runtime: drop chan circular linked list in favor of circular buffer

The list elements are already being allocated out of a
single memory buffer.  We can drop the Link* pointer
following and the memory it requires, replacing it with
index operations.

The change also keeps a channel from containing a pointer
back into its own allocation block, which would create a
cycle.  Blocks involved in cycles are not guaranteed to be
finalized properly, and channels depend on finalizers to
free OS-level locks on some systems.  The self-reference
was keeping channels from being garbage collected.

runtime-gdb.py will need to be updated in order to dump
the content of buffered channels with the new data structure.

Fixes #1676.

R=ken2, r
CC=golang-dev
https://golang.org/cl/4411045

1 files changed, 1 insertions, 12 deletions

diff --git a/src/cmd/ld/dwarf.c b/src/cmd/ld/dwarf.c
index d0b6407796..fa55fcbb4a 100644
--- a/src/cmd/ld/dwarf.c
+++ b/src/cmd/ld/dwarf.c
@@ -1377,7 +1377,7 @@ static void
 synthesizechantypes(DWDie *die)
 {
 	DWDie *sudog, *waitq, *link, *hchan,
-		*dws, *dww, *dwl, *dwh, *elemtype;
+		*dws, *dww, *dwh, *elemtype;
 	DWAttr *a;
 	int elemsize, linksize, sudogsize;
 
@@ -1416,21 +1416,10 @@ synthesizechantypes(DWDie *die)
 		newattr(dww, DW_AT_byte_size, DW_CLS_CONSTANT,
 			getattr(waitq, DW_AT_byte_size)->value, NULL);
 
-		// link<T>
-		dwl = newdie(&dwtypes, DW_ABRV_STRUCTTYPE,
-			mkinternaltypename("link", getattr(elemtype, DW_AT_name)->data, NULL));
-		copychildren(dwl, link);
-		substitutetype(dwl, "link", defptrto(dwl));
-		substitutetype(dwl, "elem", elemtype);
-		newattr(dwl, DW_AT_byte_size, DW_CLS_CONSTANT,
-			linksize + (elemsize > 8 ? elemsize - 8 : 0), NULL);
-
 		// hchan<T>
 		dwh = newdie(&dwtypes, DW_ABRV_STRUCTTYPE,
 			mkinternaltypename("hchan", getattr(elemtype, DW_AT_name)->data, NULL));
 		copychildren(dwh, hchan);
-		substitutetype(dwh, "senddataq", defptrto(dwl));
-		substitutetype(dwh, "recvdataq", defptrto(dwl));
 		substitutetype(dwh, "recvq", dww);
 		substitutetype(dwh, "sendq", dww);
 		substitutetype(dwh, "free", dws);