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Fix a race in casuword() exposed by csup. casuword() non-atomically read

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the current value of its argument before atomically replacing it, which
could occasionally return the wrong value on an SMP system. This resulted
in user mutex operations hanging when using threaded applications.
This commit is contained in:
Nathan Whitehorn 2009-10-31 17:59:24 +00:00
parent ad26477892
commit e2cd4c2a65
1 changed files with 13 additions and 2 deletions
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15
sys/powerpc/aim/copyinout.c
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@ -347,8 +347,19 @@ casuword(volatile u_long *addr, u_long old, u_long new)
return (-1);
}
val = *p;
(void) atomic_cmpset_32((volatile uint32_t *)p, old, new);
__asm __volatile (
"1:\tlwarx %0, 0, %2\n\t" /* load old value */
"cmplw %3, %0\n\t" /* compare */
"bne 2f\n\t" /* exit if not equal */
"stwcx. %4, 0, %2\n\t" /* attempt to store */
"bne- 1b\n\t" /* spin if failed */
"b 3f\n\t" /* we've succeeded */
"2:\n\t"
"stwcx. %0, 0, %2\n\t" /* clear reservation (74xx) */
"3:\n\t"
: "=&r" (val), "=m" (*p)
: "r" (p), "r" (old), "r" (new), "m" (*p)
: "cc", "memory");
td->td_pcb->pcb_onfault = NULL;