Here’s the JDK issue that Fangmin mentioned: https://bugs.openjdk.java.net/browse/JDK-8207746
It’s a JDK 10 & 11 bug which has already been fixed since JDK11 b27. Andor > On 2019. Oct 28., at 8:00, Enrico Olivelli <[email protected]> wrote: > > Fangmin, > > Il lun 28 ott 2019, 02:23 Fangmin Lv <[email protected]> ha scritto: > >> Hey everyone, >> >> (Forgot to add subject in the previous email, resent with clear subject.) >> >> I'd like to share some weird inconsistency bugs we saw recently on prod, >> the root cause and potential fixes of it. It took us around a month to >> investigate, reproduce and find out the root cause, hopefully the >> informations here will help people avoid hitting this same potential issue. >> >> [Trigger conditions and behavior] >> >> The inconsistency issue only happened when running ZK with OpenJDK 10 on >> SKL machines, and it's not because of bugs inside ZK but due to a >> macro-assembly bug inside JDK. >> >> And the behavior of the issues might be: >> >> * NONODE returned when getData from a child exist when queried with >> getChildren, and there is no delete issued >> * NONODE error returned when try to create a child based on the parent node >> just successfully created, and there is no delete issued >> * No client is able to acquire the lock even though the previous session >> who hold the lock already dead >> >> [Root cause] >> >> The direct cause of the misbehavior above is due to the key/value put into >> the ZooKeeperServer.outstandingChangesForPath HashMap or the >> DataNode.children HashSet are not visible to the future get or remove, >> which caused the outstanding changes not visible when leader prepare the >> following txns, or node being deleted but not removed from >> DataNode.children. >> >> And the 'bad' HashMap/HashSet behavior is not because of concurrency bugs >> inside ZK, but due to a macro-assembly bug which is used to generate the >> String.equals intrinsic assembly code in JDK 9 and 10. The bug was >> introduced in JDK-8144771 when adding AVX-512 instructions support in JDK >> to optimize the String.equals intrinsic performance with 512 bit vector op >> support. Due to the bug, the String.equals method may return false result >> when using high band of CPU register (xmm16 - xmm31) with non-empty stack >> on SKL machines where AVX-512 is available. >> >> The macro-assembly bug we hit is in vptest which is used in the >> string_compare macro assembly code >> < >> http://hg.openjdk.java.net/jdk/jdk10/file/b09e56145e11/src/hotspot/cpu/x86/macroAssembler_x86.cpp#l4933 >>> . >> It uses add/sub instruction when saving/resuming register values >> temporarily from stack, which will affect and distort the ZF (zero flag) in >> FLAGS register from the previous test instruction. >> >> For our case, if the key exist in the DataNode.children HashSet, the test >> instruction result will be zero, ZF bit will be set to 1, if the RSP value >> is not 0 (e.g stack is not empty) after addptr code here, then the ZF bit >> will be changed to 0, so String.equals compare during removeNode will >> return false result, and the key won't be removed. >> >> There is bug reported in JDK-8207746, the behavior is different, we've >> confirmed the issue by adding assembly code to log the issue in JDK 10. >> >> [Solutions] >> >> The possible mitigations are: >> >> 1. Disabling the AVX-512 with JVM option -XX:UseAVX=2 >> 2. Using OpenJDK version higher than 10, which has fixed the issue in >> JDK-8207746 >> >> Upgrading to OpenJDK 11+ is a better option, since 10 is not well >> supported, and AVX-512 do helps improving performance. >> >> We use JDK 10 due to SSL quorum socket close stall issue mentioned in >> ZOOKEEPER-3384 <https://issues.apache.org/jira/browse/ZOOKEEPER-3384>, and >> the SO_LINGER option is not honored in JDK 11. We've unblocked JDK 11 by >> asynchronously closing the quorum socket, and we're upstreaming that in >> ZOOKEEPER-3574 <https://issues.apache.org/jira/browse/ZOOKEEPER-3574>. >> >> Thanks, >> Fangmin >> > > > Thank you for sharing this. > Do you have any pointer to the jdk11 bugs? Is it solved in 12+? > > I am running with jdk11-13 but without ssl, so never seen problems. > > Enrico > >>
