cmd/compile, runtime: add go:yeswritebarrierrec pragma

This pragma cancels the effect of go:nowritebarrierrec. This is useful
in the scheduler because there are places where we enter a function
without a valid P (and hence cannot have write barriers), but then
obtain a P. This allows us to annotate the function with
go:nowritebarrierrec and split out the part after we've obtained a P
into a go:yeswritebarrierrec function.

Change-Id: Ic8ce4b6d3c074a1ecd8280ad90eaf39f0ffbcc2a
Reviewed-on: https://go-review.googlesource.com/30938
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Keith Randall <khr@golang.org>

src/cmd/compile/internal/gc/dcl.go

@@ -1317,7 +1317,7 @@ func checknowritebarrierrec() {
 	visitBottomUp(xtop, func(list []*Node, recursive bool) {
 		// Functions with write barriers have depth 0.
 		for _, n := range list {
-			if n.Func.WBLineno != 0 {
+			if n.Func.WBLineno != 0 && n.Func.Pragma&Yeswritebarrierrec == 0 {
 				c.best[n] = nowritebarrierrecCall{target: nil, depth: 0, lineno: n.Func.WBLineno}
 			}
 		}
@@ -1329,6 +1329,12 @@ func checknowritebarrierrec() {
 		for _ = range list {
 			c.stable = false
 			for _, n := range list {
+				if n.Func.Pragma&Yeswritebarrierrec != 0 {
+					// Don't propagate write
+					// barrier up to a
+					// yeswritebarrierrec function.
+					continue
+				}
 				if n.Func.WBLineno == 0 {
 					c.curfn = n
 					c.visitcodelist(n.Nbody)
@@ -1393,9 +1399,6 @@ func (c *nowritebarrierrecChecker) visitcall(n *Node) {
 	if fn == nil || fn.Op != ONAME || fn.Class != PFUNC || fn.Name.Defn == nil {
 		return
 	}
-	if (compiling_runtime || fn.Sym.Pkg == Runtimepkg) && fn.Sym.Name == "allocm" {
-		return
-	}
 	defn := fn.Name.Defn
 
 	fnbest, ok := c.best[defn]

src/cmd/compile/internal/gc/lex.go

@@ -69,11 +69,16 @@ const (
 	Norace                // func must not have race detector annotations
 	Nosplit               // func should not execute on separate stack
 	Noinline              // func should not be inlined
+	CgoUnsafeArgs         // treat a pointer to one arg as a pointer to them all
+	UintptrEscapes        // pointers converted to uintptr escape
+
+	// Runtime-only pragmas.
+	// See ../../../../runtime/README.md for detailed descriptions.
+
 	Systemstack        // func must run on system stack
 	Nowritebarrier     // emit compiler error instead of write barrier
 	Nowritebarrierrec  // error on write barrier in this or recursive callees
-	CgoUnsafeArgs            // treat a pointer to one arg as a pointer to them all
-	UintptrEscapes           // pointers converted to uintptr escape
+	Yeswritebarrierrec // cancels Nowritebarrierrec in this function and callees
 )
 
 func pragmaValue(verb string) Pragma {
@@ -105,6 +110,11 @@ func pragmaValue(verb string) Pragma {
 			yyerror("//go:nowritebarrierrec only allowed in runtime")
 		}
 		return Nowritebarrierrec | Nowritebarrier // implies Nowritebarrier
+	case "go:yeswritebarrierrec":
+		if !compiling_runtime {
+			yyerror("//go:yeswritebarrierrec only allowed in runtime")
+		}
+		return Yeswritebarrierrec
 	case "go:cgo_unsafe_args":
 		return CgoUnsafeArgs
 	case "go:uintptrescapes":

src/cmd/compile/internal/gc/ssa.go

@@ -3312,7 +3312,7 @@ func (s *state) insertWBmove(t *Type, left, right *ssa.Value, line int32, rightI
 	// }
 
 	if s.noWB {
-		s.Fatalf("write barrier prohibited")
+		s.Error("write barrier prohibited")
 	}
 	if s.WBLineno == 0 {
 		s.WBLineno = left.Line
@@ -3377,7 +3377,7 @@ func (s *state) insertWBstore(t *Type, left, right *ssa.Value, line int32, skip
 	// }
 
 	if s.noWB {
-		s.Fatalf("write barrier prohibited")
+		s.Error("write barrier prohibited")
 	}
 	if s.WBLineno == 0 {
 		s.WBLineno = left.Line

src/runtime/HACKING.md

@@ -0,0 +1,49 @@
+This is a very incomplete and probably out-of-date guide to
+programming in the Go runtime and how it differs from writing normal
+Go.
+
+Runtime-only compiler directives
+================================
+
+In addition to the "//go:" directives documented in "go doc compile",
+the compiler supports additional directives only in the runtime.
+
+go:systemstack
+--------------
+
+`go:systemstack` indicates that a function must run on the system
+stack. This is checked dynamically by a special function prologue.
+
+go:nowritebarrier
+-----------------
+
+`go:nowritebarrier` directs the compiler to emit an error if the
+following function contains any write barriers. (It *does not*
+suppress the generation of write barriers; it is simply an assertion.)
+
+Usually you want `go:nowritebarrierrec`. `go:nowritebarrier` is
+primarily useful in situations where it's "nice" not to have write
+barriers, but not required for correctness.
+
+go:nowritebarrierrec and go:yeswritebarrierrec
+----------------------------------------------
+
+`go:nowritebarrierrec` directs the compiler to emit an error if the
+following function or any function it calls recursively, up to a
+`go:yeswritebarrierrec`, contains a write barrier.
+
+Logically, the compiler floods the call graph starting from each
+`go:nowritebarrierrec` function and produces an error if it encounters
+a function containing a write barrier. This flood stops at
+`go:yeswritebarrierrec` functions.
+
+`go:nowritebarrierrec` is used in the implementation of the write
+barrier to prevent infinite loops.
+
+Both directives are used in the scheduler. The write barrier requires
+an active P (`getg().m.p != nil`) and scheduler code often runs
+without an active P. In this case, `go:nowritebarrierrec` is used on
+functions that release the P or may run without a P and
+`go:yeswritebarrierrec` is used when code re-acquires an active P.
+Since these are function-level annotations, code that releases or
+acquires a P may need to be split across two functions.

