Alan mentioned I should have sent this to net-dev@. Instead, I submitted
a new bug:
https://bugs.openjdk.java.net/browse/JDK-8151505
-Aleksey
On 03/09/2016 02:06 PM, Aleksey Shipilev wrote:
> Hi,
>
> In recently committed java.net.http.ExecutorWrapper, there is a
> synchronize() method [1], which is used as "memory fence" [2]:
>
> public synchronized void synchronize() {}
>
> public void execute(Runnable r, Supplier<AccessControlContext>
> ctxSupplier) {
> synchronize();
> Runnable r1 = () -> {
> try {
> r.run();
> } catch (Throwable t) {
> Log.logError(t);
> }
> };
>
> ...
>
> executor.execute(r1);
> }
>
>
> How's that supposed to work? Is that supposed to guard from bad Runnable
> $r?
>
> The problem is, once you get $r via the race, there is no way to recover
> with local synchronization (IOW: There is no way to sanitize a racy
> input, once it happened. Races are bad like that) And if $r got to you
> properly, you don't need to do anything special too (IOW: API may as
> well assume it is coming from the current thread).
>
> Therefore, I think synchronize() method there is superfluous.
>
> In fact, assuming that a synchronized method has any *detached* memory
> semantics is wrong too -- compilers are known to elide associated
> fences. E.g. if ExecutorWrapper is known to never escape a thread, or a
> single thread locks on it, and biases a lock towards itself.
>
> Thanks,
> -Aleksey
>
> [1]
> http://hg.openjdk.java.net/jdk9/jdk9/jdk/file/e0da6c2a5c32/src/java.httpclient/share/classes/java/net/http/ExecutorWrapper.java#l74
> [2]
> http://hg.openjdk.java.net/jdk9/jdk9/jdk/file/e0da6c2a5c32/src/java.httpclient/share/classes/java/net/http/ExecutorWrapper.java#l77
>
