On 6/24/20 12:01 AM, Yasumasa Suenaga wrote:
On 2020/06/24 15:32, Chris Plummer wrote:
Hi Yasumasa ,
I think LinuxAMD64CFrame is used for pstack and what I've been
looking at has been jstack, and in particular AMD64CurrentFrameGuess,
which does use "last java frame". I'm not sure why LinuxAMD64CFrame
does not look at "last java frame". Maybe it should.
I thought both pattern (jstack, mixed stack) for this change.
As you know, mixed jstack (jstack --mixed) attempt to find top of
native stack via LinuxAMD64CFrame, register values are needed for it
(so it depends on ptrace() call). So I guess mixed mode jstack (jhsdb
jstack --mixed) would not show any stacks (cannot find "last java
frame").
Hi Yasumasa,
I should have been more clear on what I meant by jstack and pstack. For
jstack I meant using StackTrace.java, which is what you get by default
with "jhsdb jstack" and also the clhsdb jstack command. For pstack I
meant PStack.java, which is what you get with "jhsdb jstack --mixed" or
the clhsdb pstack command.
So this CR impacts both types of stack traces in that they will get null
registers when the the lower level API fails to get the register set.
For StackTrace.java it will then defer to "last java frame" if
available. For PStack.java it will not, and will always result in no
stack trace. The code of interest is here:
AMD64ThreadContext context = (AMD64ThreadContext)
thread.getContext();
Address pc = context.getRegisterAsAddress(AMD64ThreadContext.RIP);
if (pc == null) return null;
return LinuxAMD64CFrame.getTopFrame(dbg, pc, context);
So the question is should "last java frame" be used if pc == null. If
so, then getTopFrame() would also need to be modified to use "last java
frame" when fetching RBP.
thanks,
Chris
Thanks,
Yasumasa
thanks,
Chris
On 6/23/20 11:04 PM, Yasumasa Suenaga wrote:
Hi Chris,
Thanks you for explanation.
Your change looks good (but "last java frame" would not be found in
Linux AMD64 because RSP is NULL - cf. LinuxAMD64CFrame.java)
Thanks,
Yasumasa
On 2020/06/24 12:09, Chris Plummer wrote:
On 6/23/20 6:05 PM, Yasumasa Suenaga wrote:
Hi Chris,
Skillful troubleshooters who use jhsdb will aware this warnings,
and they will take other appropriate methods.
However, I'm not sure it is worth to continue to perform even if
SA cannot get register values.
For example, Linux AMD64 depends on RIP and RSP values to find top
frame.
According to your change, The caller of
getThreadIntegerRegisterSet() has responsible for dealing with the
set of null registers. However X86ThreadContext::data (it includes
raw register values) would still be zero when it happens.
This is what I intended to have happen. Just end up with a
register set of all nulls. Then when stack walking code gets a
null, it will revert to "last java frame" if available, otherwise
no stack dump is done.
So I think register holder (e.g. X86ThreadContext) should have
tri-state (have registers, fail to get registers, not yet attempt
to get registers).
OTOH it might be over-engineering. What do you think?
Before implementing this I looked at the what would be the easier
approach to get the desired effect of stack walking code simply
failing over to using "last java frame", and decided the null set
of registers was easiest. Other approaches involved more changes
and impacted more files.
thanks,
Chris
Thanks,
Yasumasa
On 2020/06/24 3:16, Chris Plummer wrote:
On 6/20/20 12:53 AM, Yasumasa Suenaga wrote:
Hi Chris,
On 2020/06/20 15:20, Chris Plummer wrote:
Hi Yasumasa,
ptrace is not used for core files, so the EFAULT for a bad core
file is not a possibility. However, get_lwp_regs() does
redirect to core_get_lwp_regs() for core files. It can fail,
but the only reason it ever does is if the LWP can't be found
in the core (which is never suppose to happen). I would think
if this happened due to the core being truncated, SA would be
blowing up all over the place with exceptions, probably before
we ever get to this code, but in any cast what we do here
wouldn't really make a difference.
You are right, sorry.
I'm not sure why you prefer an exception for errors other than
ESRCH. Why should they be treated differently?
getThreadIntegerRegisterSet0() is used for finding the current
frame for stack tracing. With my changes any failure will
result in deferring to "last java frame" if set, and otherwise
just not produce a stack trace (and the WARNING will be present
in the output). This seems preferable to completely abandoning
any further thread stack tracking.
I'm not sure we can trust call stack when ptrace() returns any
errors other than ESRCH even if "last java frame" is available.
For example, don't ptrace() return EFAULT or EIO when something
wrong? (e.g. stack corruption) If so, it may lead to a wrong
analysis for troubleshooter.
I think it should be abort dumping call stack for its thread at
least.
