runtime/proc.go:2478 +0x519
runtime.goyield_m(0xc0002ce480)
	/tmp/workdir-host-solaris-oracle-amd64-oraclerel/go/src/runtime/proc.go:2772 +0x9a
runtime.mcall(0x200000)
	/tmp/workdir-host-solaris-oracle-amd64-oraclerel/go/src/runtime/asm_amd64.s:318 +0x64

[gopherbot]

Change https://golang.org/cl/205817 mentions this issue: Revert "sync: yield to the waiter when unlocking a starving mutex"