RFR(XXS): 8149803: Adjust lock rankings for some Event-based tracing locks

[David Holmes]

Hi Markus,

On 20/02/2016 1:55 AM, Markus Gronlund wrote:
> Greetings,
>
> Please review this small change lowering the lock rankings  of some locks.

Have we actually verified the new ranking constraints (ie that special 
guarantees not to block) by code inspection?

> This is done in order to reduce the risk for potential deadlocks and to
> increase the surface area for event generation.
>
> Bug: https://bugs.openjdk.java.net/browse/JDK-8149803
>
> Patch of this tiny change is attached.

-  def(JfrStream_lock               , Mutex,   nonleaf,     true, 
Monitor::_safepoint_check_never);
+  def(JfrStream_lock               , Mutex,   special+1,   true, 
Monitor::_safepoint_check_never);


Not clear what "special+1" is supposed to signify here - doesn't that 
make it the same rank as suspend_resume?

   enum lock_types {
        event,
        special,
        suspend_resume,
        leaf        = suspend_resume +   2,
        safepoint   = leaf           +  10,
        barrier     = safepoint      +   1,
        nonleaf     = barrier        +   1,
        max_nonleaf = nonleaf        + 900,
        native      = max_nonleaf    +   1
   };


Thanks,
David


> Thanks in advance
>
> Markus
>