linux-kernel - [PATCH v2 for-4.17 2/3] powerpc: Remove smp_mb() from arch_spin_is

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Date:   Wed, 28 Mar 2018 11:47:37 +0200
From:   Andrea Parri <andrea.parri@...rulasolutions.com>
To:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-kernel@...r.kernel.org,
        Andrea Parri <andrea.parri@...rulasolutions.com>
Subject: [PATCH v2 for-4.17 2/3] powerpc: Remove smp_mb() from arch_spin_is_locked()

Commit 51d7d5205d338 ("powerpc: Add smp_mb() to arch_spin_is_locked()")
added an smp_mb() to arch_spin_is_locked(), in order to ensure that

	Thread 0			Thread 1

	spin_lock(A);			spin_lock(B);
	r0 = spin_is_locked(B)		r1 = spin_is_locked(A);

never ends up with r0 = r1 = 0, and reported one example (in ipc/sem.c)
relying on such guarantee.

It's however understood (and undocumented) that spin_is_locked() is not
required to ensure such ordering guarantee, guarantee that is currently
_not_ provided by all implementations/arch, and that callers relying on
such ordering should instead use suitable memory barriers before acting
on the result of spin_is_locked().

Following a recent auditing[1] of the callers of {,raw_}spin_is_locked()
revealing that none of them are relying on this guarantee anymore, this
commit removes the leading smp_mb() from the primitive thus effectively
reverting 51d7d5205d338.

[1] https://marc.info/?l=linux-kernel&m=151981440005264&w=2

Signed-off-by: Andrea Parri <andrea.parri@...rulasolutions.com>
Cc: Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc: Paul Mackerras <paulus@...ba.org>
Cc: Michael Ellerman <mpe@...erman.id.au>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
---
 arch/powerpc/include/asm/spinlock.h | 1 -
 1 file changed, 1 deletion(-)

diff --git a/arch/powerpc/include/asm/spinlock.h b/arch/powerpc/include/asm/spinlock.h
index b9ebc3085fb79..ecc141e3f1a73 100644
--- a/arch/powerpc/include/asm/spinlock.h
+++ b/arch/powerpc/include/asm/spinlock.h
@@ -67,7 +67,6 @@ static __always_inline int arch_spin_value_unlocked(arch_spinlock_t lock)
 
 static inline int arch_spin_is_locked(arch_spinlock_t *lock)
 {
-	smp_mb();
 	return !arch_spin_value_unlocked(*lock);
 }
 
-- 
2.7.4