[PATCH v4] printk: fix delayed messages from CPU hotplug events

[Ralf Baechle]

From:   Kevin Cernekee <cernekee@xxxxxxxxx>

printk: fix delayed 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.

The issue was seen on 2.6.34.

[Ralf: Cleanup patch posted by akpm merged into the patch]

Signed-off-by: Kevin Cernekee <cernekee@xxxxxxxxx>
To: <mingo@xxxxxxx>
To: <akpm@xxxxxxxxxxxxxxxxxxxx>
To: <simon.kagstrom@xxxxxxxxxxxxxx>
To: <David.Woodhouse@xxxxxxxxx>
To: <lethal@xxxxxxxxxxxx>
Cc: <linux-kernel@xxxxxxxxxxxxxxx>
Cc: <linux-mips@xxxxxxxxxxxxxx>
Patchwork: http://patchwork.linux-mips.org/patch/1355/
Reviewed-by: Paul Mundt <lethal@xxxxxxxxxxxx>
Patchwork: http://patchwork.linux-mips.org/patch/1356/
Signed-off-by: Ralf Baechle <ralf@xxxxxxxxxxxxxx>

 kernel/printk.c |   30 ++++++++++++++++++++++++++++--
 1 file changed, 28 insertions(+), 2 deletions(-)

Index: linux-queue/kernel/printk.c
===================================================================
--- linux-queue.orig/kernel/printk.c
+++ linux-queue/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 +987,29 @@ void resume_console(void)
 }
 
 /**
+ * console_cpu_notify - print deferred console messages after CPU hotplug
+ * @self: notifier struct
+ * @action: CPU hotplug event
+ * @hcpu: unused
+ *
+ * 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 fails to come up), 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_UP_CANCELED:
+		acquire_console_sem();
+		release_console_sem();
+	}
+	return NOTIFY_OK;
+}
+
+/**
  * acquire_console_sem - lock the console system for exclusive use.
  *
  * Acquires a semaphore which guarantees that the caller has
@@ -1371,7 +1396,7 @@ int unregister_console(struct console *c
 }
 EXPORT_SYMBOL(unregister_console);
 
-static int __init disable_boot_consoles(void)
+static int __init printk_late_init(void)
 {
 	struct console *con;
 
@@ -1382,9 +1407,10 @@ static int __init disable_boot_consoles(
 			unregister_console(con);
 		}
 	}
+	hotcpu_notifier(console_cpu_notify, 0);
 	return 0;
 }
-late_initcall(disable_boot_consoles);
+late_initcall(printk_late_init);
 
 #if defined CONFIG_PRINTK
 
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