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From:   Alexander A Sverdlin <alexander.sverdlin@nokia.com>
To:     Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>, Will Deacon <will@kernel.org>,
        linux-kernel@vger.kernel.org
Cc:     Alexander Sverdlin <alexander.sverdlin@nokia.com>,
        Russell King <linux@armlinux.org.uk>,
        linux-arm-kernel@lists.infradead.org
Subject: [PATCH 1/2] qspinlock: Ensure writes are pushed out of core write buffer
Date:   Wed, 27 Jan 2021 21:01:08 +0100
Message-Id: <20210127200109.16412-1-alexander.sverdlin@nokia.com>
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From: Alexander Sverdlin <alexander.sverdlin@nokia.com>

Ensure writes are pushed out of core write buffer to prevent waiting code
on another cores from spinning longer than necessary.

6 threads running tight spinlock loop competing for the same lock
on 6 cores on MIPS/Octeon do 1000000 iterations...

before the patch in:	4.3 sec
after the patch in:	1.2 sec

Same 6-core Octeon machine:
sysbench --test=mutex --num-threads=64 --memory-scope=local run

w/o patch:	1.53s
with patch:	1.28s

This will also allow to remove the smp_wmb() in
arch/arm/include/asm/mcs_spinlock.h (was it actually addressing the same
issue?).

Finally our internal quite diverse test suite of different IPC/network
aspects didn't detect any regressions on ARM/ARM64/x86_64.

Signed-off-by: Alexander Sverdlin <alexander.sverdlin@nokia.com>
---
 kernel/locking/mcs_spinlock.h | 5 +++++
 kernel/locking/qspinlock.c    | 6 ++++++
 2 files changed, 11 insertions(+)

diff --git a/kernel/locking/mcs_spinlock.h b/kernel/locking/mcs_spinlock.h
index 5e10153..10e497a 100644
--- a/kernel/locking/mcs_spinlock.h
+++ b/kernel/locking/mcs_spinlock.h
@@ -89,6 +89,11 @@ void mcs_spin_lock(struct mcs_spinlock **lock, struct mcs_spinlock *node)
 		return;
 	}
 	WRITE_ONCE(prev->next, node);
+	/*
+	 * This is necessary to make sure that the corresponding "while" in the
+	 * mcs_spin_unlock() doesn't loop forever
+	 */
+	smp_wmb();
 
 	/* Wait until the lock holder passes the lock down. */
 	arch_mcs_spin_lock_contended(&node->locked);
diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
index cbff6ba..577fe01 100644
--- a/kernel/locking/qspinlock.c
+++ b/kernel/locking/qspinlock.c
@@ -469,6 +469,12 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
 
 		/* Link @node into the waitqueue. */
 		WRITE_ONCE(prev->next, node);
+		/*
+		 * This is necessary to make sure that the corresponding
+		 * smp_cond_load_relaxed() below (running on another core)
+		 * doesn't spin forever.
+		 */
+		smp_wmb();
 
 		pv_wait_node(node, prev);
 		arch_mcs_spin_lock_contended(&node->locked);
-- 
2.10.2