From afdc34a3d3b823a12a93b822ee1efb566f884032 Mon Sep 17 00:00:00 2001 From: "Steven Rostedt (Red Hat)" Date: Thu, 19 Jun 2014 17:33:31 -0400 Subject: printk: Add per_cpu printk func to allow printk to be diverted Being able to divert printk to call another function besides the normal logging is useful for such things like NMI handling. If some functions are to be called from NMI that does printk() it is possible to lock up the box if the nmi handler triggers when another printk is happening. One example of this use is to perform a stack trace on all CPUs via NMI. But if the NMI is to do the printk() it can cause the system to lock up. By allowing the printk to be diverted to another function that can safely record the printk output and then print it when it in a safe context then NMIs will be safe to call these functions like show_regs(). Link: http://lkml.kernel.org/p/20140619213952.209176403@goodmis.org Tested-by: Jiri Kosina Acked-by: Jiri Kosina Acked-by: Paul E. McKenney Reviewed-by: Petr Mladek Signed-off-by: Steven Rostedt --- kernel/printk/printk.c | 38 +++++++++++++++++++++++++++++--------- 1 file changed, 29 insertions(+), 9 deletions(-) (limited to 'kernel/printk') diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c index ced2b84b1cb7..f7b723f98cb9 100644 --- a/kernel/printk/printk.c +++ b/kernel/printk/printk.c @@ -1807,6 +1807,30 @@ asmlinkage int printk_emit(int facility, int level, } EXPORT_SYMBOL(printk_emit); +int vprintk_default(const char *fmt, va_list args) +{ + int r; + +#ifdef CONFIG_KGDB_KDB + if (unlikely(kdb_trap_printk)) { + r = vkdb_printf(fmt, args); + return r; + } +#endif + r = vprintk_emit(0, -1, NULL, 0, fmt, args); + + return r; +} +EXPORT_SYMBOL_GPL(vprintk_default); + +/* + * This allows printk to be diverted to another function per cpu. + * This is useful for calling printk functions from within NMI + * without worrying about race conditions that can lock up the + * box. + */ +DEFINE_PER_CPU(printk_func_t, printk_func) = vprintk_default; + /** * printk - print a kernel message * @fmt: format string @@ -1830,19 +1854,15 @@ EXPORT_SYMBOL(printk_emit); */ asmlinkage __visible int printk(const char *fmt, ...) { + printk_func_t vprintk_func; va_list args; int r; -#ifdef CONFIG_KGDB_KDB - if (unlikely(kdb_trap_printk)) { - va_start(args, fmt); - r = vkdb_printf(fmt, args); - va_end(args); - return r; - } -#endif va_start(args, fmt); - r = vprintk_emit(0, -1, NULL, 0, fmt, args); + preempt_disable(); + vprintk_func = this_cpu_read(printk_func); + r = vprintk_func(fmt, args); + preempt_enable(); va_end(args); return r; -- cgit v1.2.3 From 04b74b27c2941e5d62120f6fee3a0a9388a30613 Mon Sep 17 00:00:00 2001 From: "Steven Rostedt (Red Hat)" Date: Fri, 21 Nov 2014 09:16:58 -0500 Subject: printk/percpu: Define printk_func when printk is not defined To avoid include hell, the per_cpu variable printk_func was declared in percpu.h. But it is only defined if printk is defined. As users of printk may also use the printk_func variable, it needs to be defined even if CONFIG_PRINTK is not. Also add a printk.h include in percpu.h just to be safe. Link: http://lkml.kernel.org/r/20141121183215.01ba539c@canb.auug.org.au Reported-by: Stephen Rothwell Signed-off-by: Steven Rostedt --- kernel/printk/printk.c | 3 +++ 1 file changed, 3 insertions(+) (limited to 'kernel/printk') diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c index f7b723f98cb9..5af2b8bc88f0 100644 --- a/kernel/printk/printk.c +++ b/kernel/printk/printk.c @@ -1896,6 +1896,9 @@ static size_t msg_print_text(const struct printk_log *msg, enum log_flags prev, bool syslog, char *buf, size_t size) { return 0; } static size_t cont_print_text(char *text, size_t size) { return 0; } +/* Still needs to be defined for users */ +DEFINE_PER_CPU(printk_func_t, printk_func); + #endif /* CONFIG_PRINTK */ #ifdef CONFIG_EARLY_PRINTK -- cgit v1.2.3