Если вы планируете потратить какое-то значительное время на поиск или понимание ядра Linux, я рекомендую загрузить копию и использовать Cscope .
Использование Cscope в больших проектах (например, ядро Linux)
Я нашел следующее в копии ядра Linux 2.4.18.
Ключом является комментарий перед последним фрагментом кода ниже. Похоже, что возвращаемое значение sched_find_first_bit не определено, если бит не установлен.
С linux-2.4 / include / linux / sched.h: 185
/*
* The maximum RT priority is configurable. If the resulting
* bitmap is 160-bits , we can use a hand-coded routine which
* is optimal. Otherwise, we fall back on a generic routine for
* finding the first set bit from an arbitrarily-sized bitmap.
*/
#if MAX_PRIO 127
#define sched_find_first_bit(map) _sched_find_first_bit(map)
#else
#define sched_find_first_bit(map) find_first_bit(map, MAX_PRIO)
#endif
Начиная с linux-2.4 / include / asm-i386 / bitops.h: 303
/**
* find_first_bit - find the first set bit in a memory region
* @addr: The address to start the search at
* @size: The maximum size to search
*
* Returns the bit-number of the first set bit, not the number of the byte
* containing a bit.
*/
static __inline__ int find_first_bit(void * addr, unsigned size)
{
int d0, d1;
int res;
/* This looks at memory. Mark it volatile to tell gcc not to move it around */
__asm__ __volatile__(
"xorl %%eax,%%eax\n\t"
"repe; scasl\n\t"
"jz 1f\n\t"
"leal -4(%%edi),%%edi\n\t"
"bsfl (%%edi),%%eax\n"
"1:\tsubl %%ebx,%%edi\n\t"
"shll $3,%%edi\n\t"
"addl %%edi,%%eax"
:"=a" (res), "=&c" (d0), "=&D" (d1)
:"1" ((size + 31) >> 5), "2" (addr), "b" (addr));
return res;
}
С linux-2.4 / include / asm-i386 / bitops.h: 425
/*
* Every architecture must define this function. It's the fastest
* way of searching a 140-bit bitmap where the first 100 bits are
* unlikely to be set. It's guaranteed that at least one of the 140
* bits is cleared.
*/
static inline int _sched_find_first_bit(unsigned long *b)
{
if (unlikely(b[0]))
return __ffs(b[0]);
if (unlikely(b[1]))
return __ffs(b[1]) + 32;
if (unlikely(b[2]))
return __ffs(b[2]) + 64;
if (b[3])
return __ffs(b[3]) + 96;
return __ffs(b[4]) + 128;
}
Начиная с linux-2.4 / include / asm-i386 / bitops.h: 409
/**
* __ffs - find first bit in word.
* @word: The word to search
*
* Undefined if no bit exists, so code should check against 0 first.
*/
static __inline__ unsigned long __ffs(unsigned long word)
{
__asm__("bsfl %1,%0"
:"=r" (word)
:"rm" (word));
return word;
}