[PATCH 1/3] arm/arm64: spinlocks: fix memory barriers

[Andrew Jones]

It shouldn't be necessary to use a barrier on the way into
spin_lock. We'll be focused on a single address until we get
it (exclusively) set, and then we'll do a barrier on the way
out. Also, it does make sense to do a barrier on the way in
to spin_unlock, i.e. ensure what we did in the critical section
is ordered wrt to what we do outside it, before we announce that
we're outside.

Signed-off-by: Andrew Jones <drjo...@redhat.com>
---
 lib/arm/spinlock.c   | 8 ++++----
 lib/arm64/spinlock.c | 5 ++---
 2 files changed, 6 insertions(+), 7 deletions(-)

diff --git a/lib/arm/spinlock.c b/lib/arm/spinlock.c
index 3b023ceaebf71..116ea5d7db930 100644
--- a/lib/arm/spinlock.c
+++ b/lib/arm/spinlock.c
@@ -7,10 +7,9 @@ void spin_lock(struct spinlock *lock)
 {
        u32 val, fail;
 
-       dmb();
-
        if (!mmu_enabled()) {
                lock->v = 1;
+               smp_mb();
                return;
        }
 
@@ -25,11 +24,12 @@ void spin_lock(struct spinlock *lock)
                : "r" (&lock->v)
                : "cc" );
        } while (fail);
-       dmb();
+
+       smp_mb();
 }
 
 void spin_unlock(struct spinlock *lock)
 {
+       smp_mb();
        lock->v = 0;
-       dmb();
 }
diff --git a/lib/arm64/spinlock.c b/lib/arm64/spinlock.c
index 68b68b75ba60d..a3907f03cacda 100644
--- a/lib/arm64/spinlock.c
+++ b/lib/arm64/spinlock.c
@@ -13,10 +13,9 @@ void spin_lock(struct spinlock *lock)
 {
        u32 val, fail;
 
-       smp_mb();
-
        if (!mmu_enabled()) {
                lock->v = 1;
+               smp_mb();
                return;
        }
 
@@ -35,9 +34,9 @@ void spin_lock(struct spinlock *lock)
 
 void spin_unlock(struct spinlock *lock)
 {
+       smp_mb();
        if (mmu_enabled())
                asm volatile("stlrh wzr, [%0]" :: "r" (&lock->v));
        else
                lock->v = 0;
-       smp_mb();
 }
-- 
2.4.3

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