atomic: implement generic atomic_dec_if_positive()

The x86 implementation of atomic_dec_if_positive is quite generic, so make
it available to all architectures.

This is needed for "swap: add a simple detector for inappropriate swapin
readahead".

[akpm@linux-foundation.org: do the "#define foo foo" trick in the conventional manner]
Signed-off-by: Shaohua Li <shli@fusionio.com>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Rik van Riel <riel@redhat.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Michal Simek <monstr@monstr.eu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 25 insertions, 0 deletions

diff --git a/include/linux/atomic.h b/include/linux/atomic.h
index 70cfcb2d63c..5b08a8540ec 100644
--- a/include/linux/atomic.h
+++ b/include/linux/atomic.h
@@ -86,6 +86,31 @@ static inline int atomic_dec_unless_positive(atomic_t *p)
 }
 #endif
 
+/*
+ * atomic_dec_if_positive - decrement by 1 if old value positive
+ * @v: pointer of type atomic_t
+ *
+ * The function returns the old value of *v minus 1, even if
+ * the atomic variable, v, was not decremented.
+ */
+#ifndef atomic_dec_if_positive
+static inline int atomic_dec_if_positive(atomic_t *v)
+{
+	int c, old, dec;
+	c = atomic_read(v);
+	for (;;) {
+		dec = c - 1;
+		if (unlikely(dec < 0))
+			break;
+		old = atomic_cmpxchg((v), c, dec);
+		if (likely(old == c))
+			break;
+		c = old;
+	}
+	return dec;
+}
+#endif
+
 #ifndef CONFIG_ARCH_HAS_ATOMIC_OR
 static inline void atomic_or(int i, atomic_t *v)
 {