src/runtime/proc.go

@@ -1275,8 +1275,10 @@ type cgothreadstart struct {
 // Can use p for allocation context if needed.
 // fn is recorded as the new m's m.mstartfn.
 //
-// This function it known to the compiler to inhibit the
-// go:nowritebarrierrec annotation because it uses P for allocation.
+// This function is allowed to have write barriers even if the caller
+// isn't because it borrows _p_.
+//
+//go:yeswritebarrierrec
 func allocm(_p_ *p, fn func()) *m {
 	_g_ := getg()
 	_g_.m.locks++ // disable GC because it can be called from sysmon
@@ -2459,7 +2461,11 @@ func entersyscallblock_handoff() {
 // Arrange for it to run on a cpu again.
 // This is called only from the go syscall library, not
 // from the low-level system calls used by the runtime.
+//
+// Write barriers are not allowed because our P may have been stolen.
+//
 //go:nosplit
+//go:nowritebarrierrec
 func exitsyscall(dummy int32) {
 	_g_ := getg()
 
@@ -2552,22 +2558,7 @@ func exitsyscallfast() bool {
 	// Try to re-acquire the last P.
 	if _g_.m.p != 0 && _g_.m.p.ptr().status == _Psyscall && atomic.Cas(&_g_.m.p.ptr().status, _Psyscall, _Prunning) {
 		// There's a cpu for us, so we can run.
-		_g_.m.mcache = _g_.m.p.ptr().mcache
-		_g_.m.p.ptr().m.set(_g_.m)
-		if _g_.m.syscalltick != _g_.m.p.ptr().syscalltick {
-			if trace.enabled {
-				// The p was retaken and then enter into syscall again (since _g_.m.syscalltick has changed).
-				// traceGoSysBlock for this syscall was already emitted,
-				// but here we effectively retake the p from the new syscall running on the same p.
-				systemstack(func() {
-					// Denote blocking of the new syscall.
-					traceGoSysBlock(_g_.m.p.ptr())
-					// Denote completion of the current syscall.
-					traceGoSysExit(0)
-				})
-			}
-			_g_.m.p.ptr().syscalltick++
-		}
+		exitsyscallfast_reacquired()
 		return true
 	}
 
@@ -2597,6 +2588,35 @@ func exitsyscallfast() bool {
 	return false
 }
 
+// exitsyscallfast_reacquired is the exitsyscall path on which this G
+// has successfully reacquired the P it was running on before the
+// syscall.
+//
+// This function is allowed to have write barriers because exitsyscall
+// has acquired a P at this point.
+//
+//go:yeswritebarrierrec
+//go:nosplit
+func exitsyscallfast_reacquired() {
+	_g_ := getg()
+	_g_.m.mcache = _g_.m.p.ptr().mcache
+	_g_.m.p.ptr().m.set(_g_.m)
+	if _g_.m.syscalltick != _g_.m.p.ptr().syscalltick {
+		if trace.enabled {
+			// The p was retaken and then enter into syscall again (since _g_.m.syscalltick has changed).
+			// traceGoSysBlock for this syscall was already emitted,
+			// but here we effectively retake the p from the new syscall running on the same p.
+			systemstack(func() {
+				// Denote blocking of the new syscall.
+				traceGoSysBlock(_g_.m.p.ptr())
+				// Denote completion of the current syscall.
+				traceGoSysExit(0)
+			})
+		}
+		_g_.m.p.ptr().syscalltick++
+	}
+}
+
 func exitsyscallfast_pidle() bool {
 	lock(&sched.lock)
 	_p_ := pidleget()

test/nowritebarrier.go

@@ -0,0 +1,78 @@
+// errorcheck -+
+
+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Test go:nowritebarrier and related directives.
+
+package p
+
+type t struct {
+	f *t
+}
+
+var x t
+var y *t
+
+//go:nowritebarrier
+func a1() {
+	x.f = y // ERROR "write barrier prohibited"
+	a2()    // no error
+}
+
+//go:noinline
+func a2() {
+	x.f = y
+}
+
+//go:nowritebarrierrec
+func b1() {
+	b2()
+}
+
+//go:noinline
+func b2() {
+	x.f = y // ERROR "write barrier prohibited by caller"
+}
+
+// Test recursive cycles through nowritebarrierrec and yeswritebarrierrec.
+
+//go:nowritebarrierrec
+func c1() {
+	c2()
+}
+
+//go:yeswritebarrierrec
+func c2() {
+	c3()
+}
+
+func c3() {
+	x.f = y
+	c4()
+}
+
+//go:nowritebarrierrec
+func c4() {
+	c2()
+}
+
+//go:nowritebarrierrec
+func d1() {
+	d2()
+}
+
+func d2() {
+	d3()
+}
+
+func d3() {
+	x.f = y // ERROR "write barrier prohibited by caller"
+	d4()
+}
+
+//go:yeswritebarrierrec
+func d4() {
+	d2()
+}