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SubjectRe: [lkp] [ipc/sem.c] 5864a2fd30: aim9.shared_memory.ops_per_sec -13.0%

as discussed before:
The root cause for the performance regression is the smp_mb() that was
added into the fast path.

I see two options:
1) switch to full spin_lock()/spin_unlock() for the rare codepath,
then the fast path doesn't need the smp_mb() anymore.

2) confirm that no arch needs the smp_mb(), then remove it.
- powerpc is ok after commit
6262db7c088b ("powerpc/spinlock: Fix spin_unlock_wait()")
- arm is ok after commit
d86b8da04dfa ("arm64: spinlock: serialise spin_unlock_wait against concurrent lockers")
- for x86 is ok after commit
2c6100227116 ("locking/qspinlock: Fix spin_unlock_wait() some more")
- for the remaining SMP architectures, I don't have a status.

I would prefer the approach 1:
The memory ordering provided by spin_lock()/spin_unlock() is clear.

Attached are patches for approach 1:

- Patch 1 replaces spin_unlock_wait() with spin_lock()/spin_unlock() and
removes all memory barriers that are then unnecessary.

- Patch 2 adds the hysteresis code: This makes the rare codepath
extremely rare.
It also corrects some wrong comments, e.g. regarding switching
from global lock to per-sem lock (we "must' switch, not we "can"
switch as written right now).

The patches passed stress-testing.

What do you think?
My initial idea was to aim for 4.10, then we have more time to decide.


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