On 26/02/2019 4:32 pm, Yasumasa Suenaga wrote:
Hi David,
2019年2月26日(火) 15:14 David Holmes <david.hol...@oracle.com>:
On 26/02/2019 4:01 pm, Yasumasa Suenaga wrote:
Hi all,
When I attached to VM which is after 8215622 change via jcmd in JDK 8
on Linux, jcmd hanged.
What command are you trying to execute through jcmd?
I tried VM.info, Thread.print, and GC.run .
Right a simple "jcmd <pid> help" hangs.
Do we support connection to later VMs from earlier JDK tools?
I could not find the spec about this.
So I asked to serviceability folks before filing this to JBS :-)
I'm not certain either but this is a huge compatibility change even if
not officially supported.
I checked the status of target VM and jcmd,
LinuxAttachListener::read_request() on target VM was waiting for
subsequent argument.
8215622 increased number of arguments of Attach API [1].
Isn't that supposed to be a maximum number of arguments? So if less are
sent it should still work okay?
This change affects to request reading on AttachListener.
AttachListener set expected string (includes arguments) count from
maximum number of arguments.
http://hg.openjdk.java.net/jdk/jdk/file/616a32d6b463/src/hotspot/os/linux/attachListener_linux.cpp#l247
This value uses as loop condition.
http://hg.openjdk.java.net/jdk/jdk/file/616a32d6b463/src/hotspot/os/linux/attachListener_linux.cpp#l297
That code looks wrong to me. It's a maximum number of args not the
expected number. As per the comment:
// Read until all (expected) strings have been read, the buffer is
// full, or EOF.
if we have fewer than max args we should get EOF and handle it fine!
Of course it is my mistake, but I think we can increase version number
(PROTOCOL_VERSION) because Attach API has been updated in 8215622.
What do you think?
That will just cause it be rejected rather than hang - right?
No, jcmd is hanged.
You can reproduce it on Linux box.
My environment:
Target VM: jdk/jdk fastdebug build (changeset: 53918:616a32d6b463)
jcmd: OpenJDK 8u201 (java-1.8.0-openjdk-1.8.0.201.b09-2.fc29.x86_64)
I checked the value of `str_count` and `left` on
LinuxAttachListener::read_request() in target VM via GDB.
Target VM waited subsequent arguments.
Yes but if we change the protocol version that will cause the incoming
request from the earlier jcmd to be rejected - right? Or are we going to
adjust the expected number of args based on the protocol version?
I'm trying to understand how changing the protocol version will fix the
problem.
Thanks,
David
-----
Thanks,
Yasumasa
Thanks,
David
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If it is accepted, I will file it to JBS.
Thanks,
Yasumasa
[1] http://hg.openjdk.java.net/jdk/jdk/rev/e7cf035682e3#l2.1
