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
