[PATCH] console: flush delayed log messages from cpu-hotplug event=
From: Santosh Shilimkar
Date: Tue Aug 03 2010 - 03:28:22 EST
When a secondary CPU is being brought up, it is not uncommon for
printk() to be invoked when cpu_online(smp_processor_id()) =3D=3D 0. The
case that I witnessed personally was on MIPS:
http://lkml.org/lkml/2010/5/30/4
If (can_use_console() =3D=3D 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@xxxxxx>
Signed-off-by: Kevin Cernekee <cernekee@xxxxxxxxx>
---
kernel/printk.c | 36 ++++++++++++++++++++++++++++++++++++
1 files changed, 36 insertions(+), 0 deletions(-)
diff --git a/kernel/printk.c b/kernel/printk.c
index 444b770..a884d81 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>
=20
#include <asm/uaccess.h>
=20
@@ -985,6 +987,40 @@ void resume_console(void)
}
=20
/**
+ * 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)
+{
+ switch (action) {
+ case CPU_ONLINE:
+ case CPU_DEAD:
+ case CPU_DYING:
+ case CPU_DOWN_FAILED:
+ case CPU_UP_CANCELED:
+ if (try_acquire_console_sem() =3D=3D 0)
+ release_console_sem();
+ }
+ return NOTIFY_OK;
+}
+
+static struct notifier_block __cpuinitdata console_nb =3D {
+ .notifier_call =3D 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
--=20
1.6.0.4
-----------------------------------------------
--_002_EAF47CD23C76F840A9E7FCE10091EFAB02C641D8ECdbde02enttico_
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