Re: [PATCH] printk/nmi: avoid direct printk()-s from __printk_nmi_flush()

[Sergey Senozhatsky]

Petr,
one more question. Not related to the patch, but still related to NMI.

can NMI nest?


        CPU0

->      NMI#0
                printk_nmi_enter()
                        this_cpu_write(printk_func, vprintk_nmi)
                ...
=>              NMI#1
                :       printk_nmi_enter()
                :               this_cpu_write(printk_func, vprintk_nmi)
                :       this_cpu_read(printk_func)(fmt, args)
                :       ^^^^^^^^^^^^^ OK, vprintk_nmi() from NMI
                :       printk_nmi_exit()
                :               this_cpu_write(printk_func, vprintk_default) <<
                return NMI#1

                this_cpu_read(printk_func)(fmt, args)
                ^^^^^^^^^^^^^ vprintk_default() from NMI?
                printk_nmi_exit()
                        this_cpu_write(printk_func, vprintk_default)
        return NMI#0

can this happen?
shouldn't we do something like this then? /* not tested */

Signed-off-by: Sergey Senozhatsky <sergey.senozhat...@gmail.com>

---
 kernel/printk/internal.h | 2 ++
 kernel/printk/nmi.c      | 9 ++++++++-
 2 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/kernel/printk/internal.h b/kernel/printk/internal.h
index 7fd2838..5b7508f 100644
--- a/kernel/printk/internal.h
+++ b/kernel/printk/internal.h
@@ -31,6 +31,8 @@ extern raw_spinlock_t logbuf_lock;
  * via per-CPU variable.
  */
 DECLARE_PER_CPU(printk_func_t, printk_func);
+DECLARE_PER_CPU(printk_func_t, printk_func_saved);
+
 static inline __printf(1, 0) int vprintk_func(const char *fmt, va_list args)
 {
        return this_cpu_read(printk_func)(fmt, args);
diff --git a/kernel/printk/nmi.c b/kernel/printk/nmi.c
index 16bab47..9d83929 100644
--- a/kernel/printk/nmi.c
+++ b/kernel/printk/nmi.c
@@ -39,6 +39,7 @@
  * were handled or when IRQs are blocked.
  */
 DEFINE_PER_CPU(printk_func_t, printk_func) = vprintk_default;
+DEFINE_PER_CPU(printk_func_t, printk_func_saved);
 static int printk_nmi_irq_ready;
 atomic_t nmi_message_lost;
 
@@ -259,10 +260,16 @@ void __init printk_nmi_init(void)
 
 void printk_nmi_enter(void)
 {
+       printk_func_t func = this_cpu_read(printk_func);
+
+       if (func != vprintk_nmi)
+               this_cpu_write(printk_func_saved, func);
        this_cpu_write(printk_func, vprintk_nmi);
 }
 
 void printk_nmi_exit(void)
 {
-       this_cpu_write(printk_func, vprintk_default);
+       printk_func_t func = this_cpu_read(printk_func_saved);
+
+       this_cpu_write(printk_func, func);
 }
-- 
2.9.3