linux-kernel - [PATCH 3.14 192/238] powerpc: Add smp_mb()s to arch_spin_unlock

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Message-Id: <20141003212919.803167810@linuxfoundation.org>
Date:	Fri,  3 Oct 2014 14:31:47 -0700
From:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:	linux-kernel@...r.kernel.org
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	stable@...r.kernel.org, Michael Ellerman <mpe@...erman.id.au>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Subject: [PATCH 3.14 192/238] powerpc: Add smp_mb()s to arch_spin_unlock_wait()

3.14-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Michael Ellerman <mpe@...erman.id.au>

commit 78e05b1421fa41ae8457701140933baa5e7d9479 upstream.

Similar to the previous commit which described why we need to add a
barrier to arch_spin_is_locked(), we have a similar problem with
spin_unlock_wait().

We need a barrier on entry to ensure any spinlock we have previously
taken is visibly locked prior to the load of lock->slock.

It's also not clear if spin_unlock_wait() is intended to have ACQUIRE
semantics. For now be conservative and add a barrier on exit to give it
ACQUIRE semantics.

Signed-off-by: Michael Ellerman <mpe@...erman.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@...nel.crashing.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 arch/powerpc/lib/locks.c |    4 ++++
 1 file changed, 4 insertions(+)

--- a/arch/powerpc/lib/locks.c
+++ b/arch/powerpc/lib/locks.c
@@ -70,12 +70,16 @@ void __rw_yield(arch_rwlock_t *rw)
 
 void arch_spin_unlock_wait(arch_spinlock_t *lock)
 {
+	smp_mb();
+
 	while (lock->slock) {
 		HMT_low();
 		if (SHARED_PROCESSOR)
 			__spin_yield(lock);
 	}
 	HMT_medium();
+
+	smp_mb();
 }
 
 EXPORT_SYMBOL(arch_spin_unlock_wait);


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