[PATCH/RFC 3/7] x86: Rework ACCESS_ONCE for spinlock code

[Christian Borntraeger]

ACCESS_ONCE does not work reliably on non-scalar types. For
example gcc 4.6 and 4.7 might remove the volatile tag for such
accesses during the SRA (scalar replacement of aggregates) step
(https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58145)

Change the spinlock code to access the union members with
ACCESS_ONCE to avoid this problem.

Signed-off-by: Christian Borntraeger <borntraeger@xxxxxxxxxx>
---
 arch/x86/include/asm/spinlock.h | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/arch/x86/include/asm/spinlock.h b/arch/x86/include/asm/spinlock.h
index 9295016..168eb59 100644
--- a/arch/x86/include/asm/spinlock.h
+++ b/arch/x86/include/asm/spinlock.h
@@ -8,6 +8,7 @@
 #include <linux/compiler.h>
 #include <asm/paravirt.h>
 #include <asm/bitops.h>
+#include <asm/spinlock_types.h>
 
 /*
  * Your basic SMP spinlocks, allowing only a single CPU anywhere
@@ -105,7 +106,7 @@ static __always_inline int arch_spin_trylock(arch_spinlock_t *lock)
 {
 	arch_spinlock_t old, new;
 
-	old.tickets = ACCESS_ONCE(lock->tickets);
+	old.head_tail = ACCESS_ONCE(lock->head_tail);
 	if (old.tickets.head != (old.tickets.tail & ~TICKET_SLOWPATH_FLAG))
 		return 0;
 
@@ -162,16 +163,19 @@ static __always_inline void arch_spin_unlock(arch_spinlock_t *lock)
 
 static inline int arch_spin_is_locked(arch_spinlock_t *lock)
 {
-	struct __raw_tickets tmp = ACCESS_ONCE(lock->tickets);
+	arch_spinlock_t tmp = {};
 
-	return tmp.tail != tmp.head;
+	tmp.head_tail =ACCESS_ONCE(lock->head_tail);
+	return tmp.tickets.tail != tmp.tickets.head;
 }
 
 static inline int arch_spin_is_contended(arch_spinlock_t *lock)
 {
-	struct __raw_tickets tmp = ACCESS_ONCE(lock->tickets);
+	arch_spinlock_t tmp = {};
+
+	tmp.head_tail = ACCESS_ONCE(lock->head_tail);
+	return (__ticket_t)(tmp.tickets.tail - tmp.tickets.head) > TICKET_LOCK_INC;
 
-	return (__ticket_t)(tmp.tail - tmp.head) > TICKET_LOCK_INC;
 }
 #define arch_spin_is_contended	arch_spin_is_contended
 
-- 
1.9.3

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