runtime: avoid write barrier in startpanic_m

startpanic_m could be called correctly in a context where there's a
valid G, a valid M, but no P, for example in a signal handler which
panics. Currently, startpanic_m has write barriers enabled because
write barriers are permitted if a G's M is dying. However, all the
current write barrier implementations assume the current G has a P.

Therefore, in this change we disable write barriers in startpanic_m,
remove the only pointer write which clears g.writebuf, and fix up gwrite
to ignore the writebuf if the current G's M is dying, rather than
relying on it being nil in the dying case.

Fixes #26575.

Change-Id: I9b29e6b9edf00d8e99ffc71770c287142ebae086
Reviewed-on: https://go-review.googlesource.com/c/154837
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

src/runtime/panic.go

@@ -729,10 +729,13 @@ func fatalpanic(msgs *_panic) {
 // It returns true if panic messages should be printed, or false if
 // the runtime is in bad shape and should just print stacks.
 //
-// It can have write barriers because the write barrier explicitly
+// It must not have write barriers even though the write barrier
-// ignores writes once dying > 0.
+// explicitly ignores writes once dying > 0. Write barriers still
+// assume that g.m.p != nil, and this function may not have P
+// in some contexts (e.g. a panic in a signal handler for a signal
+// sent to an M with no P).
 //
-//go:yeswritebarrierrec
+//go:nowritebarrierrec
 func startpanic_m() bool {
 	_g_ := getg()
 	if mheap_.cachealloc.size == 0 { // very early
@@ -752,8 +755,8 @@ func startpanic_m() bool {
 
 	switch _g_.m.dying {
 	case 0:
+		// Setting dying >0 has the side-effect of disabling this G's writebuf.
 		_g_.m.dying = 1
-		_g_.writebuf = nil
 		atomic.Xadd(&panicking, 1)
 		lock(&paniclk)
 		if debug.schedtrace > 0 || debug.scheddetail > 0 {

src/runtime/print.go

@@ -89,7 +89,12 @@ func gwrite(b []byte) {
 	}
 	recordForPanic(b)
 	gp := getg()
-	if gp == nil || gp.writebuf == nil {
+	// Don't use the writebuf if gp.m is dying. We want anything
+	// written through gwrite to appear in the terminal rather
+	// than be written to in some buffer, if we're in a panicking state.
+	// Note that we can't just clear writebuf in the gp.m.dying case
+	// because a panic isn't allowed to have any write barriers.
+	if gp == nil || gp.writebuf == nil || gp.m.dying > 0 {
 		writeErr(b)
 		return
 	}