RFR(S): JDK-8146546 assert(fr->safe_for_sender(thread)) failed: Safety check

Frederic Parain frederic.parain at oracle.com
Fri Sep 23 17:16:00 UTC 2016


Thank you for the review.

My answers are in-lined below.

On 09/15/2016 11:33 AM, Daniel D. Daugherty wrote:
> On 9/15/16 8:44 AM, Frederic Parain wrote:
>> Greetings,
>> Please review this small fix for bug JDK-8146546:
>> https://bugs.openjdk.java.net/browse/JDK-8146546
>> Initial bug report is about an assertion failure in the reserved
>> stack code. The failing assertion calls safe_for_sender() after
>> the reconstruction of the first frame to initiate the stack
>> walking.
>> After investigation, it appears that the issue is that
>> safe_for_sender() is used for different purposes in different contexts.
>> JFR uses this method to check if it is safe to walk the stack, if the
>> method returns false, JFR simply records the current event without
>> stack information. JFR has to be very conservative on the conditions to
>> be satisfied to safely walk the stack, because JFR events could occur
>> at any time.
>> In the current case, safe_for_sender() is not called by JFR, but by the
>> reserved stack management code. The implementation of the reserved
>> stack requires to walk the stack too, but always on well defined points
>> in execution: when the stack banging is performed to detect potential
>> stack overflow ahead of time. Because the reserved stack code knows
>> exactly the state of the stack when it has to browse it, it has less
>> constraints than the JFR code. The condition that makes
>> safe_for_sender() to return false here, and by consequence causes the
>> assertion failure, are harmless for the reserved stack code.
>> Removing the condition in safe_for_sender() doesn't seem a good idea,
>> as it could be harmful for JFR code.
>> Modifying safe_for_sender() to support both usages would make this
>> method even more ugly.
>> However, removing the assertion in the reserved stack code would be
>> harmless, this is the solution proposed by this fix:
>> http://cr.openjdk.java.net/~fparain/8146546/webrev.00/index.html
> src/os/windows/vm/os_windows.cpp
>     No comments on the assert removal.
> src/os_cpu/linux_x86/vm/os_linux_x86.cpp
>     No comments on the assert removal.
> src/os_cpu/solaris_x86/vm/os_solaris_x86.cpp
>     No comments on the assert removal.
> We're used to seeing this code pattern in stack walking:
>     assert(fr->safe_for_sender(thread), "Safety check");
>     *fr = fr->java_sender();
> so seeing a java_sender() call without the assert is a bit
> troubling without an explanation. Perhaps you can add a
> comment like this above each get_frame_at_stack_banging_point()
> function definition:
>     // get_frame_at_stack_banging_point() is only called when we
>     // have well defined stacks so java_sender() calls do not need
>     // to assert safe_for_sender() first.

Done, comment has been added at each place where the assert has
been removed.

> And if we are really saying that the above comment is true, then
> hotspot/src/os_cpu/solaris_sparc/vm/os_solaris_sparc.cpp should
> also be updated... Are there others? Are they also safe?

We support the reserved stack area only on x86 and SPARC, and
the SPARC version of safe_for_sender() doesn't contain the checks
that are causing issues on the x86 platform. So this fix is
specific to the x86 platform.

New webrev:

Thank you,


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