we're seen some mutex occasional failures in CI, for a while now.
turns out maybe not false alarms, but the implementation regressing (since 9.2.1)

the issue can be spotted under concurrent threading with some user use-cases.
e.g. a .rb CountDownLatch https://raw.githubusercontent.com/benlangfeld/countdownlatch/master/lib/countdownlatch.rb ... guessing AR's connection pool might be also affected by this.

already bisected and find the cause to be: 66d2905

which makes no sense ... going to try setup a PR with a concurrent test-case and a proposed fix.