From 034260d6779087431a8b2f67589c68b919299e5c Mon Sep 17 00:00:00 2001 From: Kevin Cernekee Date: Thu, 3 Jun 2010 22:11:25 -0700 Subject: 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. Original patch by Kevin Cernekee with cleanups by akpm and additional fixes by Santosh Shilimkar. This patch superseeds https://patchwork.linux-mips.org/patch/1357/. Signed-off-by: Kevin Cernekee To: To: To: To: To: Cc: Cc: Reviewed-by: Paul Mundt Signed-off-by: Kevin Cernekee Patchwork: https://patchwork.linux-mips.org/patch/1534/ LKML-Reference: LKML-Reference: Signed-off-by: Ralf Baechle --- kernel/printk.c | 33 +++++++++++++++++++++++++++++++-- 1 file changed, 31 insertions(+), 2 deletions(-) (limited to 'kernel/printk.c') diff --git a/kernel/printk.c b/kernel/printk.c index 444b770..4ab0164 100644 --- a/kernel/printk.c +++ b/kernel/printk.c @@ -37,6 +37,8 @@ #include #include #include +#include +#include #include @@ -985,6 +987,32 @@ 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_DEAD: + case CPU_DYING: + case CPU_DOWN_FAILED: + 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 +1399,7 @@ int unregister_console(struct console *console) } EXPORT_SYMBOL(unregister_console); -static int __init disable_boot_consoles(void) +static int __init printk_late_init(void) { struct console *con; @@ -1382,9 +1410,10 @@ static int __init disable_boot_consoles(void) 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 -- cgit v1.1 From 8c4af38e9b2c2a78369c4e2e5706fe539ac64eb2 Mon Sep 17 00:00:00 2001 From: Andi Kleen Date: Mon, 9 Aug 2010 17:20:36 -0700 Subject: gcc-4.6: printk: use stable variable to dump kmsg buffer kmsg_dump takes care to sample the global variables inside a spinlock, but then goes on to use the same variables outside the spinlock region too. Use the correct variable. This will make the race window smaller. Found by gcc 4.6's new warnings. Signed-off-by: Andi Kleen Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- kernel/printk.c | 10 +++++----- 1 file changed, 5 insertions(+), 5 deletions(-) (limited to 'kernel/printk.c') diff --git a/kernel/printk.c b/kernel/printk.c index 4ab0164..8fe465a 100644 --- a/kernel/printk.c +++ b/kernel/printk.c @@ -1549,9 +1549,9 @@ void kmsg_dump(enum kmsg_dump_reason reason) chars = logged_chars; spin_unlock_irqrestore(&logbuf_lock, flags); - if (logged_chars > end) { - s1 = log_buf + log_buf_len - logged_chars + end; - l1 = logged_chars - end; + if (chars > end) { + s1 = log_buf + log_buf_len - chars + end; + l1 = chars - end; s2 = log_buf; l2 = end; @@ -1559,8 +1559,8 @@ void kmsg_dump(enum kmsg_dump_reason reason) s1 = ""; l1 = 0; - s2 = log_buf + end - logged_chars; - l2 = logged_chars; + s2 = log_buf + end - chars; + l2 = chars; } if (!spin_trylock_irqsave(&dump_list_lock, flags)) { -- cgit v1.1