tree 356d4d6319dbe0999aa259982f89ea5e20683552
parent 19d609b06c76702e29da84eaecee1f43fd5c7f9b
author Hans Boehm <hboehm@google.com> 1672977513 +0000
committer Hans Boehm <hboehm@google.com> 1680048736 +0000

Revert^8 "Thread suspension cleanup and deadlock fix"

This reverts commit c85ae17f8267ac528e58892099dcefcc73bb8a26.

PS1: Identical to aosp/2266238

PS2: Address the lint failure that was the primary cause of the revert.
     Don't print information about what caused a SuspendAll loop
     unless we actually gathered the information.

     Restructure thread flip to drop the assumption that certain threads
     will shortly become Runnable. That added a lot fo complexity and
     was deadlock-prone. We now simply try to run the flip function both
     in the target thread when it tries to become runnable again, and in
     the requesting thread, without paying attention to the target
     thread's state. The first attempt succeeds. Which means the
     originating thread will only succeed if the target is still
     suspended.

     This adds some complexity to deal with threads terminating in the
     meantime, but it avoids several issues:

     1) RequestSynchronousCheckpoint blocked with thread_list_lock
     and suspend_count_lock, while waiting for flip_function to become
     non-null. This made it at best hard to reason about deadlock
     freedom. Several other functions also had to wait for a null
     flip_function, complicating the code.

     2) Synchronization in FlipThreadRoots was questionable. In order to
     tell when to treat a thread as previously runnable, it looked at
     thread state bits that could change asynchronously. AFAICT, this
     was probably correct under sequential consistency, but not with
     the actual specified memory ordering. That code was deleted.

     3) If a thread was intended to become runnable shortly after
     the start of the flip, we paused it until all thread flips were
     completed. This probably occasionally added latency that
     escaped our measurements.

     Weaken several assertions involving IsSuspended() to merely
     claim the thread is not runnable, to be consistent with the
     above change. The stringer assertion no longer holds in the
     flip function.

     Assert that we never suspend while running the GC, to ensure
     the GC never acts on a thread suspension request, which would
     likely result in deadlock.

     Update mutator_gc_coord.md to reflect additional insights about
     this code.

     Change the last parameter of DecrementSuspendCount to be a boolean
     rather than SuspendReason, since we mostly ignore the precise
     SuspendReason.

     Add NotifyOnThreadExit mechanism so that we can tell whether
     a thread exited, even if we release thread_list_lock_.

     Rewrite RequestSynchronousCheckpoint to take advantage of the
     above. The new thread-flip code also uses it.

     Remove now unnecessary checks that we do not suspend with a thread
     flip in progress.

     Various secondary changes and simplifications that follow from the
     above.

     Reduce DefaultThreadSuspendTimeout to something below ANR timeout.

     Explicitly ensure that when FlipThreadRoots() returns, all thread
     flips have completed. Previously that was mostly true, but actually
     guaranteed by barrier code in the collector. Remove that code.
     (The old version was hard to fix in light of potential exiting
     threads.)

PS3: Rebase

PS4: Fix and complete PS2 changes.

PS5: Edit commit message.

PS6: Update entry_points_order_test, again.

PS7-8: Address many minor reviewer comments. Remove more dead code,
     including all the IsTransitioningToRunnable stuff.

PS9: Slightly messy rebase

PS10: Address comments. Most notably:
     SuspendAll now ensures that the caller is not left with a pending
     flip function.
     GetPeerFromOtherThread() sometimes runs the flip function instead
     of calling mark. The old way would not work for CMC. This makes it
     no longer const.

PS11: Fix a PS10 oversight, mostly in the CMC collector code.

PS12: Fix comment and documentation typos.

Test: Run host run tests. TreeHugger.
Bug: 240742796
Bug: 203363895
Bug: 238032384
Bug: 253671779

Change-Id: I81e366d4b739c5b48bd3c1509acb90d2a14e18d1