linux-kernel - [PATCH 0/5 V5] Clarify/standardize memory barriers for lock/unlock

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Date:   Wed, 31 Aug 2016 15:42:25 +0200
From:   Manfred Spraul <manfred@...orfullife.com>
To:     benh@...nel.crashing.org, paulmck@...ux.vnet.ibm.com,
        Ingo Molnar <mingo@...e.hu>, Boqun Feng <boqun.feng@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     LKML <linux-kernel@...r.kernel.org>, 1vier1@....de,
        Davidlohr Bueso <dave@...olabs.net>,
        Manfred Spraul <manfred@...orfullife.com>
Subject: [PATCH 0/5 V5] Clarify/standardize memory barriers for lock/unlock

Hi,

V5: Major restructuring based on input from Peter and Davidlohr.

As discussed before:
If a high-scalability locking scheme is built with multiple
spinlocks, then often additional memory barriers are required.

The documentation was not as clear as possible, and memory
barriers were missing / superfluous in the implementation.

Patch 1: sem.c: Remove the smp_rmb() after spin_unlock_wait().
Patch 2: Documentation
Patch 3: Add spinlock_store_acquire(), update ipc/sem.c
Patch 4: Move smp_mb__after_unlock_lock to <linux/spinlock.h>
Patch 5: Fix memory ordering for nf_conntrack

The patches are safe for all architectures, the default is smp_mb().

Patch 5 is larger than required, it rewrites the conntrack logic
with the code from ipc/sem.c. I think the new code is simpler
and more realtime-friendly.

@Peter: A hint if qspinlocks can omit the smp_mb() would be
appreciated (everywhere or x86-only).
The comments on top of queued_spin_unlock_wait() had convinced
me that no additional barrier is required.
But the comment also convinced me that on x86, no further
barrier would be required for a full smp_mb() - and there you
wrote that this is wrong.

@Andrew: The patches are relative to mmots.
Could you include them in your tree, with the target of including in
linux-next?

--
	Manfred