From 1e91e2a25a0058cc25b3836b59a0954c6c75a62c Mon Sep 17 00:00:00 2001 From: Austin Clements Date: Mon, 15 Feb 2016 16:50:12 -0500 Subject: [PATCH] runtime: document non-obvious requirement on sudog.elem The channel code must not allow stack splits between when it assigns a potential stack pointer to sudog.elem (or sudog.selectdone) and when it makes the sudog visible to copystack by putting it on the g.waiting list. We do get this right everywhere, but add a comment about this subtlety for future eyes. Change-Id: I941da150437167acff37b0e56983c793f40fcf79 Reviewed-on: https://go-review.googlesource.com/19632 Reviewed-by: Rick Hudson Run-TryBot: Austin Clements TryBot-Result: Gobot Gobot --- src/runtime/chan.go | 4 ++++ src/runtime/select.go | 2 ++ 2 files changed, 6 insertions(+) diff --git a/src/runtime/chan.go b/src/runtime/chan.go index 063c5ce391..f6f3ce4d90 100644 --- a/src/runtime/chan.go +++ b/src/runtime/chan.go @@ -203,6 +203,8 @@ func chansend(t *chantype, c *hchan, ep unsafe.Pointer, block bool, callerpc uin if t0 != 0 { mysg.releasetime = -1 } + // No stack splits between assigning elem and enqueuing mysg + // on gp.waiting where copystack can find it. mysg.elem = ep mysg.waitlink = nil mysg.g = gp @@ -460,6 +462,8 @@ func chanrecv(t *chantype, c *hchan, ep unsafe.Pointer, block bool) (selected, r if t0 != 0 { mysg.releasetime = -1 } + // No stack splits between assigning elem and enqueuing mysg + // on gp.waiting where copystack can find it. mysg.elem = ep mysg.waitlink = nil gp.waiting = mysg diff --git a/src/runtime/select.go b/src/runtime/select.go index b6c3fea001..b315dde6c6 100644 --- a/src/runtime/select.go +++ b/src/runtime/select.go @@ -370,6 +370,8 @@ loop: sg.g = gp // Note: selectdone is adjusted for stack copies in stack1.go:adjustsudogs sg.selectdone = (*uint32)(noescape(unsafe.Pointer(&done))) + // No stack splits between assigning elem and enqueuing + // sg on gp.waiting where copystack can find it. sg.elem = cas.elem sg.releasetime = 0 if t0 != 0 {