tree 500123257432cbdad469d6796a4e9e65b554b021
parent 3cc24aa81d5fa2681bab920128f1173e54f8dfd8
author David Brazdil <dbrazdil@google.com> 1569590272 +0000
committer David Brazdil <dbrazdil@google.com> 1571223025 +0100

Revert^2: Improve spinlock implementation for ARMv8.0

Current implementation of spinlocks using C11 atomics compiles into an
LDAXR/STXR pair which we observe does not always make forward progress
on Cortex A72 (concurrent_save_restore test livelocks).

This patch inserts a WFE instruction when the LDAXR returns that the
lock is currently taken, pausing the thread until an event is
triggered and easing off the contention. This is observed to help
forward progress on A72 and should also improve power consumption
of our spinlocks.

Note that there is still no guarantee of forward progress and threads
can be shown to livelock on the A72 if the loops are extremely tight.
This should be revisited in the future, possibly by leveraging ARMv8.1
LSE atomics.

This reverts commit c4dfdb789421c7658ce59e24eb60cda38335c2f2.

Test: vcpu_state.concurrent_save_restore
Bug: 141087046
Change-Id: I0435632e4994e74457102ae7133bf77b47c0283f