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Date:   Wed, 9 Dec 2020 05:34:19 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
To:     Petr Mladek <pmladek@suse.com>
Cc:     John Ogness <john.ogness@linutronix.de>,
        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>,
        Peter Zijlstra <peterz@infradead.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
Message-ID: <X8/jS9k/eMIL+Acw@jagdpanzerIV.localdomain>
References: <20201201205341.3871-1-john.ogness@linutronix.de>
 <20201201205341.3871-3-john.ogness@linutronix.de>
 <X8n9a2DWUFE/giyB@alley>
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On (20/12/04 10:12), Petr Mladek wrote:
> On Tue 2020-12-01 21:59:40, John Ogness wrote:
> > Currently @clear_seq access is protected by @logbuf_lock. Once
> > @logbuf_lock is removed some other form of synchronization will be
> > required. Change the type of @clear_seq to atomic64_t to provide the
> > synchronization.
> > 
> > diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> > index fc5e3a7d6d89..e9018c4e1b66 100644
> > --- a/kernel/printk/printk.c
> > +++ b/kernel/printk/printk.c
> > @@ -3412,7 +3418,7 @@ EXPORT_SYMBOL_GPL(kmsg_dump_get_buffer);
> >   */
> >  void kmsg_dump_rewind_nolock(struct kmsg_dumper *dumper)
> >  {
> > -	dumper->cur_seq = clear_seq;
> > +	dumper->cur_seq = atomic64_read(&clear_seq);
> 
> Sigh, atomic64_read() uses a spin lock in the generic implementation
> that is used on some architectures.

Oh... So on those archs prb is not lockless in fact, it actually
takes the spin_lock each time we read the descriptor state?

	desc_read()
	  atomic_long_read(state_var)
	    atomic64_read()
	      raw_spin_lock_irqsave(lock, flags)
	        << NMI panic >>

Am I missing something?

	-ss