runtime: in traceback, only jump stack if M doesn't change

CL 424257 modified gentraceback to switch gp when jumping from a
system stack to a user stack to simplify reasoning through the rest of
the function. This has the unintended side-effect of also switching
all references to gp.m. The vast majority of the time, g0.m and curg.m
are the same across a stack switch, making this a no-op, but there's
at least one case where this isn't true: if a profiling signal happens
in execute between setting mp.curg and setting gp.m. In this case,
mp.curg.m is briefly nil, which can cause gentraceback to crash with a
nil pointer dereference. We see this failure (surprisingly
frequently!) in profiling tests in the morestack=mayMoreStackPreempt
testing mode (#48297).

Fix this by making only jumping stacks if doing so will not switch Ms.
This restores the original property that gp.m doesn't change across
the stack jump, and makes gentraceback a little more conservative
about jumping stacks.

Fixes #54885.

Change-Id: Ib1524c41c748eeff35896e0f3abf9a7efbe5969f
Reviewed-on: https://go-review.googlesource.com/c/go/+/428656
Reviewed-by: Michael Pratt <[email protected]>
Run-TryBot: Austin Clements <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Auto-Submit: Austin Clements <[email protected]>

Author: aclements

src/runtime/traceback.go

@@ -159,7 +159,10 @@ func gentraceback(pc0, sp0, lr0 uintptr, gp *g, skip int, pcbuf *uintptr, max in
 		if frame.fp == 0 {
 			// Jump over system stack transitions. If we're on g0 and there's a user
 			// goroutine, try to jump. Otherwise this is a regular call.
-			if flags&_TraceJumpStack != 0 && gp == gp.m.g0 && gp.m.curg != nil {
+			// We also defensively check that this won't switch M's on us,
+			// which could happen at critical points in the scheduler.
+			// This ensures gp.m doesn't change from a stack jump.
+			if flags&_TraceJumpStack != 0 && gp == gp.m.g0 && gp.m.curg != nil && gp.m.curg.m == gp.m {
 				switch f.funcID {
 				case funcID_morestack:
 					// morestack does not return normally -- newstack()