runtime: switch gp when jumping stacks during traceback

Currently, when traceback jumps from the system stack to a user stack
(e.g., during profiling tracebacks), it leaves gp pointing at the g0.
This is currently harmless since it's only used during profiling, so
the code paths in gentraceback that care about gp aren't used, but
it's really confusing and would certainly break if _TraceJumpStack
were ever used in a context other than profiling.

Fix this by updating gp to point to the user g when we switch stacks.

For #54466.

Change-Id: I1541e004667a52e37671803ce45c91d8c5308830
Reviewed-on: https://go-review.googlesource.com/c/go/+/424257
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: Austin Clements <austin@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Austin Clements <austin@google.com>

src/runtime/traceback.go

@@ -167,20 +167,22 @@ func gentraceback(pc0, sp0, lr0 uintptr, gp *g, skip int, pcbuf *uintptr, max in
 					// This keeps morestack() from showing up in the backtrace,
 					// but that makes some sense since it'll never be returned
 					// to.
-					frame.pc = gp.m.curg.sched.pc
+					gp = gp.m.curg
+					frame.pc = gp.sched.pc
 					frame.fn = findfunc(frame.pc)
 					f = frame.fn
 					flag = f.flag
-					frame.lr = gp.m.curg.sched.lr
-					frame.sp = gp.m.curg.sched.sp
-					stack = gp.m.curg.stack
-					cgoCtxt = gp.m.curg.cgoCtxt
+					frame.lr = gp.sched.lr
+					frame.sp = gp.sched.sp
+					stack = gp.stack
+					cgoCtxt = gp.cgoCtxt
 				case funcID_systemstack:
 					// systemstack returns normally, so just follow the
 					// stack transition.
-					frame.sp = gp.m.curg.sched.sp
-					stack = gp.m.curg.stack
-					cgoCtxt = gp.m.curg.cgoCtxt
+					gp = gp.m.curg
+					frame.sp = gp.sched.sp
+					stack = gp.stack
+					cgoCtxt = gp.cgoCtxt
 					flag &^= funcFlag_SPWRITE
 				}
 			}