On 2020-12-07, Peter Zijlstra <[email protected]> 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);
That will not work. We are talking about a situation where the writer is preempted. So seq will never equal seq_copy in that situation. I expect that the seqcount_latch is necessary. John Ogness
