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

[Frederic Parain]

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