sodonnel commented on code in PR #10109:
URL: https://github.com/apache/ozone/pull/10109#discussion_r3233368271
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hadoop-hdds/container-service/src/main/java/org/apache/hadoop/ozone/container/common/impl/ContainerSet.java:
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@@ -279,6 +285,49 @@ public Container<?> getContainer(long containerId) {
return containerMap.get(containerId);
}
+ /**
+ * Returns the max retry for a container map swap while acquiring container
lock.
+ * @return max retry count
+ */
+ public static int maxContainerMapSwapRetries() {
+ return MAX_CONTAINER_MAP_SWAP_RETRIES;
+ }
+
+ /**
+ * Locks the container mapped to {@code containerId} for write, and verifies
that the instance locked is still
+ * the one stored in this set. If the mapping is swapped or the container no
longer exists, unlocks and retries up to
+ * {@link #maxContainerMapSwapRetries()} times, then returns {@code null}.
+ *
+ * @return the locked container, or {@code null} if none mapped, or mapping
could not be stabilized
+ */
+ @Nullable
+ public Container<?> acquireContainerLock(long containerId) {
+ for (int retry = 0; retry < MAX_CONTAINER_MAP_SWAP_RETRIES; retry++) {
+ Container<?> candidate = getContainer(containerId);
+ if (candidate == null) {
+ LOG.info("Container {} no longer present in ContainerSet, skipping.",
containerId);
+ return null;
+ }
+ candidate.writeLock();
+ Container<?> current = getContainer(containerId);
+ if (current == null) {
+ candidate.writeUnlock();
+ LOG.info("Container {} no longer exists in ContainerSet while
acquiring lock.", containerId);
+ return null;
+ }
+ if (current != candidate) {
+ candidate.writeUnlock();
+ if (LOG.isDebugEnabled()) {
+ LOG.debug("Container {} mapping changed during lock acquisition
(attempt {}); retrying.",
Review Comment:
> You can first lock the current, then unlock the candidate, so no retry is
needed.
Are you sure that will work? What if someone swaps the current again while
you are trying to get the lock? You need to refetch and check again:
1. Get container
2. Lock it
3. Check get it again and check it has not changed.
If changed:
1. Lock new copy
2. Need to check if it has changed while waiting for the lock, so its kind
of back to the start.
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