commit 2817e77024a53aae31cc02c01a26fce0ebb11a79
author Austin Clements <austin@google.com> Mon Dec 19 22:55:53 2016 -0500
committer Austin Clements <austin@google.com> Tue Jan 10 15:59:39 2017 +0000
tree 272b664f6f89986d8327651838df6f5ec9b6bb67
parent 7f1ff65c3947b916cc4d0827fd8c1307d7efd7bf

runtime: debug prints for spanBytesAlloc underflow

Updates #18043.

Change-Id: I24e687fdd5521c48b672987f15f0d5de9f308884
Reviewed-on: https://go-review.googlesource.com/34612
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Rick Hudson <rlh@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/mgcsweep.go

diff --git a/src/runtime/mgcsweep.go b/src/runtime/mgcsweep.go
index e74a451..fb5c488 100644
--- a/src/runtime/mgcsweep.go
+++ b/src/runtime/mgcsweep.go
@@ -405,7 +405,10 @@
 		// Nobody cares about the credit. Avoid the atomic.
 		return
 	}
-	if int64(atomic.Xadd64(&mheap_.spanBytesAlloc, -int64(unusableBytes))) < 0 {
+	nval := atomic.Xadd64(&mheap_.spanBytesAlloc, -int64(unusableBytes))
+	if int64(nval) < 0 {
+		// Debugging for #18043.
+		print("runtime: bad spanBytesAlloc=", nval, " (was ", nval+uint64(unusableBytes), ") unusableBytes=", unusableBytes, " sweepPagesPerByte=", mheap_.sweepPagesPerByte, "\n")
 		throw("spanBytesAlloc underflow")
 	}
 }