Re: [Qemu-devel] [PATCH 17/22] bitmap: add atomic test and clear

[Paolo Bonzini <pbonzini@redhat.com>]

On 27/03/2015 07:37, Fam Zheng wrote:
> On Thu, 03/26 18:38, Paolo Bonzini wrote:
>> From: Stefan Hajnoczi <stefanha@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 <stefanha@redhat.com>
>> Message-Id: <1417519399-3166-3-git-send-email-stefanha@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 <pbonzini@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