Ping #2. I still need one more reviewer (Thanks for the review, Dan). I
updated the webrev based on Dan's comments:
http://cr.openjdk.java.net/~cjplummer/8244383/webrev.01/
I can still make the simplification mentioned below if necessary.
thanks,
Chris
On 6/23/20 11:29 AM, Chris Plummer wrote:
Ping!
If this fix is too complicated, there is a simplification I can make,
but at the cost of abandoning some attempts to determine the current
frame when this error condition pops up. At the start of
validateInterpreterFrame() it attempts to verify that the frame is
valid by verifying that frame->method and frame->bcp are valid. This
part is pretty simple. The complicated part is everything that follows
if the verification fails. It attempts to error correct the situation
by looking at various register contents and stack contents. I could
just abandon this complicated code and return false if frame->method
and frame->bcp don't check out. Upon return, the caller's code would
be simplified to:
if (validateInterpreterFrame(sp, fp, pc)) {
return true; // We're done. setValues() has been called
for valid interpreter frame.
} else {
return checkLastJavaSP();
}
So there's still a chance we can determine a valid current frame if
"last java frame" has been setup. However, if not setup we would not
be able to. This is where the complicated code in
validateInterpreterFrame() is useful because it can usually determine
the current frame, even if "last java frame" is not setup, but it's
rare enough that we run into this situation that I think failing to
get the current frame is ok.
So if I can get a couple promises for reviews if I make this change,
I'll go ahead and do it and send out a new RFR.
thanks,
Chris
On 6/18/20 5:54 PM, Chris Plummer wrote:
[I've added runtime-dev to this SA review since understanding
interpreter invokes (code generated by
TemplateInterpreterGenerator::generate_normal_entry()) and stack
walking is probably more important than understanding SA.]
Hello,
Please help review the following:
https://bugs.openjdk.java.net/browse/JDK-8244383
http://cr.openjdk.java.net/~cjplummer/8244383/webrev.00/index.html
The crux of the bug is when doing stack walking the topmost frame is
in an inconsistent state because we are in the middle of pushing a
new interpreter frame. Basically we are executing code generated by
TemplateInterpreterGenerator::generate_normal_entry(). Since the PC
register is in this code, SA assumes the topmost frame is an
interpreter frame.
The first issue with this interpreter frame assumption is if we
haven't actually pushed the frame yet, then the current frame is the
caller's frame, and could be compiled. But since SA thinks it's
interpreted, later on it tries to convert the frame->bcp to a BCI,
but frame->bcp is only valid for interpreter frames. Thus the
"illegal BCI" failures. If the previous frame happened to be
interpreted, then the existing SA code works fine.
The other state of frame pushing that was problematic was when the
new frame had been pushed, but frame->method and frame->bcp were not
setup yet. This also would lead to "illegal BCI" later on because
garbage would be stored in these locations.
Fixing the above problems requires trying to determine the state of
the frame push through a series of checks, and then adapting what is
considered to be the current frame based on the outcome of the
checks. The first things checked is that frame->method is valid (we
can successfully instantiate a wrapper for the Method* without
failure) and that frame->bcp is within the method. If both these pass
then we can use the frame as-is.
If the above checks fail, then we try to determine whether the issue
is that the frame is not yet pushed and the current frame is actually
compiled, or the frame has been pushed but not yet initialized. This
is done by first getting the return address from the stack or RAX
(it's location depends on how far along we are in the entry code) and
comparing this to what is stored in frame->return_addr. If they are
the same, then we have pushed the frame but not yet initialized it.
In this case we use the previous frame (senderSP() and senderFP()) as
the current frame since the current frame is not yet initialized. If
the return address check fails, then we assume the new frame is not
yet pushed, and and treat the current frame as compiled, even though
PC points into the interpreter (we replace PC with RAX in this case).
Comments in the code pretty well explain all the above, so it is
probably easier to follow the logic in the code along with the
comments rather than apply my above description to the code.
I should add that it's very rare that we ever get into this special
error handling code. This bug was very hard to reproduce initially. I
was only able to make progress with reproducing and debugging by
inserting delay loops in various spots in the code generated by
TemplateInterpreterGenerator::generate_normal_entry(). By doing this
I was able to reproduce the issue quite easily and hit all the logic
in the new code I've added.
The fix is basically entirely contained within
AMD64CurrentFrameGuess.java. The rest of the changes are minor:
src/jdk.hotspot.agent/share/classes/sun/jvm/hotspot/runtime/amd64/AMD64CurrentFrameGuess.java
-Main fix for CR
src/jdk.hotspot.agent/share/classes/sun/jvm/hotspot/runtime/x86/X86Frame.java
-Added getInterpreterFrameBCP(), which is now needed by
AMD64CurrentFrameGuess.java
-I also simplified some code by using the existing
getInterpreterFrameMethod()
rather than replicating inline what it does.
src/jdk.hotspot.agent/share/classes/sun/jvm/hotspot/debugger/bsd/amd64/BsdAMD64CFrame.java
-I noticed the windows version of this code had some extra checks
that were missing
from the bsd version. I then looked at the linux version, but it had
been heavily modified
a short while back to leverage DWARF info to determine frames. So I
looked at the previous
rev and it too had these extra checks. I decided to add them to the
BSD port. I'm not sure
if it helps at all, but it certainly doesn't seem to do any harm.
thanks,
Chris
