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Date:   Mon, 7 Dec 2020 13:56:45 +0100
From:   Petr Mladek <pmladek@suse.com>
To:     John Ogness <john.ogness@linutronix.de>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
        Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH next v2 2/3] printk: change @clear_seq to atomic64_t
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On Mon 2020-12-07 11:09:39, John Ogness wrote:
> 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.

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