RFR: 8175010: ImageReader is not thread-safe

Peter Levart peter.levart at gmail.com
Thu Feb 16 08:17:15 UTC 2017

Hi Claes,

On 02/15/2017 10:41 PM, Claes Redestad wrote:
> Hi Peter,
> are you suggesting that if I have class Foo { Bar b; }, then creating
> and putting Foo b in a CHM before returning it to a consumer which is
> then read from another thread is enough to force b to be safely
> published even when the other thread does *not* get the object via a
> call to the same CHM (which would go via the same volatile read and add
> the necessary happens before relationship)?  I recalled having seen
> examples to the effect that a non-volatile b isn't safely published in
> this case.

No, I'm not suggesting that. I'm merely observing (and hoping that my 
IDE didn't miss places in code where this is not the case) that there is 
no unsafe publication going on here. Obtaining ImageReader from 
ImageReaderFactory.get() is safe as it either returns an instance 
constructed in the same thread or it returns an instance constructed in 
a different thread, but in that case, the instance is handed through 
CHM.set/get which enforces ordering. The only other place where I found 
ImageReader.open() call is in constructing the SystemImage 
implementation as an anonymous inner instance which holds an ImageReader 
in its synthetic final field which ensures safe publication of 
ImageReader even if SystemImage instance is published unsafely.

> The (very shaky) hypothesis is thus that this could be what's happening
> in any of the places which load and locally cache the system ImageReader
> for anyone to use, e.g., SystemModuleFinder.SystemImage 

...here the ImageReader instance is obtained via 

      * Holder class for the ImageReader
     private static class SystemImage {
         static final ImageReader READER;
         static {
             long t0 = System.nanoTime();
             READER = ImageReaderFactory.getImageReader();

         static ImageReader reader() {
             return READER;

...which is just a shortcut for 
ImageReaderFactory.get(BOOT_MODULES_JIMAGE) and I believe 
ImageReaderFactory.get() publishes (and did publish even before the 
patch) ImageReader instances safely (old code):

   52     private static final Map<Path, ImageReader> readers = new 
   54     /**
   55      * Returns an {@code ImageReader} to read from the given image 
   56      */
   57     public static ImageReader get(Path jimage) throws IOException {
   58         Objects.requireNonNull(jimage);
   59         ImageReader reader = readers.get(jimage);
   60         if (reader != null) {
   61             return reader; // <<-- HERE the instance was handed 
through CHM
   62         }
   63         reader = ImageReader.open(jimage);
   64         // potential race with other threads opening the same URL
   65         ImageReader r = readers.putIfAbsent(jimage, reader);
   66         if (r == null) {
   67             return reader; // <<-- HERE the instance was 
constructed in same thread
   68         } else {
   69             reader.close();
   70             return r; // <<-- HERE the instance was handed through CHM
   71         }
   72     }

> or
> JavaRuntimeURLConnection (which is implicitly called when there's a
> security manager installed).

...here the instance is also obtained via 

     // ImageReader to access resources in jimage
     private static final ImageReader reader;
     static {
         PrivilegedAction<ImageReader> pa = 
         reader = AccessController.doPrivileged(pa);

In both above places the ImageReader instance is obtained in <clinit> 
and this is another "layer" of synchronization between consumer and 
producer threads.

> I might be (in fact likely am) wrong, but
> we discussed this offline and came to the conclusion that there was no
> harm in implementing these improvements regardless of whether they
> actually resolve 8174817 or not.

No, there's no harm and I support the changes. It's just that they might 
not help in resolving the problem.

> I think prior to this patch a concurrent ImageReader.close() could
> happen if there was a race between 3 or more threads to resolve the
> same Path from ImageReaderFactory.get (especially since there might be
> a longish time window there since we might block to load a library
> etc), so I don't think we lose anything from plugging that hole by
> using computeIfAbsent here.

I don't believe this was the the source of concurrent close(). If you 
look at the code (above), you can see that reader.close() (line 69) is 
called only if the reader was not successfully published and it is 
called by the thread that constructed it. But computeIfAbsent is a good 
choice here to avoid optimistic constructions followed by close(s) when 
there is concurrency.

> I think your observations about potential issues in JRTFS is correct,
> but there was nothing to suggest JRTFS code was involved in JDK-8174817
> (as it's not code that's used by the BuiltinClassLoader).

The only remaining ImageReader.close() invocation is now in 
jdk.internal.jrtfs.SystemImage anonymous inner class delegated from 
SystemImage.close(), which is only called from JrtFileSystem.close() or 
.finalize(). So you think JRTFS is not the source of the problem? What 
if JRTFS is accessing the same image as JavaRuntimeURLConnection in the 
same VM. Different instances of ImageReader would be used for them, but 
they would share the same underlying SharedImageReader. Currently I 
don't see a problem in SharedImageReader code, but I haven't studied it 
carefully yet. Maybe there is something to be found there...

