On 27/03/2015 07:37, Fam Zheng wrote:
> On Thu, 03/26 18:38, Paolo Bonzini wrote:
>> From: Stefan Hajnoczi <stefa...@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 <stefa...@redhat.com>
>> Message-Id: <1417519399-3166-3-git-send-email-stefa...@redhat.com>
>> [Test before xchg; then a full barrier is needed at the end just like
>> in the previous patch. The barrier can be avoided if we did at least
>> one xchg. - Paolo]
>>
>> Signed-off-by: Paolo Bonzini <pbonz...@redhat.com>
>> ---
>> include/qemu/bitmap.h | 2 ++
>> util/bitmap.c | 43 +++++++++++++++++++++++++++++++++++++++++++
>> 2 files changed, 45 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 e4957da..570758a 100644
>> --- a/util/bitmap.c
>> +++ b/util/bitmap.c
>> @@ -232,6 +232,49 @@ 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;
>> +
>> + /* First word */
>> + if (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++;
>> + }
>> +
>> + /* Full words */
>> + while (nr - bits_to_clear >= 0) {
>> + if (*p) {
>> + old_bits = atomic_xchg(p, 0);
>> + dirty |= old_bits;
>> + }
>> + nr -= bits_to_clear;
>> + p++;
>> + }
>> +
>> + /* Last word */
>> + 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;
>> + } else {
>> + if (!dirty) {
>> + smp_mb();
>
> Is this for the "*p" in the while loop?
Yes, this is why it's within "if (!dirty)". If any bit was set, an
atomic_xchg was done and the memory barrier is not needed.
> If so, and if the while loop is not
> executed (bits contained in the first word, and clean), isn't this barrier
> superfluous then?
Yes, I can add an "if (*p)" in the first if too.
Paolo
