On Sun, Aug 28, 2016 at 01:56:13PM +0200, Manfred Spraul wrote:
Right now, the spinlock machinery tries to guarantee barriers even forWhy? This does not explain why..
unorthodox locking cases, which ends up as a constant stream of updates
as the architectures try to support new unorthodox ideas.
The patch proposes to reverse that:
spin_lock is ACQUIRE, spin_unlock is RELEASE.
spin_unlock_wait is also ACQUIRE.
Code that needs further guarantees must use appropriate explicit barriers.
Architectures that can implement some barriers for free can define the
barriers as NOPs.
As the initial step, the patch converts ipc/sem.c to the new defines:
- no more smp_rmb() after spin_unlock_wait(), that is part of
spin_unlock_wait()
- smp_mb__after_spin_lock() instead of a direct smp_mb().
- With commit 2c6100227116- Why smp_mb is required after spin_lock? See Patch 02, I added the race that exists on real hardware.
("locking/qspinlock: Fix spin_unlock_wait() some more"),
(and the commits for the other archs), spin_unlock_wait() is an
ACQUIRE.
Therefore the smp_rmb() after spin_unlock_wait() can be removed.
- smp_mb__after_spin_lock() instead of a direct smp_mb().
This allows that architectures override it with a less expensive
barrier if this is sufficient for their hardware.