From: Petr Mladek <pmladek@suse.com>
To: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
Steven Rostedt <rostedt@goodmis.org>, Jan Kara <jack@suse.cz>,
Andrew Morton <akpm@linux-foundation.org>,
Linus Torvalds <torvalds@linux-foundation.org>,
"Rafael J . Wysocki" <rjw@rjwysocki.net>,
Eric Biederman <ebiederm@xmission.com>,
Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
Jiri Slaby <jslaby@suse.com>, Pavel Machek <pavel@ucw.cz>,
Len Brown <len.brown@intel.com>,
linux-kernel@vger.kernel.org
Subject: Re: [RFC][PATCHv2 1/8] printk: move printk_pending out of per-cpu
Date: Mon, 3 Apr 2017 14:43:42 +0200 [thread overview]
Message-ID: <20170403124342.GE3452@pathway.suse.cz> (raw)
In-Reply-To: <20170403112301.GB7867@jagdpanzerIV.localdomain>
On Mon 2017-04-03 20:23:01, Sergey Senozhatsky wrote:
> On (03/31/17 15:33), Peter Zijlstra wrote:
> > 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.
>
> Petr and Peter, thanks for the review.
>
> can you educate me, what exactly is broken there?
Good point!
> when called from console_unlock(), we have something as follows
>
> console_unlock()
> {
> for (;;) {
> spin_lock_irqsave();
> ...
> spin_unlock_irqrestore();
> ...
> }
>
> spin_unlock_irqrestore();
>
> <<IRQs enabled>>
>
> if (wake_klogd)
> wake_up_klogd()
> {
> set_bit(PRINTK_PENDING_WAKEUP, &printk_pending);
> irq_work_queue(this_cpu_ptr(&wake_up_klogd_work));
> }
> }
>
>
> we queue a per-CPU irq_work.
Ah, I forgot that irq_work is still per-CPU. In this case, everything
seems to be safe even without the barrier. The important thing is that
there always will be queued an irq_work that will see and handle the
bit. I believe that the barrier would be needed if the irq_work was
global.
I am sorry for the noise.
Best Regards,
Petr
next prev parent reply other threads:[~2017-04-03 12:43 UTC|newest]
Thread overview: 75+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-03-29 9:25 [RFC][PATCHv2 0/8] printk: introduce printing kernel thread Sergey Senozhatsky
2017-03-29 9:25 ` [RFC][PATCHv2 1/8] printk: move printk_pending out of per-cpu Sergey Senozhatsky
2017-03-31 13:09 ` Petr Mladek
2017-03-31 13:33 ` Peter Zijlstra
2017-04-03 11:23 ` Sergey Senozhatsky
2017-04-03 12:43 ` Petr Mladek [this message]
2017-03-29 9:25 ` [RFC][PATCHv2 2/8] printk: introduce printing kernel thread Sergey Senozhatsky
2017-04-04 9:01 ` Petr Mladek
2017-04-04 9:36 ` Sergey Senozhatsky
2017-04-06 17:14 ` Pavel Machek
2017-04-07 5:12 ` Sergey Senozhatsky
2017-04-07 7:21 ` Pavel Machek
2017-04-07 8:15 ` Sergey Senozhatsky
2017-04-07 12:06 ` Pavel Machek
2017-03-29 9:25 ` [RFC][PATCHv2 3/8] printk: offload printing from wake_up_klogd_work_func() Sergey Senozhatsky
2017-03-31 14:56 ` Petr Mladek
2017-04-04 16:15 ` Sergey Senozhatsky
2017-03-29 9:25 ` [RFC][PATCHv2 4/8] pm: switch to printk.emergency mode in unsafe places Sergey Senozhatsky
2017-03-31 15:06 ` Petr Mladek
2017-04-06 17:20 ` Pavel Machek
2017-04-09 10:59 ` Andreas Mohr
2017-04-10 12:20 ` Petr Mladek
2017-04-10 14:38 ` Sergey Senozhatsky
2017-03-29 9:25 ` [RFC][PATCHv2 5/8] sysrq: " Sergey Senozhatsky
2017-03-31 15:37 ` Petr Mladek
2017-04-01 0:04 ` Sergey Senozhatsky
2017-03-29 9:25 ` [RFC][PATCHv2 6/8] kexec: " Sergey Senozhatsky
2017-03-31 15:39 ` Petr Mladek
2017-03-29 9:25 ` [RFC][PATCHv2 7/8] printk: add printk emergency_mode parameter Sergey Senozhatsky
2017-04-03 15:29 ` Petr Mladek
2017-04-04 8:29 ` Sergey Senozhatsky
2017-03-29 9:25 ` [RFC][PATCHv2 8/8] printk: enable printk offloading Sergey Senozhatsky
2017-03-30 21:38 ` [printk] fbc14616f4: BUG:kernel_reboot-without-warning_in_test_stage kernel test robot
2017-03-31 2:35 ` Sergey Senozhatsky
2017-03-31 4:04 ` Sergey Senozhatsky
2017-03-31 6:39 ` Ye Xiaolong
2017-03-31 14:47 ` Sergey Senozhatsky
2017-03-31 15:28 ` Eric W. Biederman
2017-04-03 9:31 ` Jan Kara
2017-04-03 10:06 ` Petr Mladek
2017-04-06 17:33 ` Pavel Machek
2017-04-07 4:44 ` Sergey Senozhatsky
2017-04-07 7:15 ` Pavel Machek
2017-04-07 7:46 ` Sergey Senozhatsky
2017-04-07 8:14 ` Pavel Machek
2017-04-07 12:10 ` Sergey Senozhatsky
2017-04-07 12:44 ` Pavel Machek
2017-04-07 14:40 ` Steven Rostedt
2017-05-08 6:37 ` Sergey Senozhatsky
2017-05-17 13:13 ` Petr Mladek
2017-04-07 15:13 ` Sergey Senozhatsky
2017-04-07 15:23 ` Peter Zijlstra
2017-04-07 15:40 ` Sergey Senozhatsky
2017-04-09 18:21 ` Eric W. Biederman
2017-04-10 4:46 ` Sergey Senozhatsky
2017-04-09 10:12 ` Pavel Machek
2017-04-10 4:53 ` Sergey Senozhatsky
2017-04-10 11:54 ` Petr Mladek
2017-04-10 15:08 ` Sergey Senozhatsky
2017-04-10 18:48 ` Pavel Machek
2017-04-11 1:46 ` Sergey Senozhatsky
2017-04-11 16:19 ` Sergey Senozhatsky
2017-04-12 18:43 ` Pavel Machek
2017-04-13 4:34 ` Sergey Senozhatsky
2017-04-13 5:50 ` Sergey Senozhatsky
2017-04-13 8:19 ` Sergey Senozhatsky
2017-04-13 14:03 ` Petr Mladek
2017-04-14 4:42 ` Sergey Senozhatsky
2017-04-07 14:29 ` Steven Rostedt
2017-04-09 9:57 ` Pavel Machek
2017-04-03 10:51 ` Sergey Senozhatsky
2017-04-05 7:29 ` Ye Xiaolong
2017-04-05 8:40 ` Sergey Senozhatsky
2017-04-03 15:42 ` [RFC][PATCHv2 8/8] printk: enable printk offloading Petr Mladek
2017-04-04 13:20 ` Sergey Senozhatsky
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