Hi list,
I was reading rcu_nest.h and the code snippet (lines 59 - 63), which I believe
is to detect the overflow of variable rcu_gp_ctr, confused me a lot. Can some
of you shed light on that?
Specifically, what's the goal of line 60? Why should we compare the value of
(rcu_gp_ctr - tmp) against "111 1111 0000 0000"? If I understand correctly,
line 60 is to detect the scenario where rcu_gp_ctr has wrapped around and is
currently smaller than variable tmp. If that is correct, should line 60 be the
following?
60 ((tmp - READ_ONCE(rcu_gp_ctr)) > RCU_GP_CTR_BOTTOM_BIT) {
Or did I misunderstand anything here?
40 static void rcu_read_lock(void)
41 {
42 long tmp;
43 long *rrgp;
44
45 /*
46 * If this is the outermost RCU read-side critical section,
47 * copy the global grace-period counter. In either case,
48 * increment the nesting count held in the low-order bits.
49 */
50
51 rrgp = &__get_thread_var(rcu_reader_gp);
52 retry:
53 tmp = *rrgp;
54 if ((tmp & RCU_GP_CTR_NEST_MASK) == 0)
55 tmp = READ_ONCE(rcu_gp_ctr);
56 tmp++;
57 *rrgp = tmp;
58 smp_mb();
59 if (((tmp & RCU_GP_CTR_NEST_MASK) == 1) &&
60 ((rcu_gp_ctr - tmp) > (RCU_GP_CTR_NEST_MASK << 8)) != 0) {
61 (*rrgp)--;
62 goto retry;
63 }
64 }
Thanks,
--Junchang
