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

[dean.long at oracle.com]

Hi Frederic.  I see in the bug report that Tobias was wondering why 
safe_for_sender() checks for deoptimized frames on x86.  My guess is 
that it was to avoid bug JDK-8161598, which is now fixed.  If that's the 
case, then it would be safe to remove that code from safe_for_sender().  
It looks like Staffan added that code, so maybe he remembers the reason 
it was added.

dl


On 9/15/16 7:44 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
>
> Thank you,
>
> Fred