Re: [Qemu-devel] [PATCH v3 23/29] bitmap: add atomic set functions

[Paolo Bonzini <pbonzini@redhat.com>]

On 27/05/2015 09:58, Fam Zheng wrote:
> On Tue, 05/26 18:54, Paolo Bonzini wrote:
>> From: Stefan Hajnoczi <stefanha@redhat.com>
>>
>> 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]
>> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
>> ---
>>  include/qemu/bitmap.h |  2 ++
>>  include/qemu/bitops.h | 14 ++++++++++++++
>>  util/bitmap.c         | 38 ++++++++++++++++++++++++++++++++++++++
>>  3 files changed, 54 insertions(+)
>>
>> diff --git a/include/qemu/bitmap.h b/include/qemu/bitmap.h
>> index f0273c9..3e0a4f3 100644
>> --- 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
> 
> The tab/space mix is not consistent. Fixing can be on top.
> 
>>   * 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 8abdcf9..8164225 100644
>> --- 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)
>> @@ -39,6 +40,19 @@ static inline void set_bit(long nr, unsigned long *addr)
>>  }
>>  
>>  /**
>> + * 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);
>> +}
> 
> Where is this function used?

In cpu_physical_memory_set_dirty_flag (patch 25).

Paolo