Hi Peter, Thank you for your comments and the fix. It's a good idea to mark cachedLoader with the Thread object.
I think we need to check the marking thread of cachedLoader before updating it. Otherwise, there is a scenario that can leak a CachedLoader object: //1. Thread-A enters readObject() and then call resolveClass() outerCL-A <- null cachedLoader <- Thread-A cachedLoader <- CachedLoader-A //2. Thread-B enters readObject() and then call resolveClass() outerCL-B <- CachedLoader-A cachedLoader <- Thread-B cachedLoader <- CachedLoader-B1 //3. Thread-B returns from readObject() cachedLoader is unchanged because outerCL.thread == Thread-A //4. Thread-B enters readObject() again and then call resolveClass() outerCL-B <- CachedLoader-B1 cachedLoader <- Thread-B cachedLoader <- CachedLoader-B2 //5. Thread-A returns from readObject() cachedLoader <- null //6. Thread-B returns from readObject() cachedLoader <- CachedLoader-B1 // Because outerCL-B.thread is Thread-B By adding checking before updating the mark, Thread-B won't update cachedLoader, or it only saves null when race occurs (and always restores to null on exit). Here is the updated webrev: http://cr.openjdk.java.net/~horii/8188858/webrev.03/ I also made minor changes to reduce the number of invocation of the JNI method Thread.currentThread(). Regards, Ogata From: Peter Levart <[email protected]> To: Kazunori Ogata <[email protected]>, Alan Bateman <[email protected]> Cc: [email protected] Date: 2017/10/16 19:58 Subject: Re: RFR: 8188858: Caching latestUserDefinedLoader() results in ObjectInputStream.readObject() Hi Ogata, I found a problem in my last suggestion. See below... On 10/16/2017 11:36 AM, Peter Levart wrote: > > > On 10/16/2017 11:02 AM, Peter Levart wrote: >> For example: >> - let public readObject() / readUnshared() entry and exit points just >> clear the cached loader (set it to null). > > An alternative would be for entry point to save and clear the cached > loader while exit point would restore / clear it if it is from correct > thread / when the call was not nested. Like the following: > > public Object readObject() { > CachedLoader outerCL = cachedLoader; > cachedLoader = null; > try { > ... > } finally { > if (outerCL == null || outerCL.thread == > Thread.currentThread()) { > // restore/clear cached loader when nested/outer call ends > cachedLoader = outerCL; > } > } > } > > with resolveClass() fragment repeated here for comparison: > > CachedLoader cl = cachedLoader; > Thread curThread = Thread.currentThread(); > ClassLoader loader; > if (cl == null) { > loader = latestUserDefinedLoader(); > cachedLoader = new CachedLoader(loader, curThread); > } else if (cl.thread == curThread) { > loader = cl.loader; > } else { > // multi threaded use > loader = latestUserDefinedLoader(); > } > > // and then... > return Class.forName(name, false, loader); > > > There are all sorts of races possible when called concurrently from > multiple threads, but the worst consequence is that the loader is not > cached. I also think that even in the presence of races, the > cachedLoader is eventually cleared when all calls to OIS complete. I > couldn't think of a situation where such cached loader would remain > hanging off the completed OIS because of races. > > Well, there is one such situation but for a different reason. For > example, if an OIS subclass is constructed solely to override > resolveClass method to make it accessible to custom code (for example, > make it public and call super.resolveClass()) in order to provide a > utility for resolving classes with the default OIS semantics, but such > OIS instance is never used for deserialization itself > (readObject()/readUnshared() is never called). > > To solve this problem, resolveClass() logic, including lazy caching, > should be moved to a private method (resolveClass0()) with protected > resolveClass() treated like public readObject()/readUnshared() with > before/after treatment of cached loader around delegation to > resolveClass0(). All OIS internal uses of resolveClass() should then > be redirected to resolveClass0(). Oops, this would not work for subclasses that override resolveClass() with custom logic. Hm... The correct and optimal solution is a little bit more involved, I think. Here's what I think should work (did not run any tests yet): https://urldefense.proofpoint.com/v2/url?u=http-3A__cr.openjdk.java.net_-7Eplevart_jdk10-2Ddev_8188858-5FOIS.latestUserDefinedLoader.caching_webrev.01_&d=DwIDaQ&c=jf_iaSHvJObTbx-siA1ZOg&r=p-FJcrbNvnCOLkbIdmQ2tigCrcpdU77tlI2EIdaEcJw&m=PbaGqOdJOR6jMQkXDVYmjn6832m7o0LU2bzwt2awUgQ&s=gKz_rwcTcGIw8JvmRqlg1-OtjqFNXmIs4oQmIXlF3Wc&e= Regards, Peter
