linux-kernel - Re: [PATCH] printk/nmi: avoid direct printk()-s from __printk_nmi

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Date:   Tue, 30 Aug 2016 16:58:34 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jan Kara <jack@...e.cz>, linux-kernel@...r.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Subject: Re: [PATCH] printk/nmi: avoid direct printk()-s from
 __printk_nmi_flush()

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.senozhatsky@...il.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