linux-kernel - Re: [RFC][PATCHv2 1/8] printk: move printk

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Date:   Fri, 31 Mar 2017 15:33:05 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>, Jan Kara <jack@...e.cz>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        "Rafael J . Wysocki" <rjw@...ysocki.net>,
        Eric Biederman <ebiederm@...ssion.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>, Pavel Machek <pavel@....cz>,
        Len Brown <len.brown@...el.com>, linux-kernel@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Subject: Re: [RFC][PATCHv2 1/8] printk: move printk_pending out of per-cpu

On Fri, Mar 31, 2017 at 03:09:50PM +0200, Petr Mladek wrote:
> On Wed 2017-03-29 18:25:04, Sergey Senozhatsky wrote:

> >  	if (waitqueue_active(&log_wait)) {
> > -		this_cpu_or(printk_pending, PRINTK_PENDING_WAKEUP);
> > +		set_bit(PRINTK_PENDING_WAKEUP, &printk_pending);
> 
> We should add here a write barrier:
> 
> 	/*
> 	 * irq_work_queue() uses cmpxchg() and implies the memory
> 	 * barrier only when the work is queued. An explicit barrier
> 	 * is needed here to make sure that wake_up_klogd_work_func()
> 	 * sees printk_pending set even when the work was already queued
> 	 * because of an other pending event.
> 	 */
> 	 smp_wmb();
> 
> >  		irq_work_queue(this_cpu_ptr(&wake_up_klogd_work));
> >  	}
> >  	preempt_enable();

smp_mb__after_atomic() is probably better, because if you're not
ordering with the cmpxchg, you're ordering against a load done by
cmpxchg to see it doesn't need to do anything.