On 7/29/07, James Im <[EMAIL PROTECTED]> wrote:
> In org.apache.mina.transport.socket.nio.SocketIoProcessor I have a bad
> _feeling_ about the concurrency of these 2 methods:
>
> void flush(SocketSessionImpl session) {
> scheduleFlush(session);
> Selector selector = this.selector;
> if (selector != null) {
> selector.wakeup();
> }
> }
> void updateTrafficMask(SocketSessionImpl session) {
> scheduleTrafficControl(session);
> Selector selector = this.selector;
> if (selector != null) {
> selector.wakeup();
> }
> }
>
>
> I have a problem specifically with this piece of code:
>
> Selector selector = this.selector;
> if (selector != null) {
> selector.wakeup();
> }
>
> The code access 'this.selector' in an unsynchronized manner from other
> threads and I don't know if some visibility/safe publication issues
> might arise here.
>
> The problem is not that you call "selector.wakeup();" from other
> threads, the problem is rather the fact that you are accessing the
> variable 'selector' without synchronization.
>
> In the same class when you change the variable 'selector' you
> synchronize around the 'lock' object to take care of synchronization
> issues. For me that seems to indicate that on one hand you are taking
> care of visibility/safe publication issues and on the other one it is
> not taken care of.
>
> If you happen to have the book "Java Concurrency in Practice", please
> read again "3.5 Safe publication" on page 49-54. It seems to me that
> the gray rectangle (in 3.5.3) hints that to avoid the problem we might
> need to store the reference to the selector into a volatile field.
>
> private volatile Selector selector;
>
> What do you think? From a visibility/safe publication stand point, do
> you see a problem here?
Right, there's a visibility problem, but it won't hurt MINA because
Selector is called with timeout (1 second). At worst case, wakeup
will be delayed for 1 second. And the callers of wakeup() calls
wakeup() mostly after some synchronized block (e.g.
IoSession.write()), so the possibility that something goes wrong is
very close to zero.
Trustin
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