On Sun, Dec 06, 2020 at 09:29:59PM +0106, John Ogness wrote:
> Yes, and it is read-only access. Perhaps atomic64_t is the wrong thing
> to use here. We could use a seqcount_latch and a shadow variable so that
> if a writer has been preempted, we can use the previous value. (Only
> kmsg_dump would need to use the lockless variant to read the value.)
>
> void clear_seq_set(u64 val)
> {
> spin_lock_irq(&clear_lock);
> raw_write_seqcount_latch(&clear_latch);
> clear_seq[0] = val;
> raw_write_seqcount_latch(&clear_latch);
> clear_seq[1] = val;
> spin_unlock_irq(&clear_lock);
> }
>
> u64 clear_seq_get_nolock(void)
> {
> unsigned int seq, idx;
> u64 val;
>
> do {
> seq = raw_read_seqcount_latch(&clear_latch);
> idx = seq & 0x1;
> val = clear_seq[idx];
> } while (read_seqcount_latch_retry(&clear_latch, seq));
>
> return val;
> }
That's overly complicated.
If you're going to double the storage you can simply do:
seq = val
smp_wmb();
seq_copy = val;
vs
do {
tmp = seq_copy;
smp_rmb();
val = seq;
} while (val != tmp);
Also look for CONFIG_64_BIT in kernel/sched/fair.c.
