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Comment: update description of soft handshakes with respect to the new behaviour (collector threads do not take part)

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Various VM services, such as the garbage collector and locking, may wish to request a thread to block. In some cases, we want to block all threads except for the thread that makes the request. As well, some VM services may only wish for a "soft handshake", where we wait for each non-collector thread to perform some action exactly once and then continue (in this case, the only thread that blocks is the thread requesting the soft handshake, but all other non-collector threads must "yield" in order to perform the requested action; in most cases that action is non-blocking). A unified facility for performing all of these requests is provided by RVMThread.

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  • RVMThread.block(). This is a low-level facility for requesting that a particular thread blocks. It is inherently unsafe to use this facility directly - for example, if thread A calls B.block() while thread B calls A.block(), the two threads may mutually deadlock.
  • RVMThread.beginPairHandshake(). This implements a safe pair-handshaking mechanism, in which two threads become bound to each other for a short time. The thread requesting the pair handshake waits until the other thread is at a safe point or else is "effectively safe", and prevents it from going back to executing Java code. Note that at this point, neither thread will respond to any other handshake requests until RVMThread.endPairHandshake() is called. This is useful for implementing biased locking, but it has general utility anytime one thread needs to manipulate something another thread's execution state.
  • RVMThread.softHandshake(). This implements soft handshakes. In a soft handshake, the requesting thread waits for all non-collector threads to perform some action exactly once, and then returns. If any of those threads are effectively safe, then the requesting thread performs the action on their behalf. softHandshake() is invoked with a SoftHandshakeVisitor that determines which threads are to be affected, and what the requested action is. An example of how this is used is found in org.jikesrvm.mm.mmtk.Collection and org.jikesrvm.compilers.opt.runtimesupport.OptCompiledMethod.
  • RVMThread.hardHandshakeSuspend(). This stops all threads except for the garbage collector threads and the thread making the request. It returns once all Java threads are stopped. This is used by the garbage collector itself, but may be of utility elsewhere (for example, dynamic software updating). To resume all stopped threads, call RVMThread.hardHandshakeResume(). Note that this mechanism is carefully designed so that even after the world is stopped, it is safe to request a garbage collection (in that case, the garbage collector will itself call a variant of hardHandshakeSuspend(), but it will only affect the one remaining running Java thread).

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