Hi Yasumasa,
In general stack walking makes a best effort and can be wrong,
even when not getting errors like this. For any actively
executing thread SA needs to determine where the stack starts,
with register contents being the starting point (SP, FP, and PC).
These registers could contain anything, and SA makes a best
effort to determine a current frame from them. However, the
verification steps it takes are not 100% guaranteed, and can lead
to an incorrect assumption of the current frame, which in turn
can result in an exception later on when walking the stack. See
JDK-8247641.
Keep in mind that the WARNING message will always be there. This
should be enough to put the troubleshooter on alert that the
stack trace may not be accurate. I think it's better to make an
attempt at a stack trace then to just abandon it and not attempt
to do something that may be useful.
thanks,
Chris
Thanks,
Yasumasa
thanks,
Chris
On 6/19/20 6:33 PM, Yasumasa Suenaga wrote:
Hi Chris,
I checked Linux kernel code at a glance, ESRCH seems to be set
to errno by default.
So I guess it is similar to "generic" error code.
https://github.com/torvalds/linux/blob/master/kernel/ptrace.c
According to manpage of ptrace(2), it might return errno other
than ESRCH.
For example, if we analyze broken core (e.g. the core was
dumped with disk full), we might get EFAULT.
Thus I prefer to handle ESRCH only in your patch, and also I
think SA should throw DebuggerException if other error is
occurred.
https://www.man7.org/linux/man-pages/man2/ptrace.2.html
Thanks,
Yasumasa
On 2020/06/20 5:51, Chris Plummer wrote:
Hello,
I've updated with webrev based on the new finding that a
JavaThread cannot be on the ThreadList after its OS thread
has been destroyed since the JavaThread removes itself from
the ThreadList, and therefore must be running on its OS
thread. The logic of the fix is unchanged from the first
webrev, but I updated the comments to better reflect what is
going on. I also updated the CR:
https://bugs.openjdk.java.net/browse/JDK-8247533
http://cr.openjdk.java.net/~cjplummer/8247533/webrev.01/index.html
thanks,
Chris
On 6/19/20 12:24 AM, David Holmes wrote:
Hi Chris,
On 19/06/2020 8:55 am, Chris Plummer wrote:
On 6/18/20 1:43 AM, David Holmes wrote:
On 18/06/2020 4:49 pm, Chris Plummer wrote:
On 6/17/20 10:29 PM, David Holmes wrote:
On 18/06/2020 3:13 pm, Chris Plummer wrote:
On 6/17/20 10:09 PM, David Holmes wrote:
On 18/06/2020 2:33 pm, Chris Plummer wrote:
On 6/17/20 7:43 PM, David Holmes wrote:
Hi Chris,
On 18/06/2020 6:34 am, Chris Plummer wrote:
Hello,
Please help review the following:
https://bugs.openjdk.java.net/browse/JDK-8247533
http://cr.openjdk.java.net/~cjplummer/8247533/webrev.00/index.html
The CR contains all the needed details. Here's a
summary of changes in each file:
The problem sounds to me like a variation of the
more general problem of not ensuring a thread is
kept alive whilst acting upon it. I don't know how
the SA finds these references to the threads it is
going to stackwalk, but is it possible to fix this
via appropriate uses of ThreadsListHandle/Iterator?
It fetches ThreadsSMRSupport::_java_thread_list.
Keep in mind that once SA attaches, nothing in the VM
changes. For example, SA can't create a wrapper to a
JavaThread, only to have the JavaThread be freed
later on. It's just not possible.
Then how does it obtain a reference to a JavaThread
for which the native OS thread id is invalid? Any
thread found in _java_thread_list is either live or
still to be started. In the latter case the
JavaThread->osThread does not have its thread_id set yet.
My assumption was that the JavaThread is in the process
of being destroyed, and it has freed its OS thread but
is itself still in the thread list. I did notice that
the OS thread id being used looked to be in the range
of thread id #'s you would expect for the running app,
so that to me indicated it was once valid, but is no more.
Keep in mind that although hotspot may have
synchronization code that prevents you from pulling a
JavaThread off the thread list when it is in the
process of being destroyed (I'm guessing it does), SA
has no such protections.
But you stated that once the SA has attached, the target
VM can't change. If the SA gets its set of thread from
one attach then tries to make queries about those
threads in a separate attach, then obviously it could be
providing garbage thread information. So you would need
to re-validate the JavaThread in the target VM before
trying to do anything with it.
That's not what is going on here. It's attaching and
doing a stack trace, which involves getting the thread
list and iterating through all threads without detaching.
Okay so I restate my original comment - all the
JavaThreads must be alive or not yet started, so how are
you encountering an invalid thread id? Any thread you find
via the ThreadsList can't have destroyed its osThread. In
any case the logic should be checking thread->osThread()
for NULL, and then osThread()->get_state() to ensure it is
>= INITIALIZED before using the thread_id().
