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From:   David Laight <David.Laight@ACULAB.COM>
To:     'John Ogness' <john.ogness@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>
CC:     Petr Mladek <pmladek@suse.com>,
        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" <linux-kernel@vger.kernel.org>
Subject: RE: [PATCH next v2 2/3] printk: change @clear_seq to atomic64_t
Thread-Topic: [PATCH next v2 2/3] printk: change @clear_seq to atomic64_t
Thread-Index: AQHWzICKswdVD7bXr0KkK6JFsMrIzKnr1zJA
Date:   Mon, 7 Dec 2020 16:46:40 +0000
Message-ID: <d7d870a0081d4375945d169ee5850b9d@AcuMS.aculab.com>
References: <20201201205341.3871-1-john.ogness@linutronix.de>
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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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