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authorSteven Rostedt (Red Hat) <rostedt@goodmis.org>2014-06-19 17:33:31 -0400
committerSteven Rostedt <rostedt@goodmis.org>2014-11-19 22:01:21 -0500
commitafdc34a3d3b823a12a93b822ee1efb566f884032 (patch)
treed770a2806476751adba361918edb8826762db730 /kernel/printk
parent8d58e99af5980d444948720977b0976455885391 (diff)
downloadlinux-afdc34a3d3b823a12a93b822ee1efb566f884032.tar.bz2
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 <jkosina@suse.cz> Acked-by: Jiri Kosina <jkosina@suse.cz> Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Reviewed-by: Petr Mladek <pmladek@suse.cz> Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Diffstat (limited to 'kernel/printk')
-rw-r--r--kernel/printk/printk.c38
1 files changed, 29 insertions, 9 deletions
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;