Messages in this thread | | | Date | Wed, 17 Jun 2009 18:42:14 +0800 | Subject | surpring results of test_and_set_bit() and test_and_clear_bit() functions in Linux-2.6.25 kernel at x86_64 architecture | From | qingbo yuan <> |
| |
hi,all I recently encountered a strange phenomenon about test_and_set_bit() and test_and_clear_bit(). Here are the comments about these functions in Linux-2.6.25 kernel at x86_64 architecture:
/** * test_and_set_bit - Set a bit and return its old value */
/** * test_and_clear_bit - Clear a bit and return its old value */
According to the comments, if the old value is '0', then these functions will return 0 to the caller, and if the old value is '1', then these functions will return 1. However, in my experiment, if the old value is '1', I got -1 instead of 1. Here is the code:
void generos_test_bitmap(void) { int cur,ret_clear,ret_set; unsigned long bitmap =0x3721; for(cur = find_first_bit(&bitmap,BITS_PER_LONG);cur < BITS_PER_LONG;cur = find_next_bit(&bitmap, BITS_PER_LONG, cur + 1)){ ret_clear = test_and_set_bit(cur, &bitmap); ret_set = test_and_clear_bit(cur, &bitmap); printk("test_and_set bit [%d] ret [%d]\n",cur,ret_set); printk("test_and_clear bit [%d] ret [%d]\n\n\n",cur,ret_clear); } }
And here is the results: test_and_set bit [0] ret [-1] test_and_clear bit [0] ret [-1] test_and_set bit [5] ret [-1] test_and_clear bit [5] ret [-1] test_and_set bit [8] ret [-1] test_and_clear bit [8] ret [-1] test_and_set bit [9] ret [-1] test_and_clear bit [9] ret [-1] test_and_set bit [10] ret [-1] test_and_clear bit [10] ret [-1] test_and_set bit [12] ret [-1] test_and_clear bit [12] ret [-1] test_and_set bit [13] ret [-1] test_and_clear bit [13] ret [-1]
Interesting! Let's take a look at the definition of these funtions in Linux-2.6.25: static inline int test_and_set_bit(int nr, volatile void *addr) { int oldbit; asm volatile(LOCK_PREFIX "bts %2,%1\n\t" "sbb %0,%0" : "=r" (oldbit), ADDR : "Ir" (nr) : "memory"); return oldbit; }
static inline int test_and_clear_bit(int nr, volatile void *addr) { int oldbit; asm volatile(LOCK_PREFIX "btr %2,%1\n\t" "sbb %0,%0" : "=r" (oldbit), ADDR : "Ir" (nr) : "memory"); return oldbit; }
1. bts and btr set/clear the bit and copy the old value to CF flag. 2. sbb %0,%0 equals (%0-(%0+CF)) -> %0 3. %0 -> oldbit
So, if the old bit is '0', these functions return 0; if the old bit is '1', these functions return -1; Both of these functions were right if the comments modified as "test_and_set_bit - Set a bit and return its opposite old value".
Can anybody point out what problems exist in my analysis? -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
| |