RFR 8012326: Deadlock occurs when Charset.availableCharsets() is called by several threads at the same time

[Xueming Shen]

Thanks Ulf!

There is another version with a new ExtendedCharsets.java at

http://cr.openjdk.java.net/~sherman/8012326/webrev.newECP/

I merged the stuff in AbstractCharsetProvider into ExtendedCharsets.java.
The standardcharset provider now uses the FastCharsetProvider, so there
is no need to have an abstract class anymore, as long as we are not going
to add a new or separate the charsets.jar. I kinda remember there was
a plan to further divide the charsets.jar in the past though...

I took the chance to "clean up" the synchronization mechanism in
ExtendedCharsets. It appears there are two sync needs here.

One is to protect the "cache" inside lookup(), which triggers the race
condition if the lookup() gets invoked by multiple threads and the
"cahce" map gets accessed/updated at the same time, this is reported
and fixed by 4685305 [1], the original fix is to put the sync block in
AbstractCharsetProvider.charsetForName().  We put in another sync
block in iterator.next() for 6898310 [2], which is the trigger of this bug.
In the new version, I "consolidated" them together into lookup()

Another sync need is for the "init()", in which it may update the aliasMap,
classMap and aliasNameMap via charset() and deleteCharset() if those
special properties are defined. There are two sources for the charset()/
deleteCharset(), one is from the constructor, one is from the init(), given
ExtendedCharsets is now singleton and get initialized at class init, these
should be no race concern between these two sources, so no need to
have any sync block inside charset() and deleteCharset(), the only thing
need to defend is inside init(), and all three public methods invoke the
init() at the beginning of the method body.

It appears I will still have to keep the same logic in Charset to access
the ExtendedCharset, as it is need to be "probed", just in case it is not
"installed"...

Yes, there is also room to improve in FastCharsetProvider...I know I
need pick some time on it.

-Sherman

On 5/4/13 10:09 AM, Ulf Zibis wrote:
> Hi Sherman,
>
> looks good to me.
>
> Maybe you like to compare with webrevs from:
> https://bugs.openjdk.java.net/show_bug.cgi?id=100092
> https://bugs.openjdk.java.net/show_bug.cgi?id=100095
>
> -Ulf
>
> Am 03.05.2013 06:29, schrieb Xueming Shen:
>> Hi,
>>
>> Please help review the proposed fix for 8012326.
>>
>> The direct trigger is the fix we put in #6898310 [1], in which we 
>> added the
>> synchronization to prevent a possible race condition as described in 
>> $4685305.
>>
>> However it appears the added synchronization triggers another race 
>> condition as
>> showed in this stack trace [4] when run the test case [2][3].
>>
>> The root cause here is
>>
>> (1) The ExtendedCharsetProvider is intended to be used as a singleton 
>> (as the
>> probeExtendedProvider + lookupExtendedCharset mechanism in 
>> Charset.java),
>> however this is only true for the 
>> Charset.forName()->lookup()...scenario. Multiple
>> instances of ExtendedCharsetProvider is being created in 
>> Charset.availableCharset()
>> every time it is invoked, via providers()/ServiceLoader.load().
>>
>> (2) ISO2022_JP_2 and MSISO2022JP are provided via 
>> ExtendedCharsetProvider,
>> however both of them have two static variable 
>> DEC02{12|08}/ENC02{12|08} that
>> need to be initialized during the "class initialization", which will 
>> indirectly trigger
>> the invocation of ExtendedCharsetProvider.alisesFor(...)
>>
>> (3) static method ExtendedCharsets.aliaseFor() has a hacky 
>> implementation that
>> goes to AbstractCharsetProvider.alise() for lookup, which needs to 
>> obtain a lock
>> on its ExtendedCharesetProvider.instance. This will potential cause 
>> race condition
>> if the "instance" it tries to synchronize on is not its "own" 
>> instance. This is possible
>> because the constructor of ExtendedCharsetProvider overwrites static 
>> "instance"
>> variable with "this".
>>
>> Unfortunately as demonstrated  in [4], this appears to be what is 
>> happening.
>> The Thread-1/#9 is trying to synchronize on someone else's 
>> ExtendedCharsetProvider
>> instance <0xa4824730> (its own instance should be <0xa4835328>, which it
>> holds the monitor in the iterator.next()), it is blocked because 
>> Thread-0 already holds
>> the monitor of <0xa4824730> (in its iteratior.next()), but Thread-0 
>> is blocked at
>> Class.forName0(), which I think is because Thread-1 is inside it 
>> already...two theads
>> are deadlocked.
>>
>> A "quick fix/solution" is to remove the direct trigger in 
>> ISO2022_JP_2 and
>> MSISO2022JP to lazily-initialize those static variables as showed in the
>> webrev. However while this probably will solve the race condition, 
>> the multiple
>> instances of ExtendedCharsetProvider is really not desired. And given 
>> the
>> fact we have already hardwired ExtendedCharsetProvider (as well as the
>> StandardCharset, for performance reason) for charset 
>> lookup/forName(), the
>> availableCharsets() should also utilize the "singleton" 
>> ExtendedCharsetProvider
>> instanc (extendedCharsetProvider) as well, instead of going to the 
>> ServiceLoader
>> every time for a new instance. The suggested change is to use Charset.
>> extendedCharsetProvide via probeExtendedProvider() for extended charset
>> iteration (availableCharset()). Meanwhile, since the 
>> ExtendedCharsetProvider/
>> charsets.jar should always be in the boot classpath (if installed, 
>> and we are
>> looking up via 
>> Class.forName("sun.nio.cs.ext.ExtededCharsetProvider")), there
>> is really no need for it to be looked up/loaded via the spi mechanism 
>> (in
>> fact it's redundant to load it again after we have lookup/iterate the
>> hardwired "extendedCharsetProvider". So I removed the spi description 
>> from
>> the charsts.jar as well.
>>
>> An alternative is to really implement the singleton pattern in ECP, 
>> however
>> given the existing mechanism we have in Charset.java and the "hacky" 
>> init()
>> implementation we need in ECP, I go with the current approach.
>>
>> http://cr.openjdk.java.net/~sherman/8012326/webrev
>>
>> Thanks,
>> Sherman
>>
>> [1] http://cr.openjdk.java.net/~sherman/6898310/webrev/
>> [2] http://cr.openjdk.java.net/~sherman/8012326/runtest.bat
>> [3] http://cr.openjdk.java.net/~sherman/8012326/Test.java
>> [4] http://cr.openjdk.java.net/~sherman/8012326/stacktrace
>>
>