From bf0b605dfbc36b3555eef3315b15facca1c08bdb Mon Sep 17 00:00:00 2001 From: Alan Donovan Date: Thu, 9 May 2024 18:16:59 -0400 Subject: [PATCH] runtime: make convTstring write barrier unreachable from throw CL 581215 changed 'throw' so that instead of print(s) it called a more complicated function, printpanicval, that statically appeared to have convTstring in its call graph, even though this isn't dynamically reachable when called with a string argument. However, this caused the link-time static callgraph test to point out that throw (which is called in nowritebarrierrec contexts such as markgc) reaches a write barrier. The solution is to inline and specialize the printpanicval function for strings; it reduces to printindented. Thanks to mpratt for pointing out that the reachability check is on the fully lowered code, and is thus sensitive to optimizations such as inlining. I added an explanatory comment on the line that generates the error message to help future users confused as I was. Fixes golang/go#67274 Change-Id: Ief110d554de365ce4c09509dceee000cbee30ad9 Reviewed-on: https://go-review.googlesource.com/c/go/+/584617 Reviewed-by: Than McIntosh LUCI-TryBot-Result: Go LUCI Reviewed-by: Michael Pratt --- src/cmd/compile/internal/ssagen/nowb.go | 8 ++++++++ src/runtime/panic.go | 5 ++--- 2 files changed, 10 insertions(+), 3 deletions(-) diff --git a/src/cmd/compile/internal/ssagen/nowb.go b/src/cmd/compile/internal/ssagen/nowb.go index b8756eea61..8e776695e3 100644 --- a/src/cmd/compile/internal/ssagen/nowb.go +++ b/src/cmd/compile/internal/ssagen/nowb.go @@ -174,6 +174,14 @@ func (c *nowritebarrierrecChecker) check() { fmt.Fprintf(&err, "\n\t%v: called by %v", base.FmtPos(call.lineno), call.target.Nname) call = funcs[call.target] } + // Seeing this error in a failed CI run? It indicates that + // a function in the runtime package marked nowritebarrierrec + // (the outermost stack element) was found, by a static + // reachability analysis over the fully lowered optimized code, + // to call a function (fn) that involves a write barrier. + // + // Even if the call path is infeasable, + // you will need to reorganize the code to avoid it. base.ErrorfAt(fn.WBPos, 0, "write barrier prohibited by caller; %v%s", fn.Nname, err.String()) continue } diff --git a/src/runtime/panic.go b/src/runtime/panic.go index 122fc30df2..ff9c64113f 100644 --- a/src/runtime/panic.go +++ b/src/runtime/panic.go @@ -1014,13 +1014,12 @@ func sync_fatal(s string) { // issue #67274, so as to fix longtest builders. // //go:nosplit -//go:noinline func throw(s string) { // Everything throw does should be recursively nosplit so it // can be called even when it's unsafe to grow the stack. systemstack(func() { print("fatal error: ") - printpanicval(s) + printindented(s) // logically printpanicval(s), but avoids convTstring write barrier print("\n") }) @@ -1041,7 +1040,7 @@ func fatal(s string) { // can be called even when it's unsafe to grow the stack. systemstack(func() { print("fatal error: ") - printpanicval(s) + printindented(s) // logically printpanicval(s), but avoids convTstring write barrier print("\n") })