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SubjectRE: Additional fix : (was [v2]printk: fix delayed messages from CPU hotplug events)
> -----Original Message-----
> From: Randy Dunlap [mailto:rdunlap@xenotime.net]
> Sent: Tuesday, August 03, 2010 8:55 PM
> To: Shilimkar, Santosh
> Cc: Andrew Morton; Kevin Cernekee; linux-kernel@vger.kernel.org; Russell
> King - ARM Linux
> Subject: Re: Additional fix : (was [v2]printk: fix delayed messages from
> CPU hotplug events)
>
> On Tue, 3 Aug 2010 13:03:25 +0530 Shilimkar, Santosh wrote:
>

<snip>

> > /**
> > + * console_cpu_notify - print deferred console messages after CPU
> hotplug
> > + *
> > + * If printk() is called from a CPU that is not online yet, the
> messages
> > + * will be spooled but will not show up on the console. This function
> is
> > + * called when a new CPU comes online and ensures that any such output
> > + * gets printed.
> > + */
> > +static int __cpuinit console_cpu_notify(struct notifier_block *self,
> > + unsigned long action, void *hcpu)
> > +{
>
>
> kernel-doc notation should include function parameters also, please.
>
Sure. Here is an updated version.
--------------------------------------------
From 127c0ea58982762ad194f22d1281baaf529c9cbc Mon Sep 17 00:00:00 2001
From: Santosh Shilimkar <santosh.shilimkar@ti.com>
Date: Tue, 3 Aug 2010 12:58:22 +0530
Subject: [PATCH v2] console: flush delayed log messages from cpu-hotplug events

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.
This is true even when CPU is getting hot-plugged out(offline) so
need to add additional hotplug events.

The issue was seen on 2.6.34.

Signed-off-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
---
kernel/printk.c | 39 +++++++++++++++++++++++++++++++++++++++
1 files changed, 39 insertions(+), 0 deletions(-)

diff --git a/kernel/printk.c b/kernel/printk.c
index 444b770..c145cef 100644
--- a/kernel/printk.c
+++ b/kernel/printk.c
@@ -37,6 +37,8 @@
#include <linux/ratelimit.h>
#include <linux/kmsg_dump.h>
#include <linux/syslog.h>
+#include <linux/cpu.h>
+#include <linux/notifier.h>

#include <asm/uaccess.h>

@@ -985,6 +987,43 @@ void resume_console(void)
}

/**
+ * console_cpu_notify - print deferred console messages after CPU hotplug
+ * @self: pointer to notfier block
+ * @action: cpu-hotplug events
+ * @hcpu: void poniter to pass any data
+ *
+ * If printk() is called from a CPU that is not online yet, the messages
+ * will be spooled but will not show up on the console. This function is
+ * called when a new CPU comes online or goes offline and ensures that
+ * any such output gets printed.
+ */
+static int __cpuinit console_cpu_notify(struct notifier_block *self,
+ unsigned long action, void *hcpu)
+{
+ switch (action) {
+ case CPU_ONLINE:
+ case CPU_DEAD:
+ case CPU_DYING:
+ case CPU_DOWN_FAILED:
+ case CPU_UP_CANCELED:
+ if (try_acquire_console_sem() == 0)
+ release_console_sem();
+ }
+ return NOTIFY_OK;
+}
+
+static struct notifier_block __cpuinitdata console_nb = {
+ .notifier_call = console_cpu_notify,
+};
+
+static int __init console_notifier_init(void)
+{
+ register_cpu_notifier(&console_nb);
+ return 0;
+}
+late_initcall(console_notifier_init);
+
+/**
* acquire_console_sem - lock the console system for exclusive use.
*
* Acquires a semaphore which guarantees that the caller has
--
1.6.0.4
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