Hi David,
I chatted with Dan about this, and he said since the
JavaThread is responsible for removing itself from the
ThreadList, it is impossible to have a JavaThread still on
the ThreadList, but without and underlying OS Thread. So
I'm a bit perplexed as to how I can find a JavaThread on
the ThreadList, but that results in ESRCH when trying to
access the thread with ptrace. My only conclusion is that
this failure is somehow spurious, and maybe the issue it
just that the thread is in some temporary state that
prevents its access. If so, I still think the approach I'm
taking is the correct one, but the comments should be updated.
ESRCH can have other meanings but I don't know enough about
the broader context to know whether they are applicable in
this case.
ESRCH The specified process does not exist, or is
not currently being traced by the caller, or is not stopped
(for requests that require a stopped tracee).
I won't comment further on the fix/workaround as I don't
know the code. I'll leave that to other folk.
Cheers,
David
-----
I had one other finding. When this issue first turned up,
it prevented the thread from getting a stack trace due to
the exception being thrown. What I hadn't realize is that
after fixing it to not throw an exception, which resulted
in the stack walking code getting all nulls for register
values, I actually started to see a stack trace printed:
"JLine terminal non blocking reader thread" #26 daemon
prio=5 tid=0x00007f12f0cd6420 nid=0x1f99 runnable
[0x00007f125f0f4000]
java.lang.Thread.State: RUNNABLE
JavaThread state: _thread_in_native
WARNING: getThreadIntegerRegisterSet0: get_lwp_regs failed
for lwp (8089)
CurrentFrameGuess: choosing last Java frame: sp =
0x00007f125f0f4770, fp = 0x00007f125f0f47c0
- java.io.FileInputStream.read0() @bci=0 (Interpreted frame)
- java.io.FileInputStream.read() @bci=1, line=223
(Interpreted frame)
-
jdk.internal.org.jline.utils.NonBlockingInputStreamImpl.run()
@bci=108, line=216 (Interpreted frame)
-
jdk.internal.org.jline.utils.NonBlockingInputStreamImpl$$Lambda$536+0x0000000800daeca0.run()
@bci=4 (Interpreted frame)
- java.lang.Thread.run() @bci=11, line=832 (Interpreted
frame)
The "CurrentFrameGuess" output is some debug tracing I had
enabled, and it indicates that the stack walking code is
using the "last java frame" setting, which it will do if
current registers values don't indicate a valid frame (as
would be the case if sp was null). I had previously assumed
that without an underling valid LWP, there would be no
stack trace. Given that there is one, there must be a valid
LWP. Otherwise I don't see how the stack could have been
walked. That's another indication that the ptrace failure
is spurious in nature.
thanks,
Chris
Cheers,
David
-----
Also, even if you are using something like clhsdb to
issue commands on addresses, if the address is no longer
valid for the command you are executing, then you would
get the appropriate error when there is an attempt to
create a wrapper for it. I don't know of any command that
operates directly on a JavaThread, but I think there are
for InstanceKlass. So if you remembered the address of an
InstanceKlass, and then reattached and tried a command
that takes an InstanceKlass address, you would get an
exception when SA tries to create the wrapper for the
InsanceKlass if it were no longer a valid address for one.
Chris
David
-----
Chris
David
-----
Chris
Cheers,
David
src/jdk.hotspot.agent/linux/native/libsaproc/LinuxDebuggerLocal.cpp
src/jdk.hotspot.agent/macosx/native/libsaproc/MacosxDebuggerLocal.m
src/jdk.hotspot.agent/windows/native/libsaproc/sawindbg.cpp
-Instead of throwing an exception when the OS
ThreadID is invalid, print a warning.
src/jdk.hotspot.agent/linux/native/libsaproc/ps_proc.c
-Improve a print_debug message
src/jdk.hotspot.agent/share/classes/sun/jvm/hotspot/debugger/bsd/BsdThread.java
src/jdk.hotspot.agent/share/classes/sun/jvm/hotspot/debugger/linux/LinuxThread.java
src/jdk.hotspot.agent/share/classes/sun/jvm/hotspot/debugger/windbg/amd64/WindbgAMD64Thread.java
-Deal with the array of registers read in being
null due to the OS ThreadID not being valid.
src/jdk.hotspot.agent/share/classes/sun/jvm/hotspot/debugger/bsd/BsdDebuggerLocal.java
src/jdk.hotspot.agent/share/classes/sun/jvm/hotspot/debugger/linux/LinuxDebuggerLocal.java
-Fix issue with
"sun.jvm.hotspot.debugger.DebuggerException"
appearing twice when printing the exception.
thanks,
Chris
