linux-kernel - [PATCH] ARC: atomic64: fix atomic64_add

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Date:   Sat, 11 Aug 2018 19:08:56 +0300
From:   Eugeniy Paltsev <Eugeniy.Paltsev@...opsys.com>
To:     linux-snps-arc@...ts.infradead.org
Cc:     linux-kernel@...r.kernel.org,
        Vineet Gupta <Vineet.Gupta1@...opsys.com>,
        Alexey Brodkin <Alexey.Brodkin@...opsys.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will.deacon@....com>,
        Boqun Feng <boqun.feng@...il.com>,
        Eugeniy Paltsev <Eugeniy.Paltsev@...opsys.com>
Subject: [PATCH] ARC: atomic64: fix atomic64_add_unless function

Current implementation of 'atomic64_add_unless' function
(and hence 'atomic64_inc_not_zero') return incorrect value
if lover 32 bits of compared 64-bit number are equal and
higher 32 bits aren't.

For in following example atomic64_add_unless must return '1'
but it actually returns '0':
--------->8---------
atomic64_t val = ATOMIC64_INIT(0x4444000000000000LL);
int ret = atomic64_add_unless(&val, 1LL, 0LL)
--------->8---------

This happens because we write '0' to returned variable regardless
of higher 32 bits comparison result.

So fix it.

NOTE:
 this change was tested with atomic64_test.

Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@...opsys.com>
---
 arch/arc/include/asm/atomic.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/arc/include/asm/atomic.h b/arch/arc/include/asm/atomic.h
index 11859287c52a..e840cb1763b2 100644
--- a/arch/arc/include/asm/atomic.h
+++ b/arch/arc/include/asm/atomic.h
@@ -578,11 +578,11 @@ static inline int atomic64_add_unless(atomic64_t *v, long long a, long long u)
 
 	__asm__ __volatile__(
 	"1:	llockd  %0, [%2]	\n"
-	"	mov	%1, 1		\n"
 	"	brne	%L0, %L4, 2f	# continue to add since v != u \n"
 	"	breq.d	%H0, %H4, 3f	# return since v == u \n"
 	"	mov	%1, 0		\n"
 	"2:				\n"
+	"	mov	%1, 1		\n"
 	"	add.f   %L0, %L0, %L3	\n"
 	"	adc     %H0, %H0, %H3	\n"
 	"	scondd  %0, [%2]	\n"
-- 
2.14.4