On Mon, 12 Apr 2021 10:53:46 GMT, Robbin Ehn <r...@openjdk.org> wrote:
>> A suspend request is done by handshaking thread target thread(s). When >> executing the handshake operation we know the target mutator thread is in a >> dormant state (as in safepoint safe state). We have a guarantee that it will >> check it's poll before leaving the dormant state. To stop the thread from >> leaving the the dormant state we install a second asynchronous handshake to >> be executed by the targeted thread. The asynchronous handshake will wait on >> a monitor while the thread is suspended. The target thread cannot not leave >> the dormant state without a resume request. >> >> Per thread suspend requests are naturally serialized by the per thread >> HandshakeState lock (we can only execute one handshake at a time per thread). >> Instead of having a separate lock we use this to our advantage and use >> HandshakeState lock for serializing access to the suspend flag and for >> wait/notify. >> >> Suspend: >> Requesting thread -> synchronous handshake -> target thread >> Inside synchronus handshake (HandshakeState lock is locked while >> executing any handshake): >> - Set suspended flag >> - Install asynchronous handshake >> >> Target thread -> tries to leave dormant state -> Executes handshakes >> Target only executes asynchronous handshake: >> - While suspended >> - Go to blocked >> - Wait on HandshakeState lock >> >> Resume: >> Resuming thread: >> - Lock HandshakeState lock >> - Clear suspended flag >> - Notify HandshakeState lock >> - Unlock HandshakeState lock >> >> The "suspend requested" flag is an optimization, without it a dormant thread >> could be suspended and resumed many times and each would add a new >> asynchronous handshake. Suspend requested flag means there is already an >> asynchronous suspend handshake in queue which can be re-used, only the >> suspend flag needs to be set. >> >> ---- >> Some code can be simplified or done in a smarter way but I refrained from >> doing such changes instead tried to keep existing code as is as far as >> possible. This concerns especially raw monitors. >> >> ---- >> Regarding the changed test, the documentation says: >> "If the calling thread is specified in the request_list array, this function >> will not return until some other thread resumes it." >> >> But the code: >> LOG("suspendTestedThreads: before JVMTI SuspendThreadList"); >> err = jvmti->SuspendThreadList(threads_count, threads, results); >> ... >> // Allow the Main thread to inspect the result of tested threads >> suspension >> agent_unlock(jni); >> >> The thread will never return from SuspendThreadList until resumed, so it >> cannot unlock with agent_unlock(). >> Thus main thread is stuck forever on: >> // Block until the suspender thread competes the tested threads suspension >> agent_lock(jni); >> >> And never checks and resumes the threads. So I removed that lock instead >> just sleep and check until all thread have the expected suspended state. >> >> ---- >> >> This version already contains updates after pre-review comments from >> @dcubed-ojdk, @pchilano, @coleenp. >> (Pre-review comments here: >> https://github.com/openjdk/jdk/pull/2625) >> >> ---- >> Testing t1-t8, nsk_jdi/nsk_jvmti/jdk_jdi/tck, KS24, RunThese and >> combinations like running with -XX:ZCollectionInterval=0.01 - >> XX:ZFragmentationLimit=0. >> Running above some of above concurrently (load ~240), slow debug, >> etc... > > Robbin Ehn has updated the pull request with a new target base due to a merge > or a rebase. The pull request now contains eight commits: > > - Merge branch 'master' into SuspendInHandshake > - Review fixes 2 > - White space fixes > - Merge branch 'master' into SuspendInHandshake > - Review fixes > - Merge branch 'master' into SuspendInHandshake > - Merge branch 'master' into SuspendInHandshake > - 8257831: Suspend with handshake (review baseline) I only found nits so still looks good to me. src/hotspot/share/runtime/handshake.cpp line 466: > 464: // The exception to this rule is the asynchronous suspension > handshake. > 465: // It by-passes the NSV by manully doing the transition. > 466: // Since we can have safepoints while suspeneded we need to release > the tty locker if it is held. nit typo: s/manully/manually/ nit typo: s/suspeneded/suspended/ src/hotspot/share/runtime/handshake.hpp line 133: > 131: // check the _lock, if held go to slow path. > 132: // Since the handshakee is unsafe if _lock gets lock after this check > 133: // we know another threads cannot process any handshakes. nit typo: s/_lock gets lock/_lock gets locked/ nit typo: s/threads/thread/ src/hotspot/share/runtime/thread.cpp line 1448: > 1446: // The careful dance between thread suspension and exit is handled > here. > 1447: // Since we are in vm and suspension is done with handshakes, > 1448: // we can just put in the exiting state and it will be corrcetly > handled. Perhaps: s/Since we are in vm and/Since we are in thread_in_vm state and/ nit typo: s/corrcetly/correctly/ ------------- Marked as reviewed by dcubed (Reviewer). PR: https://git.openjdk.java.net/jdk/pull/3191