runtime: frequent "fatal error: checkdead: runnable g" on plan9 builders

[bcmills]

#!watchflakes
post <- builder ~ `^plan9` && `fatal error: checkdead: runnable g`

runtime: checkdead: find g 68 in status 1
fatal error: checkdead: runnable g

runtime stack:
runtime.throw(0x4e0c1e, 0x15)
	/tmp/workdir-gnot/go/src/runtime/panic.go:1116 +0x63
runtime.checkdead()
	/tmp/workdir-gnot/go/src/runtime/proc.go:4489 +0x381
runtime.mput(...)
	/tmp/workdir-gnot/go/src/runtime/proc.go:4906
runtime.stopm()
	/tmp/workdir-gnot/go/src/runtime/proc.go:1879 +0x85
runtime.exitsyscall0(0x10c01880)
	/tmp/workdir-gnot/go/src/runtime/proc.go:3347 +0xf0
runtime.mcall(0x0)
	/tmp/workdir-gnot/go/src/runtime/asm_386.s:340 +0x46

CC @0intro

2020-04-16T16:44:24-71a6718/plan9-386-0intro
2020-04-13T20:04:56-7b5303d/plan9-386-0intro
2020-04-08T22:19:53-b6feb03/plan9-386-0intro
2020-04-08T20:57:26-a036185/plan9-386-0intro
2020-04-08T18:35:43-da8591b/plan9-386-0intro
2020-03-28T19:37:39-45f99d8/plan9-386-0intro
2020-03-27T17:14:16-0cc1290/plan9-386-0intro
2020-03-27T15:50:43-d22086e/plan9-386-0intro
2020-03-24T11:03:07-20b46c7/plan9-386-0intro
2020-03-18T17:28:51-42dfac6/plan9-arm
2020-02-04T15:32:32-b8d339b/plan9-amd64-9front
2020-02-04T15:00:46-0fe79aa/plan9-386-0intro
2020-01-24T23:27:33-64c22b7/plan9-amd64-9front
2020-01-24T23:27:22-ad3cef1/plan9-amd64-9front
2019-11-13T20:53:39-49e05d4/plan9-amd64-9front
2019-11-08T00:07:43-33dfd35/plan9-amd64-9front
2019-10-15T16:45:31-11d7775/plan9-386-0intro
2019-07-15T20:55:12-20e4540/plan9-386-0intro
2019-07-03T23:08:27-adcb2b1/plan9-386-0intro

[bcmills]

CC @millerresearch

[gopherbot]

Timed out in state WaitingForInfo. Closing.

(I am just a bot, though. Please speak up if this is a mistake or you have the requested information.)

[gopherbot]

Found new dashboard test flakes for:

#!watchflakes
post <- builder ~ `^plan9` && `fatal error: checkdead: runnable g`

2023-09-11 20:48 plan9-arm go@06138963 (log)

runtime: checkdead: find g 10 in status 1
fatal error: checkdead: runnable g

runtime stack:
runtime.throw({0x6748ee, 0x15})
	runtime/panic.go:1018 +0x4c fp=0x3ffffbe0 sp=0x3ffffbcc pc=0x41358
runtime.checkdead.func1(0x9402f00)
	runtime/proc.go:5477 +0x12c fp=0x3ffffc00 sp=0x3ffffbe0 pc=0x50908
runtime.forEachG(0x3ffffc58)
	runtime/proc.go:608 +0x5c fp=0x3ffffc14 sp=0x3ffffc00 pc=0x45210
runtime.checkdead()
	runtime/proc.go:5463 +0x11c fp=0x3ffffc60 sp=0x3ffffc14 pc=0x50430
runtime.mput(0x97fae00)
	runtime/proc.go:5960 +0x44 fp=0x3ffffc64 sp=0x3ffffc60 pc=0x52234
runtime.stopm()
	runtime/proc.go:2533 +0x60 fp=0x3ffffc78 sp=0x3ffffc64 pc=0x48bc8
runtime.findRunnable()
	runtime/proc.go:3248 +0xdac fp=0x3ffffd28 sp=0x3ffffc78 pc=0x4a818
runtime.schedule()
	runtime/proc.go:3602 +0xac fp=0x3ffffd44 sp=0x3ffffd28 pc=0x4baa4
runtime.park_m(0x9582e00)
	runtime/proc.go:3765 +0x160 fp=0x3ffffd60 sp=0x3ffffd44 pc=0x4c330
traceback: unexpected SPWRITE function runtime.mcall
runtime.mcall(0x1)
	runtime/asm_arm.s:265 +0x48 fp=0x3ffffd68 sp=0x3ffffd60 pc=0x756c4

— watchflakes