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>

1 files changed, 38 insertions, 0 deletions

diff --git a/util/bitmap.c b/util/bitmap.c
index 9c6bb52..39994af 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 +178,43 @@ 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);