runtime: use type-based write barrier for remote stack write during chansend
A send on an unbuffered channel to a blocked receiver is the only
case in the runtime where one goroutine writes directly to the stack
of another. The garbage collector assumes that if a goroutine is
blocked, its stack contains no new pointers since the last time it ran.
The send on an unbuffered channel violates this, so it needs an
explicit write barrier. It has an explicit write barrier, but not one that
can handle a write to another stack. Use one that can (based on type bitmap
instead of heap bitmap).
To make this work, raise the limit for type bitmaps so that they are
used for all types up to 64 kB in size (256 bytes of bitmap).
(The runtime already imposes a limit of 64 kB for a channel element size.)
I have been unable to reproduce this problem in a simple test program.
Could help #11035.
Change-Id: I06ad994032d8cff3438c9b3eaa8d853915128af5
Reviewed-on: https://go-review.googlesource.com/10815
Reviewed-by: Austin Clements <austin@google.com>
diff --git a/src/cmd/compile/internal/gc/reflect.go b/src/cmd/compile/internal/gc/reflect.go
index 8ef2380..08343e8 100644
--- a/src/cmd/compile/internal/gc/reflect.go
+++ b/src/cmd/compile/internal/gc/reflect.go
@@ -1403,10 +1403,17 @@
// is 32 pointers, the bits for which fit in 4 bytes. So maxPtrmaskBytes
// must be >= 4.
//
-// We use 16 because the GC programs do have some constant overhead
+// We used to use 16 because the GC programs do have some constant overhead
// to get started, and processing 128 pointers seems to be enough to
// amortize that overhead well.
-const maxPtrmaskBytes = 16
+//
+// To make sure that the runtime's chansend can call typeBitsBulkBarrier,
+// we raised the limit to 2048, so that even 32-bit systems are guaranteed to
+// use bitmaps for objects up to 64 kB in size.
+//
+// Also known to reflect/type.go.
+//
+const maxPtrmaskBytes = 2048
// dgcsym emits and returns a data symbol containing GC information for type t,
// along with a boolean reporting whether the UseGCProg bit should be set in