Regards, Peter

> Thanks!
> /Claes
> On 2017-02-15 21:52, Peter Levart wrote:
>> Hi Claes,
>> Reading the https://bugs.openjdk.java.net/browse/JDK-8174817 and then
>> the change that was just pushed, I can't seem to figure out what was the
>> problem with original code. I can't find evidence for claims in
>> https://bugs.openjdk.java.net/browse/JDK-8175010 . Is the problem
>> publication of ImageReader via ImageReaderFactory? That can't be it,
>> since ImageReaderFactory is using ConcurrentHashMap which ensures
>> happens before relationships.
>> Is there any place else where ImageReader.open(Path) is called and then
>> the instance unsafely published to other threads? The only place I could
>> find is in jdk.internal.jrtfs.SystemImage.open():
>>     static SystemImage open() throws IOException {
>>         if (modulesImageExists) {
>>             // open a .jimage and build directory structure
>>             final ImageReader image = ImageReader.open(moduleImageFile);
>>             image.getRootDirectory();
>>             return new SystemImage() {
>>                 @Override
>>                 Node findNode(String path) throws IOException {
>>                     return image.findNode(path);
>>                 }
>>                 @Override
>>                 byte[] getResource(Node node) throws IOException {
>>                     return image.getResource(node);
>>                 }
>>                 @Override
>>                 void close() throws IOException {
>>                     image.close();
>>                 }
>>             };
>>         }
>> ...here the final 'image' local variable is captured by anonymous inner
>> SystemImage subclass into a synthetic final field, so this final field
>> ensures that ImageReader is published safely as a delegate of 
>> SystemImage.
>> ImageReader.open(Path) factory method delegates to
>> ImageReader.SharedImageReader.open(Path, ByteOrder) which creates a new
>> instance of ImageReader encapsulating a SharedImageReader object:
>>           public static ImageReader open(Path imagePath, ByteOrder
>> byteOrder) throws IOException {
>>             Objects.requireNonNull(imagePath);
>>             Objects.requireNonNull(byteOrder);
>>             synchronized (OPEN_FILES) {
>>                 SharedImageReader reader = OPEN_FILES.get(imagePath);
>>                 if (reader == null) {
>>                     // Will fail with an IOException if wrong byteOrder.
>>                     reader =  new SharedImageReader(imagePath, 
>> byteOrder);
>>                     OPEN_FILES.put(imagePath, reader);
>>                 } else if (reader.getByteOrder() != byteOrder) {
>>                     throw new IOException("\"" + reader.getName() + "\"
>> is not an image file");
>>                 }
>>                 ImageReader image = new ImageReader(reader); // <<-- 
>>                 reader.openers.add(image);
>>                 return image;
>>             }
>>         }
>> ...the ImageReader returned from this method is not safe to publish via
>> data race, but as far as I can see, there is no such publishing going
>> on. So am I right in that all this patch solves is it eliminates a
>> possibility of NPE when ImageReader is close()-d concurrently with being
>> used from some other thread. If such NPE was observed, it means that
>> close() is being called concurrently with access and there are still
>> races possible which can cause undesired effects. For example: calling
>> ImageReader.close() while using it concurrently may close underlying
>> SharedImageReader and then after closing, access it.
>> So is there a concurrent ImageReader.close() possible? The only place I
>> see ImageReader.close() being invoked is from SystemImage.close() in the
>> anonymous inner class implementation which wraps ImageReader.
>> SystemImage.close() is only being invoked from JrtFileSystem.cleanup(),
>> which is called from JrtFileSystem.close() and JrtFileSystem.finalize().
>> The following is theoretically possible:
>> FileSystem fs = FileSystems.newFileSystem(URI.create("jrt:/"), ...);
>> Path p = fs.getPath(...);
>> FileSystemProvider provider = fs.provider();
>> InputStream is = provider.newInputStream(p, ...);
>> // 'fs' and 'p' (which has a reference to 'fs') may be found finalizable
>> and be finalized while the call to obtain content of input stream is in
>> progress
>> For this to be prevented, the following method in JrtFileSystem:
>>     // returns the content of the file resource specified by the path
>>     byte[] getFileContent(JrtPath path) throws IOException {
>>         Node node = checkNode(path);
>>         if (node.isDirectory()) {
>>             throw new FileSystemException(path + " is a directory");
>>         }
>>         //assert node.isResource() : "resource node expected here";
>>         return image.getResource(node);
>>     }
>> ...would have to be changed to:
>>     byte[] getFileContent(JrtPath path) throws IOException {
>>         Node node = checkNode(path);
>>         if (node.isDirectory()) {
>>             throw new FileSystemException(path + " is a directory");
>>         }
>>         //assert node.isResource() : "resource node expected here";
>>         byte[] content = image.getResource(node);
>>         Reference.reachabilityFence(this);
>>         return content;
>>     }
>> I don't claim that this is what causes the problems and I haven't found
>> any such usages, but this is theoretically possible.
>> Regards, Peter
>> On 02/15/2017 02:22 PM, Claes Redestad wrote:
>>> Hi,
>>> a few intermittent but rare test failures[1] that has appeared
>>> since the latest jake integration, and since one of the changes
>>> in there was to make initialization of the system ImageReader
>>> lazy there appears to be cases where ImageReaders are not
>>> safely published:
>>> - Ensure ImageReader::open is called only once per Path in
>>> ImageReaderFactory by using CHM.computeIfAbsent
>>> - Ensure ImageReader.reader is safely published to a
>>> final field and signal close using a volatile boolean instead
>>> webrev: http://cr.openjdk.java.net/~redestad/8175010/webrev.02/
>>> bug: https://bugs.openjdk.java.net/browse/JDK-8175010
>>> Testing shows no issues (which admittedly doesn't mean we're
>>> actually solving the root cause for JDK-8174817), and performance
>>> numbers from adding a volatile read indicate that any overhead
>>> is lost in the noise on ImageReader-heavy workloads.
>>> Thanks!
>>> /Claes
>>> [1] https://bugs.openjdk.java.net/browse/JDK-8174817

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