Messages in this thread | | | From | David Laight <> | Subject | RE: [PATCH next v2 2/3] printk: change @clear_seq to atomic64_t | Date | Mon, 7 Dec 2020 16:46:40 +0000 |
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From: John Ogness > Sent: 07 December 2020 10:04 > > On 2020-12-07, Peter Zijlstra <peterz@infradead.org> 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.
Is the value just being incremented?? If so you can do: seq_hi_0 = val >> 32; smp_wmb(); seq_lo = val; smp_wmb(); seq_hi_1 = val >> 32;
Then the reader can assume that seq_lo is zero if seq_h1_0 and seq_hi_1 differ.
David
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