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Avoid deadlocks in cache #30461

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merged 5 commits into from
May 9, 2018
Merged

Avoid deadlocks in cache #30461

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d498e66
Avoid deadlocks in cache
jasontedor May 8, 2018
ddbe45a
Final
jasontedor May 8, 2018
6051293
Merge branch 'master' into cache-deadlock
jasontedor May 8, 2018
9344ce7
Variable names
jasontedor May 9, 2018
a3e152f
Merge remote-tracking branch 'elastic/master' into cache-deadlock
jasontedor May 9, 2018
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79 changes: 36 additions & 43 deletions server/src/main/java/org/elasticsearch/common/cache/Cache.java
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Original file line number Diff line number Diff line change
Expand Up @@ -206,34 +206,33 @@ private static class CacheSegment<K, V> {
*/
Entry<K, V> get(K key, long now, Predicate<Entry<K, V>> isExpired, Consumer<Entry<K, V>> onExpiration) {
CompletableFuture<Entry<K, V>> future;
Entry<K, V> entry = null;
try (ReleasableLock ignored = readLock.acquire()) {
future = map.get(key);
}
if (future != null) {
Entry<K, V> entry;
try {
entry = future.handle((ok, ex) -> {
if (ok != null && !isExpired.test(ok)) {
segmentStats.hit();
ok.accessTime = now;
return ok;
} else {
segmentStats.miss();
if (ok != null) {
assert isExpired.test(ok);
onExpiration.accept(ok);
}
return null;
}
}).get();
} catch (ExecutionException | InterruptedException e) {
entry = future.get();
} catch (ExecutionException e) {
assert future.isCompletedExceptionally();
segmentStats.miss();
return null;
} catch (InterruptedException e) {
throw new IllegalStateException(e);
}
}
else {
if (isExpired.test(entry)) {
segmentStats.miss();
onExpiration.accept(entry);
return null;
} else {
segmentStats.hit();
entry.accessTime = now;
return entry;
}
} else {
segmentStats.miss();
return null;
}
return entry;
}

/**
Expand Down Expand Up @@ -269,30 +268,18 @@ Tuple<Entry<K, V>, Entry<K, V>> put(K key, V value, long now) {
/**
* remove an entry from the segment
*
* @param key the key of the entry to remove from the cache
* @return the removed entry if there was one, otherwise null
* @param key the key of the entry to remove from the cache
* @param onRemoval a callback for the removed entry
*/
Entry<K, V> remove(K key) {
void remove(K key, Consumer<CompletableFuture<Entry<K, V>>> onRemoval) {
CompletableFuture<Entry<K, V>> future;
Entry<K, V> entry = null;
try (ReleasableLock ignored = writeLock.acquire()) {
future = map.remove(key);
}
if (future != null) {
try {
entry = future.handle((ok, ex) -> {
if (ok != null) {
segmentStats.eviction();
return ok;
} else {
return null;
}
}).get();
} catch (ExecutionException | InterruptedException e) {
throw new IllegalStateException(e);
}
segmentStats.eviction();
onRemoval.accept(future);
}
return entry;
}

private static class SegmentStats {
Expand Down Expand Up @@ -476,12 +463,18 @@ private void put(K key, V value, long now) {
*/
public void invalidate(K key) {
CacheSegment<K, V> segment = getCacheSegment(key);
Entry<K, V> entry = segment.remove(key);
if (entry != null) {
try (ReleasableLock ignored = lruLock.acquire()) {
delete(entry, RemovalNotification.RemovalReason.INVALIDATED);
segment.remove(key, f -> {
try {
Entry<K, V> entry = f.get();
try (ReleasableLock ignored = lruLock.acquire()) {
delete(entry, RemovalNotification.RemovalReason.INVALIDATED);
}
} catch (ExecutionException e) {
// ok
} catch (InterruptedException e) {
throw new IllegalStateException(e);
}
}
});
}

/**
Expand Down Expand Up @@ -632,7 +625,7 @@ public void remove() {
Entry<K, V> entry = current;
if (entry != null) {
CacheSegment<K, V> segment = getCacheSegment(entry.key);
segment.remove(entry.key);
segment.remove(entry.key, f -> {});
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why does this follow a different pattern than invalidate? as far as I can tell if we don't wait for the future to be completed, it may be re-inserted into the LRU by the future completion logic. I would also like to understand why this isn't a race condition even if you do complete the future (i.e., aren't we susceptible to race conditions in between the execution of the handler and the get() returning), which will cause the LRU to go out of sync (similar issue in put)

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@jasontedor and I discussed this on another channel. The reason for a different execution paths on the call backs has to do with whether we already hold a reference to the relevant entry or not. I personally prefer to not have two paths here but not enough to request a change.

I would also like to understand why this isn't a race condition even if you do complete the future (i.e., aren't we susceptible to race conditions in between the execution of the handler and the get() returning), which will cause the LRU to go out of sync (similar issue in put)

This one is guarded against by the state in the entry. Deleting an entry also changes the state to deleted and thus it will not be re-added by the handler in computeIfAbsent. That said we found another issue there where delete doesn't mark the entry as deleted if it's in the new state. This will be dealt with in a followup.

try (ReleasableLock ignored = lruLock.acquire()) {
current = null;
delete(entry, RemovalNotification.RemovalReason.INVALIDATED);
Expand Down Expand Up @@ -717,7 +710,7 @@ private void evictEntry(Entry<K, V> entry) {

CacheSegment<K, V> segment = getCacheSegment(entry.key);
if (segment != null) {
segment.remove(entry.key);
segment.remove(entry.key, f -> {});
}
delete(entry, RemovalNotification.RemovalReason.EVICTED);
}
Expand Down
1 change: 0 additions & 1 deletion server/src/test/java/org/elasticsearch/common/cache/CacheTests.java
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Original file line number Diff line number Diff line change
Expand Up @@ -344,7 +344,6 @@ protected long now() {
assertEquals(numberOfEntries, cache.stats().getEvictions());
}

@AwaitsFix(bugUrl = "https://github.com/elastic/elasticsearch/issues/30428")
public void testComputeIfAbsentDeadlock() throws BrokenBarrierException, InterruptedException {
final int numberOfThreads = randomIntBetween(2, 32);
final Cache<Integer, String> cache =
Expand Down