bitmap: add atomic set functions

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_mb() at the end.

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

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <1417519399-3166-2-git-send-email-stefanha@redhat.com>
[Avoid barrier in the single word case, use full barrier instead of write.
 - Paolo]
Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/include/qemu/bitmap.h b/include/qemu/bitmap.h
index f0273c965f263278ed4e83877b076099ffc1aa32..3e0a4f3e0c1eaf235b12a6c6c8d4be2a112b5e56 100644 (file)
--- 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 8abdcf90778aa33da5c3c4982052420ee8b79a77..81642251525ed2f06a0b1ac9f5a1712bed862d10 100644 (file)
--- 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)
@@ -38,6 +39,19 @@ static inline void set_bit(long nr, unsigned long *addr)
     *p  |= mask;
 }
 
+/**
+ * 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

diff --git a/util/bitmap.c b/util/bitmap.c
index 9c6bb526f6d020a0c60c8ed2a132e60e18adb497..39994af67547056e3b6b6081b1177aafdb7b9347 100644 (file)
--- 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 +178,43 @@ 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;
+        mask_to_set = ~0UL;
+        p++;
+    }
+
+    /* Full words */
+    if (bits_to_set == BITS_PER_LONG) {
+        while (nr >= BITS_PER_LONG) {
+            *p = ~0UL;
+            nr -= BITS_PER_LONG;
+            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(), issue a
+         * barrier to account for the assignments in the while loop.
+         */
+        smp_mb();
+    }
+}
+
 void bitmap_clear(unsigned long *map, long start, long nr)
 {
     unsigned long *p = map + BIT_WORD(start);