From 9f02cfc84b85929947b32fe1674fbc6a429f332a Mon Sep 17 00:00:00 2001 From: Stefan Hajnoczi Date: Tue, 2 Dec 2014 11:23:14 +0000 Subject: 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 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 Signed-off-by: Paolo Bonzini --- util/bitmap.c | 38 ++++++++++++++++++++++++++++++++++++++ 1 file changed, 38 insertions(+) (limited to 'util') diff --git a/util/bitmap.c b/util/bitmap.c index 9c6bb526f6..39994af675 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); -- cgit v1.2.1