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runtime: document non-obvious requirement on sudog.elem

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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 <rlh@golang.org>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
This commit is contained in:
Austin Clements 2016-02-15 16:50:12 -05:00
parent 39f2bd737b
commit 1e91e2a25a
2 changed files with 6 additions and 0 deletions
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4
src/runtime/chan.go
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@ -203,6 +203,8 @@ func chansend(t *chantype, c *hchan, ep unsafe.Pointer, block bool, callerpc uin
if t0 != 0 { if t0 != 0 {
mysg.releasetime = -1 mysg.releasetime = -1
} }
// No stack splits between assigning elem and enqueuing mysg
// on gp.waiting where copystack can find it.
mysg.elem = ep mysg.elem = ep
mysg.waitlink = nil mysg.waitlink = nil
mysg.g = gp mysg.g = gp
@ -460,6 +462,8 @@ func chanrecv(t *chantype, c *hchan, ep unsafe.Pointer, block bool) (selected, r
if t0 != 0 { if t0 != 0 {
mysg.releasetime = -1 mysg.releasetime = -1
} }
// No stack splits between assigning elem and enqueuing mysg
// on gp.waiting where copystack can find it.
mysg.elem = ep mysg.elem = ep
mysg.waitlink = nil mysg.waitlink = nil
gp.waiting = mysg gp.waiting = mysg

2
src/runtime/select.go
View File

@ -370,6 +370,8 @@ loop:
sg.g = gp sg.g = gp
// Note: selectdone is adjusted for stack copies in stack1.go:adjustsudogs // Note: selectdone is adjusted for stack copies in stack1.go:adjustsudogs
sg.selectdone = (*uint32)(noescape(unsafe.Pointer(&done))) 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.elem = cas.elem
sg.releasetime = 0 sg.releasetime = 0
if t0 != 0 { if t0 != 0 {