commit 83e460ab6ddf68874a39ffe9fbb6ebcaaeeaf7e0
author Martin Kustermann <kustermann@google.com> Wed Dec 02 14:51:07 2020 +0000
committer commit-bot@chromium.org <commit-bot@chromium.org> Wed Dec 02 14:51:07 2020 +0000
tree d901b8771595582b638f21ef3388d67256b2c310
parent 89811d9a926c11b0b6927d0f0b9441087f3d4964

[vm/concurrency] Make ICData use new type_feeback_mutex to prevent duplicate entries

Dart mutator threads cause update to runtime type feedback information
(e.g. ICData, MegamorphicCache). With multiple lightweight isolates
sharing JITed code, they will share the type feedback as well. That also
means multiple mutators can try to update type feedback at the same time
with the same (or different) feedback.

This means we can run into situations where we're trying to update type
feedback with information that's already there.
=> At a minimum we want to avoid adding duplicate entries to the ICData,
   so we have to guard against that.

Since the actual backing store to ICData is immutable, we have two
choices:

  * Let each thread read the current backing store, make a
    copy-on-write, add the new entry and update ICData with new backing
    store.
    => This can lead to an successful update being visible for a short
       period and then disappearing.

  * We can can ensure mutual exclusion when updating the backing store
    of ICData, thereby making read+grow+write appear atomic.
    => This requires taking a lock but ensures we never hit the same
       IC miss multiple times.

=> We take the latter approach for now.

Issue https://github.com/dart-lang/sdk/issues/36097

TEST=CL is purely a refactoring.

Change-Id: Ic3b2de61e25de0210323edd732424cc7d1178771
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/174465
Commit-Queue: Martin Kustermann <kustermann@google.com>
Reviewed-by: Vyacheslav Egorov <vegorov@google.com>