RE: Additional fix : (was [v2]printk: fix delayed messages from CPUhotplug events)

[Shilimkar, Santosh]

 Thanks Andrew for looking into this.
> -----Original Message-----
> From: Andrew Morton [mailto:akpm@xxxxxxxxxxxxxxxxxxxx]
> Sent: Tuesday, August 03, 2010 4:15 AM
> To: Shilimkar, Santosh
> Cc: Kevin Cernekee; linux-kernel@xxxxxxxxxxxxxxx; Russell King - ARM Linux
> Subject: Re: Additional fix : (was [v2]printk: fix delayed messages from
> CPU hotplug events)
> 
> On Tue, 29 Jun 2010 14:22:26 +0530
> "Shilimkar, Santosh" <santosh.shilimkar@xxxxxx> wrote:
> 
> > Hi,
> >
> > I have faced similar issue as what is being described in below with
> > latest kernel.
> >
> > ------------------------------------------------
> > https://patchwork.kernel.org/patch/103347/
> >
> > When a secondary CPU is being brought up, it is not uncommon for
> > printk() to be invoked when cpu_online(smp_processor_id()) == 0.  The
> > case that I witnessed personally was on MIPS:
> >
> > http://lkml.org/lkml/2010/5/30/4
> >
> > If (can_use_console() == 0), printk() will spool its output to log_buf
> > and it will be visible in "dmesg", but that output will NOT be echoed to
> > the console until somebody calls release_console_sem() from a CPU that
> > is online.  Therefore, the boot time messages from the new CPU can get
> > stuck in "limbo" for a long time, and might suddenly appear on the
> > screen when a completely unrelated event (e.g. "eth0: link is down")
> > occurs.
> >
> > This patch modifies the console code so that any pending messages are
> > automatically flushed out to the console whenever a CPU hotplug
> > operation completes successfully or aborts.
> >
> > -----------------------------------------------
> >
> > Above patch fixes only half of the problem. I mean the cpu online
> > path prints are coming on the console.
> >
> > But similar problem also exist if there are prints in the cpu offline
> > path. I got that fixed by adding below patch on top of you patch.
> >
> > diff --git a/kernel/printk.c b/kernel/printk.c
> > index d370b74..f4d7352 100644
> > --- a/kernel/printk.c
> > +++ b/kernel/printk.c
> > @@ -982,6 +982,9 @@ static int __cpuinit console_cpu_notify(struct
> notifier_bloc
> >         switch (action) {
> >         case CPU_ONLINE:
> >         case CPU_UP_CANCELED:
> > +       case CPU_DEAD:
> > +       case CPU_DYING:
> > +       case CPU_DOWN_FAILED:
> >                 if (try_acquire_console_sem() == 0)
> >                         release_console_sem();
> >         }
> 
> The patch lacked a suitable title.  I called it "console: flush log
> messages for more cpu-hotplug events".
>
This diff was on top of already posted RFC patch. I will combine them

> The patch lacks a Signed-off-by:.  Please send one.
> 
> The patch has its tabs replaced with spaces.  I fixed that.  Please
> reconfigure your email client for next time.
> 
> The code which is being patch has changed.  It now does
> 
>                 acquire_console_sem();
>                 release_console_sem();
> 
> so the code may no longer work - perhaps it now deadlocks (unlikely).
> Please retest?
Retested. No deadlock observed
> 
> Finally, I don't understand the patch :( Who is sending out CPU_DEAD,
> CPU_DYING or CPU_DOWN_FAILED events during kernel boot?  I'd have
> thought that those events simply aren't occurring, and that the patch
> has no effect.  Confused - please explain further.
These events can come during the CPU hotplug(offline). Below is the
complete patch. Also attaching it in case some email format screw
up.

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