On Mon 2020-12-07 11:09:39, John Ogness wrote: > 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.
Or we could disable interrupts around the writer. But seqcount_latch will actually be need so that it works in panic(). The writer might be on a CPU that has been stopped using NMI. And this code is used by dumpers() that are called during panic(). Sigh, I have to take a coffee and try to really understand the latch code ;-) Best Regards, Petr
