On Mon, Oct 28, 2019 at 12:06 AM 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. > 10x - thanks! Patrick > 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 > > > >
