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runtime: protect sudog.elem with hchan.lock

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Currently sudog.elem is never accessed concurrently, so in several
cases we drop the channel lock just before reading/writing the
sent/received value from/to sudog.elem. However, concurrent stack
shrinking is going to have to adjust sudog.elem to point to the new
stack, which means it needs a way to synchronize with accesses to
sudog.elem. Hence, add sudog.elem to the fields protected by
hchan.lock and scoot the unlocks down past the uses of sudog.elem.

While we're here, better document the channel synchronization rules.

For #12967.

Change-Id: I3ad0ca71f0a74b0716c261aef21b2f7f13f74917
Reviewed-on: https://go-review.googlesource.com/20040
Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
This commit is contained in:
Austin Clements 2016-02-18 09:34:43 -05:00
parent 3c2a21ff13
commit db72b41bcd
2 changed files with 18 additions and 9 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

10
src/runtime/chan.go
View File

@ -33,7 +33,10 @@ type hchan struct {
recvx uint // receive index
recvq waitq // list of recv waiters
sendq waitq // list of send waiters
lock mutex
// lock protects all fields in hchan, as well as several
// fields in sudogs blocked on this channel.
lock mutex
}
type waitq struct {
@ -261,12 +264,12 @@ func send(c *hchan, sg *sudog, ep unsafe.Pointer, unlockf func()) {
c.sendx = c.recvx // c.sendx = (c.sendx+1) % c.dataqsiz
}
}
unlockf()
if sg.elem != nil {
sendDirect(c.elemtype, sg, ep)
sg.elem = nil
}
gp := sg.g
unlockf()
gp.param = unsafe.Pointer(sg)
if sg.releasetime != 0 {
sg.releasetime = cputicks()
@ -509,7 +512,6 @@ func recv(c *hchan, sg *sudog, ep unsafe.Pointer, unlockf func()) {
if raceenabled {
racesync(c, sg)
}
unlockf()
if ep != nil {
// copy data from sender
// ep points to our own stack or heap, so nothing
@ -539,10 +541,10 @@ func recv(c *hchan, sg *sudog, ep unsafe.Pointer, unlockf func()) {
c.recvx = 0
}
c.sendx = c.recvx // c.sendx = (c.sendx+1) % c.dataqsiz
unlockf()
}
sg.elem = nil
gp := sg.g
unlockf()
gp.param = unsafe.Pointer(sg)
if sg.releasetime != 0 {
sg.releasetime = cputicks()

17
src/runtime/runtime2.go
View File

@ -214,11 +214,18 @@ type gobuf struct {
// Changes here must also be made in src/cmd/compile/internal/gc/select.go's
// selecttype.
type sudog struct {
g *g
selectdone *uint32 // CAS to 1 to win select race (may point to stack)
next *sudog
prev *sudog
elem unsafe.Pointer // data element (may point to stack)
// The following fields are protected by the hchan.lock of the
// channel this sudog is blocking on.
g *g
selectdone *uint32 // CAS to 1 to win select race (may point to stack)
next *sudog
prev *sudog
elem unsafe.Pointer // data element (may point to stack)
// The following fields are never accessed concurrently.
// waitlink is only accessed by g.
releasetime int64
ticket uint32
waitlink *sudog // g.waiting list