console_linux: Fix race: lock Cond before Signal.

gerasiov · gerasiov · commit 5d1b48d6114b · 2018-07-04T00:21:28.000+03:00
Possible race: Thread1: Thread2: line 178: Read() -> EAGAIN ... <reschedule> line 110: EpollWait() ... line 124: signalRead() ... ... line 191: Wait() ... line 110: EpollWait() Thread2 (epoll) sends signalRead() (via sync.Cond) but Thread1 is not waiting yet. Then it starts to wait, but no more signals will arrive (because Thread2 is sleeping in EpollWait() on edge-triggered epoll. To prevent this race one should held Lock when sending signalRead(). The same situation goes with Write loop. Bug was reported to docker: docker/for-linux#353 Signed-off-by: Alexander Gerasiov <gerasiov@yandex-team.ru>
diff --git a/console_linux.go b/console_linux.go
@@ -262,10 +262,14 @@ func (ec *EpollConsole) Shutdown(close func(int) error) error {
 
 // signalRead signals that the console is readable.
 func (ec *EpollConsole) signalRead() {
+	ec.readc.L.Lock()
 	ec.readc.Signal()
+	ec.readc.L.Unlock()
 }
 
 // signalWrite signals that the console is writable.
 func (ec *EpollConsole) signalWrite() {
+	ec.writec.L.Lock()
 	ec.writec.Signal()
+	ec.writec.L.Unlock()
 }

Original file line number	Diff line number	Diff line change
`@@ -262,10 +262,14 @@ func (ec *EpollConsole) Shutdown(close func(int) error) error {`
`262`	`262`
`263`	`263`	`// signalRead signals that the console is readable.`
`264`	`264`	`func (ec *EpollConsole) signalRead() {`
	`265`	`+ ec.readc.L.Lock()`
`265`	`266`	`ec.readc.Signal()`
	`267`	`+ ec.readc.L.Unlock()`
`266`	`268`	`}`
`267`	`269`
`268`	`270`	`// signalWrite signals that the console is writable.`
`269`	`271`	`func (ec *EpollConsole) signalWrite() {`
	`272`	`+ ec.writec.L.Lock()`
`270`	`273`	`ec.writec.Signal()`
	`274`	`+ ec.writec.L.Unlock()`
`271`	`275`	`}`