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

Xueming Shen xueming.shen at oracle.com
Fri May 3 04:29:03 UTC 2013

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

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