Commits on Dec 14, 2023

1. Prevent requesting exclusive, if already acquired …

   This is to prevent requestExclusiveVMAccessMetronomeTemp (used from
   alarm thread) from blocking if exclusive VM access is already requested
   prior to calling a system GC. This prevention
   (synchronizeRequestsFromExternalThread) already works for plain
   exclusive, but not if safePoint is acquired (since it does not set
   exclusiveAccessState to J9_XACCESS_EXCLUSIVE).
   
   While proper solution might be around fixing
   synchronizeRequestsFromExternalThread to account for safePointState and
   how exclusiveAccessState is changed during safePoint acquire, it would
   require more time to get it right.
   
   This solution is a workaround where there is an additional flag that is
   to be set by after safePoint is acquired and before GC is invoked. That
   flag will be checked by requestExclusiveVMAccessMetronomeTemp before
   actually proceeding with the request.
   
   There should be no timing hole where
   requestExclusiveVMAccessMetronomeTemp is called after safe point is
   acquired and the flag is set, since
   requestExclusiveVMAccessMetronomeTemp itself cannot be invoked if GC is
   not in progress (and indeed system GC has not been triggered yet).
   
   Signed-off-by: Aleksandar Micic <Aleksandar_Micic@ca.ibm.com>

   Aleksandar Micic authored and Aleksandar Micic committed Dec 14, 2023
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