[Qemu-devel] [RFC 2/6] bitmap: add atomic test and clear

[Stefan Hajnoczi <stefanha@redhat.com>]

The new bitmap_test_and_clear_atomic() function clears a range and
returns whether or not the bits were set.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
 include/qemu/bitmap.h |  2 ++
 util/bitmap.c         | 26 ++++++++++++++++++++++++++
 2 files changed, 28 insertions(+)

diff --git a/include/qemu/bitmap.h b/include/qemu/bitmap.h
index 3e0a4f3..86dd9cd 100644
--- a/include/qemu/bitmap.h
+++ b/include/qemu/bitmap.h
@@ -41,6 +41,7 @@
  * bitmap_set(dst, pos, nbits)			Set specified bit area
  * bitmap_set_atomic(dst, pos, nbits)   Set specified bit area with atomic ops
  * bitmap_clear(dst, pos, nbits)		Clear specified bit area
+ * bitmap_test_and_clear_atomic(dst, pos, nbits)    Test and clear area
  * bitmap_find_next_zero_area(buf, len, pos, n, mask)	Find bit free area
  */
 
@@ -229,6 +230,7 @@ static inline int bitmap_intersects(const unsigned long *src1,
 void bitmap_set(unsigned long *map, long i, long len);
 void bitmap_set_atomic(unsigned long *map, long i, long len);
 void bitmap_clear(unsigned long *map, long start, long nr);
+bool bitmap_test_and_clear_atomic(unsigned long *map, long start, long nr);
 unsigned long bitmap_find_next_zero_area(unsigned long *map,
                                          unsigned long size,
                                          unsigned long start,
diff --git a/util/bitmap.c b/util/bitmap.c
index 758749c..3125674 100644
--- a/util/bitmap.c
+++ b/util/bitmap.c
@@ -218,6 +218,32 @@ void bitmap_clear(unsigned long *map, long start, long nr)
     }
 }
 
+bool bitmap_test_and_clear_atomic(unsigned long *map, long start, long nr)
+{
+    unsigned long *p = map + BIT_WORD(start);
+    const long size = start + nr;
+    int bits_to_clear = BITS_PER_LONG - (start % BITS_PER_LONG);
+    unsigned long mask_to_clear = BITMAP_FIRST_WORD_MASK(start);
+    unsigned long dirty = 0;
+    unsigned long old_bits;
+
+    while (nr - bits_to_clear >= 0) {
+        old_bits = atomic_fetch_and(p, ~mask_to_clear);
+        dirty |= old_bits & mask_to_clear;
+        nr -= bits_to_clear;
+        bits_to_clear = BITS_PER_LONG;
+        mask_to_clear = ~0UL;
+        p++;
+    }
+    if (nr) {
+        mask_to_clear &= BITMAP_LAST_WORD_MASK(size);
+        old_bits = atomic_fetch_and(p, ~mask_to_clear);
+        dirty |= old_bits & mask_to_clear;
+    }
+
+    return dirty;
+}
+
 #define ALIGN_MASK(x,mask)      (((x)+(mask))&~(mask))
 
 /**
-- 
2.1.0