linux-kernel - [PATCH v2] printk: Make console tracepoint safe in NMI() context

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Message-Id: <20220718151143.32112-1-pmladek@suse.com>
Date:   Mon, 18 Jul 2022 17:11:43 +0200
From:   Petr Mladek <pmladek@...e.com>
To:     Steven Rostedt <rostedt@...dmis.org>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        John Ogness <john.ogness@...utronix.de>
Cc:     Sergey Senozhatsky <senozhatsky@...omium.org>,
        Marco Elver <elver@...gle.com>, kasan-dev@...glegroups.com,
        Thomas Gleixner <tglx@...utronix.de>,
        Johannes Berg <johannes.berg@...el.com>,
        Alexander Potapenko <glider@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Naresh Kamboju <naresh.kamboju@...aro.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Linux Kernel Functional Testing <lkft@...aro.org>,
        linux-kernel@...r.kernel.org, Petr Mladek <pmladek@...e.com>
Subject: [PATCH v2] printk: Make console tracepoint safe in NMI() context

The commit 701850dc0c31bfadf75a0 ("printk, tracing: fix console
tracepoint") moved the tracepoint from console_unlock() to
vprintk_store(). As a result, it might be called in any
context and triggered the following warning:

  WARNING: CPU: 1 PID: 16462 at include/trace/events/printk.h:10 printk_sprint+0x81/0xda
  Modules linked in: ppdev parport_pc parport
  CPU: 1 PID: 16462 Comm: event_benchmark Not tainted 5.19.0-rc5-test+ #5
  Hardware name: MSI MS-7823/CSM-H87M-G43 (MS-7823), BIOS V1.6 02/22/2014
  EIP: printk_sprint+0x81/0xda
  Code: 89 d8 e8 88 fc 33 00 e9 02 00 00 00 eb 6b 64 a1 a4 b8 91 c1 e8 fd d6 ff ff 84 c0 74 5c 64 a1 14 08 92 c1 a9 00 00 f0 00 74 02 <0f> 0b 64 ff 05 14 08 92 c1 b8 e0 c4 6b c1 e8 a5 dc 00 00 89 c7 e8
  EAX: 80110001 EBX: c20a52f8 ECX: 0000000c EDX: 6d203036
  ESI: 3df6004c EDI: 00000000 EBP: c61fbd7c ESP: c61fbd70
  DS: 007b ES: 007b FS: 00d8 GS: 0000 SS: 0068 EFLAGS: 00010006
  CR0: 80050033 CR2: b7efc000 CR3: 05b80000 CR4: 001506f0
  Call Trace:
   vprintk_store+0x24b/0x2ff
   vprintk+0x37/0x4d
   _printk+0x14/0x16
   nmi_handle+0x1ef/0x24e
   ? find_next_bit.part.0+0x13/0x13
   ? find_next_bit.part.0+0x13/0x13
   ? function_trace_call+0xd8/0xd9
   default_do_nmi+0x57/0x1af
   ? trace_hardirqs_off_finish+0x2a/0xd9
   ? to_kthread+0xf/0xf
   exc_nmi+0x9b/0xf4
   asm_exc_nmi+0xae/0x29c

It comes from:

  #define __DO_TRACE(name, args, cond, rcuidle) \
  [...]
		/* srcu can't be used from NMI */	\
		WARN_ON_ONCE(rcuidle && in_nmi());	\

It might be possible to make srcu working in NMI. But it
would be slower on some architectures. It is not worth
doing it just because of this tracepoint.

It would be possible to disable this tracepoint in NMI
or in rcuidle context. Where the rcuidle context looks
more rare and thus more acceptable to be ignored.

Alternative solution would be to move the tracepoint
back to console code. But the location is less reliable
by definition. Also the synchronization against other
tracing messages is much worse.

Let's ignore the tracepoint in rcuidle context as the least
evil solution.

Link: https://lore.kernel.org/r/20220712151655.GU1790663@paulmck-ThinkPad-P17-Gen-1

Suggested-by: Steven Rostedt <rostedt@...dmis.org>
Signed-off-by: Petr Mladek <pmladek@...e.com>
---
Changes against v1:

  + use rcu_is_watching() instead of rcu_is_idle_cpu()


 include/trace/events/printk.h | 9 ++++++++-
 kernel/printk/printk.c        | 2 +-
 2 files changed, 9 insertions(+), 2 deletions(-)

diff --git a/include/trace/events/printk.h b/include/trace/events/printk.h
index 13d405b2fd8b..5485513d8838 100644
--- a/include/trace/events/printk.h
+++ b/include/trace/events/printk.h
@@ -7,11 +7,18 @@
 
 #include <linux/tracepoint.h>
 
-TRACE_EVENT(console,
+TRACE_EVENT_CONDITION(console,
 	TP_PROTO(const char *text, size_t len),
 
 	TP_ARGS(text, len),
 
+	/*
+	 * trace_console_rcuidle() is not working in NMI. printk()
+	 * is used more often in NMI than in rcuidle context.
+	 * Choose the less evil solution here.
+	 */
+	TP_CONDITION(rcu_is_watching()),
+
 	TP_STRUCT__entry(
 		__dynamic_array(char, msg, len + 1)
 	),
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index b49c6ff6dca0..bd76a45ecc7f 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2108,7 +2108,7 @@ static u16 printk_sprint(char *text, u16 size, int facility,
 		}
 	}
 
-	trace_console_rcuidle(text, text_len);
+	trace_console(text, text_len);
 
 	return text_len;
 }
-- 
2.35.3