[Qemu-devel] [RFC v2 1/6] bitmap: add atomic set functions

[Stefan Hajnoczi]

Use atomic_or() for atomic bitmaps where several threads may set bits at
the same time.  This avoids the race condition between threads loading
an element, bitwise ORing, and then storing the element.

When setting all bits in a word we can avoid atomic ops and instead just
use an smp_wmb() write barrier at the end.

Most bitmap users don't need atomicity so introduce new functions.

Signed-off-by: Stefan Hajnoczi <address@hidden>
---
 include/qemu/bitmap.h |  2 ++
 include/qemu/bitops.h | 14 ++++++++++++++
 util/bitmap.c         | 36 ++++++++++++++++++++++++++++++++++++
 3 files changed, 52 insertions(+)

diff --git a/include/qemu/bitmap.h b/include/qemu/bitmap.h
index f0273c9..3e0a4f3 100644
--- a/include/qemu/bitmap.h
+++ b/include/qemu/bitmap.h
@@ -39,6 +39,7 @@
  * bitmap_empty(src, nbits)                    Are all bits zero in *src?
  * bitmap_full(src, nbits)                     Are all bits set in *src?
  * 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_find_next_zero_area(buf, len, pos, n, mask)  Find bit free area
  */
@@ -226,6 +227,7 @@ 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);
 unsigned long bitmap_find_next_zero_area(unsigned long *map,
                                          unsigned long size,
diff --git a/include/qemu/bitops.h b/include/qemu/bitops.h
index 181bd46..eda4132 100644
--- a/include/qemu/bitops.h
+++ b/include/qemu/bitops.h
@@ -16,6 +16,7 @@
 #include <assert.h>
 
 #include "host-utils.h"
+#include "atomic.h"
 
 #define BITS_PER_BYTE           CHAR_BIT
 #define BITS_PER_LONG           (sizeof (unsigned long) * BITS_PER_BYTE)
@@ -39,6 +40,19 @@ static inline void set_bit(long nr, unsigned long *addr)
 }
 
 /**
+ * set_bit_atomic - Set a bit in memory atomically
+ * @nr: the bit to set
+ * @addr: the address to start counting from
+ */
+static inline void set_bit_atomic(long nr, unsigned long *addr)
+{
+    unsigned long mask = BIT_MASK(nr);
+    unsigned long *p = addr + BIT_WORD(nr);
+
+    atomic_or(p, mask);
+}
+
+/**
  * clear_bit - Clears a bit in memory
  * @nr: Bit to clear
  * @addr: Address to start counting from
diff --git a/util/bitmap.c b/util/bitmap.c
index 9c6bb52..40db0d9 100644
--- a/util/bitmap.c
+++ b/util/bitmap.c
@@ -11,6 +11,7 @@
 
 #include "qemu/bitops.h"
 #include "qemu/bitmap.h"
+#include "qemu/atomic.h"
 
 /*
  * bitmaps provide an array of bits, implemented using an an
@@ -177,6 +178,41 @@ void bitmap_set(unsigned long *map, long start, long nr)
     }
 }
 
+void bitmap_set_atomic(unsigned long *map, long start, long nr)
+{
+    unsigned long *p = map + BIT_WORD(start);
+    const long size = start + nr;
+    int bits_to_set = BITS_PER_LONG - (start % BITS_PER_LONG);
+    unsigned long mask_to_set = BITMAP_FIRST_WORD_MASK(start);
+
+    /* First word */
+    if (nr - bits_to_set > 0) {
+        atomic_or(p, mask_to_set);
+        nr -= bits_to_set;
+        bits_to_set = BITS_PER_LONG;
+        p++;
+    }
+
+    /* Full words */
+    while (nr - bits_to_set >= 0) {
+        *p = ~0UL;
+        nr -= bits_to_set;
+        p++;
+    }
+
+    /* Last word */
+    if (nr) {
+        mask_to_set &= BITMAP_LAST_WORD_MASK(size);
+        atomic_or(p, mask_to_set);
+    } else {
+        /* If we avoided the full barrier in atomic_or() we should at least
+         * issue a write barrier so that other threads using barriers see
+         * up-to-date bitmap contents.
+         */
+        smp_wmb();
+    }
+}
+
 void bitmap_clear(unsigned long *map, long start, long nr)
 {
     unsigned long *p = map + BIT_WORD(start);
-- 
2.1.0