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

From: Shilimkar, Santosh
Date: Tue Jun 29 2010 - 04:52:48 EST


I have faced similar issue as what is being described in below with
latest kernel.


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:

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")

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_DEAD:
+ case CPU_DYING:
if (try_acquire_console_sem() == 0)

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