linux-kernel - Re: [PATCH next v2 2/3] printk: change @clear_seq to atomic64

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Date:   Mon, 07 Dec 2020 11:09:39 +0106
From:   John Ogness <john.ogness@...utronix.de>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH next v2 2/3] printk: change @clear_seq to atomic64_t

On 2020-12-07, Peter Zijlstra <peterz@...radead.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.

John Ogness