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LUCENE-7409: Fix comments as suggested by Dawid

This commit is contained in:
Uwe Schindler 2016-08-12 22:16:04 +02:00
parent 48cc599936
commit 97f6bb7d7f
1 changed files with 13 additions and 7 deletions
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20
lucene/core/src/java/org/apache/lucene/store/ByteBufferGuard.java
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@ -26,10 +26,8 @@ import java.util.concurrent.atomic.AtomicInteger;
* of this is used for the original and all clones, so once the original is closed and unmapped
* all clones also throw {@link AlreadyClosedException}, triggered by a {@link NullPointerException}.
* <p>
* This code uses the trick that is also used in
* {@link java.lang.invoke.MutableCallSite#syncAll(java.lang.invoke.MutableCallSite[])} to
* invalidate switch points. It also yields the current thread to give other threads a chance
* to finish in-flight requests...
* This code tries to hopefully flush any CPU caches using a store-store barrier. It also yields the
* current thread to give other threads a chance to finish in-flight requests...
*/
final class ByteBufferGuard {
@ -45,10 +43,10 @@ final class ByteBufferGuard {
private final String resourceDescription;
private final BufferCleaner cleaner;
/** not volatile, we use store-store barrier! */
/** Not volatile; see comments on visibility below! */
private boolean invalidated = false;
/** the actual store-store barrier. */
/** Used as a store-store barrier; see comments below! */
private final AtomicInteger barrier = new AtomicInteger();
/**
@ -66,9 +64,17 @@ final class ByteBufferGuard {
public void invalidateAndUnmap(ByteBuffer... bufs) throws IOException {
if (cleaner != null) {
invalidated = true;
// this should trigger a happens-before - so flushes all caches
// This call should hopefully flush any CPU caches and as a result make
// the "invalidated" field update visible to other threads. We specifically
// don't make "invalidated" field volatile for performance reasons, hoping the
// JVM won't optimize away reads of that field and hardware should ensure
// caches are in sync after this call. This isn't entirely "fool-proof"
// (see LUCENE-7409 discussion), but it has been shown to work in practice
// and we count on this behavior.
barrier.lazySet(0);
// we give other threads a bit of time to finish reads on their ByteBuffer...:
Thread.yield();
// finally unmap the ByteBuffers:
for (ByteBuffer b : bufs) {
cleaner.freeBuffer(resourceDescription, b);
}