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

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Message-ID: <20201207125605.GN3040@hirez.programming.kicks-ass.net>
Date:   Mon, 7 Dec 2020 13:56:05 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     John Ogness <john.ogness@...utronix.de>
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 Mon, Dec 07, 2020 at 11:09:39AM +0106, John Ogness wrote:
> 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.

Interrupted rather, I would think, specifically NMIs? Then yes, latch
should work. Gets you either the old or new, but never something in
between.