[PATCH v2] printk: fix delayed messages from CPU hotplug events

[PATCH v2] printk: fix delayed messages from CPU hotplug events

[Kevin Cernekee]

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.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
---
 kernel/printk.c |   33 +++++++++++++++++++++++++++++++++
 1 files changed, 33 insertions(+), 0 deletions(-)

diff --git a/kernel/printk.c b/kernel/printk.c
index 444b770..9c04719 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 +987,37 @@ void resume_console(void)
 }
 
 /**
+ * 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_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
-- 
1.7.0.4

Re: [PATCH v2] printk: fix delayed messages from CPU hotplug events

[Paul Mundt]

On Mon, May 31, 2010 at 04:57:15PM -0700, Kevin Cernekee wrote:
>  /**
> + * 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_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;
> +}

If this is to be entirely restricted to CPU hotplug then you could use
the hotcpu notifier here instead of the open-coded cpu notifier directly,
the former wraps to the latter in the CPU hotplug case and is simply a
nop for the regular SMP case.

Re: [PATCH v2] printk: fix delayed messages from CPU hotplug events

[Kevin Cernekee]

On Mon, May 31, 2010 at 8:15 PM, Paul Mundt <lethal@linux-sh.org> wrote:
> If this is to be entirely restricted to CPU hotplug then you could use
> the hotcpu notifier here instead of the open-coded cpu notifier directly,
> the former wraps to the latter in the CPU hotplug case and is simply a
> nop for the regular SMP case.

I ran some tests and saw the same problem during the regular MIPS SMP
boot.  i.e. adding "while (1) { }" at the end of __cpu_up() prevents
any of the probing/calibration messages originating on CPU1 from ever
being echoed to the console.  Adding the semaphore code before the
while loop caused the CPU1 messages to reappear.

Under normal circumstances you won't ever notice the problem at boot
time, because printing "Brought up %ld CPUs" has the undocumented side
effect of flushing out any messages that got stuck during SMP init.
And if that printk() wasn't there, the next one (from NET, PCI, SCSI,
...) would surely take its place.

But in the case of MIPS CPU hotplug, there is no such printk() at the
end, and so our luck runs out.

Re: [PATCH v2] printk: fix delayed messages from CPU hotplug events

[Andrew Morton]

On Mon, 31 May 2010 21:04:42 -0700
Kevin Cernekee <cernekee@gmail.com> wrote:

> On Mon, May 31, 2010 at 8:15 PM, Paul Mundt <lethal@linux-sh.org> wrote:
> > If this is to be entirely restricted to CPU hotplug then you could use
> > the hotcpu notifier here instead of the open-coded cpu notifier directly,
> > the former wraps to the latter in the CPU hotplug case and is simply a
> > nop for the regular SMP case.
> 
> I ran some tests and saw the same problem during the regular MIPS SMP
> boot.  i.e. adding "while (1) { }" at the end of __cpu_up() prevents
> any of the probing/calibration messages originating on CPU1 from ever
> being echoed to the console.  Adding the semaphore code before the
> while loop caused the CPU1 messages to reappear.
> 
> Under normal circumstances you won't ever notice the problem at boot
> time, because printing "Brought up %ld CPUs" has the undocumented side
> effect of flushing out any messages that got stuck during SMP init.
> And if that printk() wasn't there, the next one (from NET, PCI, SCSI,
> ...) would surely take its place.
> 
> But in the case of MIPS CPU hotplug, there is no such printk() at the
> end, and so our luck runs out.

no....  What Paul means is "please use hotcpu_notifier".  It's a
higher-level interface which yields a smaller vmlinux if
CONFIG_HOTPLUG_CPU=n.  grep around for some examples...


other comments:
 
>  /**
> + * 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.
> + */

It's conventional (although boring and usually useless) to kerneldocify
the arguments also.

> +static int __cpuinit console_cpu_notify(struct notifier_block *self,
> +	unsigned long action, void *hcpu)
> +{
> +	switch (action) {
> +	case CPU_ONLINE:
> +	case CPU_UP_CANCELED:
> +		if (try_acquire_console_sem() == 0)
> +			release_console_sem();
> +	}
> +	return NOTIFY_OK;
> +}

Would prefer to see acquire_console_sem() used here.  Because
try_acquire_console_sem() might simply fail, and the messages still get
stuck.  Possible?  If "not possible" then "needs a code comment".

> +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);

We don't really need two late_initcall() functions in printk.c.  We'd
save a few bytes by renaming disable_boot_consoles() to
printk_late_init() or something, then adding the hotcpu_notifier() call
there.

otoh, that's a bit of a reduction in source-level quality.

otoh2, perhaps late_initcall() was inappropriate for
console_notifier_init().  Why not do it earlier?

I'll let you decide ;)