tile: support atomic64_dec_if_positive()

Use the normal cmpxchg() idiom to implement this functionality.
Signed-off-by: Chris Metcalf <cmetcalf@tilera.com>

arch/tile/Kconfig

@@ -24,6 +24,7 @@ config TILE
 	select MODULES_USE_ELF_RELA
 	select HAVE_ARCH_TRACEHOOK
 	select HAVE_SYSCALL_TRACEPOINTS
+	select ARCH_HAS_ATOMIC64_DEC_IF_POSITIVE
 
 # FIXME: investigate whether we need/want these options.
 #	select HAVE_IOREMAP_PROT

arch/tile/include/asm/atomic.h

@@ -131,4 +131,25 @@ static inline int atomic_read(const atomic_t *v)
 #include <asm/atomic_64.h>
 #endif
 
+#ifndef __ASSEMBLY__
+
+static inline long long atomic64_dec_if_positive(atomic64_t *v)
+{
+	long long c, old, dec;
+
+	c = atomic64_read(v);
+	for (;;) {
+		dec = c - 1;
+		if (unlikely(dec < 0))
+			break;
+		old = atomic64_cmpxchg((v), c, dec);
+		if (likely(old == c))
+			break;
+		c = old;
+	}
+	return dec;
+}
+
+#endif /* __ASSEMBLY__ */
+
 #endif /* _ASM_TILE_ATOMIC_H */