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

[Frederic Parain]

Gerard,

Thank you for reviewing this change.

Good catch, I forgot the os_bsd_x86 platform, this is now fixed:

http://cr.openjdk.java.net/~fparain/8146546/webrev.01/

Fred

On 09/15/2016 02:25 PM, Gerard Ziemski wrote:
> hi Frederic,
>
> Why aren’t we removing the assert, which you proposed, from os_bsd_x86.cpp?
>
>
> cheers
>
>
>> On Sep 15, 2016, at 9:44 AM, Frederic Parain <frederic.parain at oracle.com> 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
>>
>> Thank you,
>>
>> Fred
>