cmd/internal/obj/x86: mark 2-instruction TLS access nonpreemptible

The 2-instruction TLS access sequence
	MOVQ TLS, BX
	MOVQ 0(BX)(TLS*1), BX
is not async preemptible, as if it is preempted and resumed on a
different thread, the TLS address may become invalid.

May fix #35349. (This is a rare failure and I haven't been able
to reproduce it.)

Change-Id: Ie1a366fd0d7d73627dc62ee2de01c0aa09365f2b
Reviewed-on: https://go-review.googlesource.com/c/go/+/206903
Run-TryBot: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Austin Clements <austin@google.com>

src/cmd/internal/obj/x86/asm6.go

@@ -1984,6 +1984,22 @@ func span6(ctxt *obj.Link, s *obj.LSym, newprog obj.ProgAlloc) {
 			fmt.Printf(" rel %#.4x/%d %s%+d\n", uint32(r.Off), r.Siz, r.Sym.Name, r.Add)
 		}
 	}
+
+	// Mark nonpreemptible instruction sequences.
+	// The 2-instruction TLS access sequence
+	//	MOVQ TLS, BX
+	//	MOVQ 0(BX)(TLS*1), BX
+	// is not async preemptible, as if it is preempted and resumed on
+	// a different thread, the TLS address may become invalid.
+	if !CanUse1InsnTLS(ctxt) {
+		useTLS := func(p *obj.Prog) bool {
+			// Only need to mark the second instruction, which has
+			// REG_TLS as Index. (It is okay to interrupt and restart
+			// the first instruction.)
+			return p.From.Index == REG_TLS
+		}
+		obj.MarkUnsafePoints(ctxt, s.Func.Text, newprog, useTLS)
+	}
 }
 
 func instinit(ctxt *obj.Link) {