diff --git a/src/runtime/panic.go b/src/runtime/panic.go index 81ff21113f..bb83be4715 100644 --- a/src/runtime/panic.go +++ b/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 -// ignores writes once dying > 0. +// It must not have write barriers even though the write barrier +// 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 { diff --git a/src/runtime/print.go b/src/runtime/print.go index 7b2e4f40ff..e605eb34cb 100644 --- a/src/runtime/print.go +++ b/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 }