From 71a76476627fb0752d1b816406da4e5a6511c2c9 Mon Sep 17 00:00:00 2001
From: Austin Clements <[email protected]>
Date: Thu, 15 Oct 2015 13:25:19 -0400
Subject: [PATCH 27/63] [release-branch.go1.5] runtime: fix recursive GC assist
 better

Commit c257dfb attempted to fix recursive allocation in gcAssistAlloc;
however, it only reduced it: setting gp.gcalloc to 0 isn't sufficient
to disable assists at the beginning of the GC cycle when gp.gcscanwork
is also small or zero.

Fix this recursion more completely by setting gcalloc to a sentinel
value that directly disables assists.

Fixes #12894 (again).

Change-Id: I9599566222d8f540d0b39806846bfc702e6666e5
Reviewed-on: https://go-review.googlesource.com/15891
Run-TryBot: Austin Clements <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Russ Cox <[email protected]>
---
 src/runtime/mgcmark.go | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/src/runtime/mgcmark.go b/src/runtime/mgcmark.go
index e5dfa72..64cc1af 100644
--- a/src/runtime/mgcmark.go
+++ b/src/runtime/mgcmark.go
@@ -152,6 +152,11 @@ func gcAssistAlloc(size uintptr, allowAssist bool) {
 	}
 
 	// Record allocation.
+	if gp.gcalloc+size < gp.gcalloc {
+		// gcalloc would overflow, or it's set to a sentinel
+		// value to prevent recursive assist.
+		return
+	}
 	gp.gcalloc += size
 
 	if !allowAssist {
@@ -295,7 +300,7 @@ retry:
 
 		// timeSleep may allocate, so avoid recursive assist.
 		gcalloc := gp.gcalloc
-		gp.gcalloc = 0
+		gp.gcalloc = ^uintptr(0)
 		timeSleep(100 * 1000)
 		gp.gcalloc = gcalloc
 		goto retry
-- 
2.6.1