On Thu, 17 Sep 2020 19:51:24 GMT, Robbin Ehn <r...@openjdk.org> wrote:
>> This patch implements asynchronous handshake, which changes how handshakes
>> works by default. Asynchronous handshakes
>> are target only executed, which they may never be executed. (target may
>> block on socket for the rest of VM lifetime)
>> Since we have several use-cases for them we can have many handshake pending.
>> (should be very rare) To be able handle an
>> arbitrary amount of handshakes this patch adds a per JavaThread queue and
>> heap allocated HandshakeOperations. It's a
>> singly linked list where you push/insert to the end and pop/get from the
>> front. Inserts are done via CAS on first
>> pointer, no lock needed. Pops are done while holding the per handshake state
>> lock, and when working on the first
>> pointer also CAS. The thread grabbing the handshake state lock for a
>> JavaThread will pop and execute all handshake
>> operations matching the filter. The JavaThread itself uses no filter and any
>> other thread uses the filter of everything
>> except asynchronous handshakes. In this initial change-set there is no need
>> to do any other filtering. If needed
>> filtering can easily be exposed as a virtual method on the HandshakeClosure,
>> but note that filtering causes handshake
>> operation to be done out-order. Since the filter determins who execute the
>> operation and not the invoked method, there
>> is now only one method to call when handshaking one thread. Some comments
>> about the changes:
>> - HandshakeClosure uses ThreadClosure, since it neat to use the same closure
>> for both alla JavThreads do and Handshake
>> all threads. With heap allocating it cannot extends StackObj. I tested
>> several ways to fix this, but those very much
>> worse then this.
>>
>> - I added a is_handshake_safe_for for checking if it's current thread is
>> operating on itself or the handshaker of that
>> thread.
>>
>> - Simplified JVM TI with a JvmtiHandshakeClosure and also made them not
>> needing a JavaThread when executing as a
>> handshaker on a JavaThread, e.g. VM Thread can execute the handshake
>> operation.
>>
>> - Added WB testing method.
>>
>> - Removed VM_HandshakeOneThread, the VM thread uses the same call path as
>> direct handshakes did.
>>
>> - Changed the handshake semaphores to mutex to be able to handle deadlocks
>> with lock ranking.
>>
>> - VM_HandshakeAllThreadsis still a VM operation, since we do support half of
>> the threads being handshaked before a
>> safepoint and half of them after, in many handshake all operations.
>>
>> - ThreadInVMForHandshake do not need to do a fenced transistion since this
>> is always a transistion from unsafe to unsafe.
>>
>> - Added NoSafepointVerifyer, we are thinking about supporting safepoints
>> inside handshake, but it's not needed at the
>> moment. To make sure that gets well tested if added the
>> NoSafepointVerifyer will raise eyebrows.
>>
>> - Added ttyLocker::break_tty_lock_for_safepoint(os::current_thread_id());
>> due to lock rank.
>>
>> - Added filtered queue and gtest for it.
>>
>> Passes multiple t1-8 runs.
>> Been through some pre-reviwing.
>
> Robbin Ehn has updated the pull request incrementally with one additional
> commit since the last revision:
>
> Removed double check, fix comment, removed not needed function, updated logs
src/hotspot/share/prims/jvmtiEnvBase.cpp line 653:
> 651: JvmtiEnvBase::get_current_contended_monitor(JavaThread *calling_thread,
> JavaThread *java_thread,
> jobject *monitor_ptr) { 652: Thread *current_thread = Thread::current();
> 653: assert(java_thread->is_handshake_safe_for(current_thread),
I like how this assert reads now!
-------------
PR: https://git.openjdk.java.net/jdk/pull/151
