RFR: 8273608: Deadlock when jcmd of OnError attaches to itself [v5]

[Xin Liu]

On Wed, 6 Oct 2021 06:30:36 GMT, Xin Liu <xliu at openjdk.org> wrote:

>> This patch allows the custom commands of OnError to attach to HotSpot itself. 
>> It sets the thread of report_and_die() to Native before os::fork_and_exec(cmd). 
>> This prevents cmds which require safepoint synchronization from deadlock.
>> eg. OnError='jcmd %p Thread.print'.
>> 
>> Without this patch, we will encounter a deadlock at safepoint synchronization. 
>> `"main" #1`  is the very thread which executes `os::fork_and_exec(cmd)`.  
>> 
>> 
>> Aborting due to java.lang.OutOfMemoryError: Java heap space
>> #
>> # A fatal error has been detected by the Java Runtime Environment:
>> #
>> #  Internal Error (debug.cpp:364), pid=94632, tid=94633
>> #  fatal error: OutOfMemory encountered: Java heap space
>> #
>> # JRE version: OpenJDK Runtime Environment (18.0) (build 18-internal+0-adhoc.xxinliu.jdk)
>> # Java VM: OpenJDK 64-Bit Server VM (18-internal+0-adhoc.xxinliu.jdk, mixed mode, tiered, compressed oops, compressed class ptrs, g1 gc, linux-amd64)
>> # No core dump will be written. Core dumps have been disabled. To enable core dumping, try "ulimit -c unlimited" before starting Java again
>> #
>> # An error report file with more information is saved as:
>> # /local/home/xxinliu/JDK-2085/hs_err_pid94632.log
>> #
>> # -XX:OnError="jcmd %p Thread.print"
>> #   Executing /bin/sh -c "jcmd 94632 Thread.print" ...
>> 94632:
>> [10.616s][warning][safepoint]
>> [10.616s][warning][safepoint] # SafepointSynchronize::begin: Timeout detected:
>> [10.616s][warning][safepoint] # SafepointSynchronize::begin: Timed out while spinning to reach a safepoint.
>> [10.616s][warning][safepoint] # SafepointSynchronize::begin: Threads which did not reach the safepoint:
>> [10.616s][warning][safepoint] # "main" #1 prio=5 os_prio=0 cpu=236.97ms elapsed=10.61s tid=0x00007f01b00232f0 nid=94633 runnable  [0x00007f01b7a08000]
>> [10.616s][warning][safepoint]    java.lang.Thread.State: RUNNABLE
>> [10.616s][warning][safepoint]
>> [10.616s][warning][safepoint] # SafepointSynchronize::begin: (End of list)
>
> Xin Liu has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Update per reviewers' feedbacks.
>   
>   1. rename the test to TestOnErrorWithSelfAttachingJCmd.java.
>   2. rename java_current_transition_into_native() and add the static qualifier.
>   3. make unlock_locks_on_error more general. ensure formal name is consistent
>   with argument name.

1. `HeapDumpOnOutOfMemoryError` can't cover OOME from Java world and thread exhaustion.  
2. I admit that unlocking mutexes is hacky.  To be fair, this problem predates this patch.   There is [Jfr::on_vm_shutdown(true)](https://github.com/openjdk/jdk/blob/master/src/hotspot/share/utilities/vmError.cpp#L1560) in VMError::report_and_die. if you use ""-XX:+FlightRecorder", "-XX:StartFlightRecording:dumponexit=true", this will tamper the coredump because it unlocks all holding mutexes. 

3. I just learned from David that "you can't unlock a lock that belongs to another thread!". I need to clean up this. 

I would like to provide a general solution to the temporary Stop-The-world. Not only for OnError=jcmd %, but also for JFR dumponexit. I think I can have a RAII object to do the bookkeeping of unlocked mutexes.  I will clean up everything for coredump.

-------------

PR: https://git.openjdk.java.net/jdk/pull/5590