runtime: "fatal error: attempt to execute system stack code on user stack" in startTemplateThread

[prattmic]

From manual runs of https://go-review.googlesource.com/c/go/+/232978/2/src/runtime/testdata/testprog/lockosthread.go, without the core runtime patch from the CL applied.

fatal error: attempt to execute system stack code on user stack                   
                                         
goroutine 8 [running]:
runtime.throw(0x50fbd9, 0x32)
        /usr/local/google/home/mpratt/src/go/src/runtime/panic.go:1116 +0x72 fp=0xc00004b6c0 sp=0xc00004b690 pc=0x4337f2                                            
runtime.morestackc()
        /usr/local/google/home/mpratt/src/go/src/runtime/stack.go:1320 +0x27 fp=0xc00004b6e0 sp=0xc00004b6c0 pc=0x462ba7                                            
runtime.stackfree(0xc000056000, 0xc000058000)                                     
        /usr/local/google/home/mpratt/src/go/src/runtime/stack.go:427 +0x36b fp=0xc00004b6e8 sp=0xc00004b6e0 pc=0x44a80b                                            
runtime.allocm(0x0, 0x511228, 0x45ff40)
        /usr/local/google/home/mpratt/src/go/src/runtime/proc.go:1440 +0xd7 fp=0xc00004b728 sp=0xc00004b6e8 pc=0x439397                                             
runtime.newm(0x511228, 0x0)
        /usr/local/google/home/mpratt/src/go/src/runtime/proc.go:1761 +0x39 fp=0xc00004b758 sp=0xc00004b728 pc=0x439b99                                             
runtime.startTemplateThread(...)         
        /usr/local/google/home/mpratt/src/go/src/runtime/proc.go:1830
runtime.LockOSThread()
        /usr/local/google/home/mpratt/src/go/src/runtime/proc.go:3773 +0x98 fp=0xc00004b778 sp=0xc00004b758 pc=0x43ee58
main.LockOSThreadTemplateThreadRace.func2(0xbfa7c99b86868dd1, 0x9c1163, 0x5ccae0, 0xc0000140d0)
        /usr/local/google/home/mpratt/src/go/src/runtime/testdata/testprog/lockosthread.go:233 +0x6d fp=0xc00004b7c0 sp=0xc00004b778 pc=0x4d786d
runtime.goexit()
        /usr/local/google/home/mpratt/src/go/src/runtime/asm_amd64.s:1374 +0x1 fp=0xc00004b7c8 sp=0xc00004b7c0 pc=0x465f81
created by main.LockOSThreadTemplateThreadRace
        /usr/local/google/home/mpratt/src/go/src/runtime/testdata/testprog/lockosthread.go:225 +0xdf

It seems that newm simply must be called on the system stack, which is not done consistently (stoplockedm -> handoffp -> startm is another case).

cc @mknyszek @aclements

[ALTree]

Also reported at #30591.

[prattmic]

Thanks, I missed that.

Duplicate of #30591.