linux-kernel - Re: [PATCH] printk/tracing: Do not trace printk_nmi

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Date:   Fri, 7 Sep 2018 09:45:31 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH] printk/tracing: Do not trace printk_nmi_enter()

On Thu, Sep 06, 2018 at 11:31:51AM +0900, Sergey Senozhatsky wrote:
> An alternative option, thus, could be re-instating back the rule that
> lockdep_off/on should be the first and the last thing we do in
> nmi_enter/nmi_exit. E.g.
> 
> nmi_enter()
> 	lockdep_off();
> 	printk_nmi_enter();
> 
> nmi_exit()
> 	printk_nmi_exit();
> 	lockdep_on();

Yes that. Also, those should probably be inline functions.

---
Subject: locking/lockdep: Fix NMI handling

Someone put code in the NMI handler before lockdep_off(). Since lockdep
is not NMI safe, this wrecks stuff.

Fixes: 42a0bb3f7138 ("printk/nmi: generic solution for safe printk in NMI")
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
 include/linux/hardirq.h  |  4 ++--
 include/linux/lockdep.h  | 11 +++++++++--
 kernel/locking/lockdep.c | 12 ------------
 3 files changed, 11 insertions(+), 16 deletions(-)

diff --git a/include/linux/hardirq.h b/include/linux/hardirq.h
index 0fbbcdf0c178..8d70270d9486 100644
--- a/include/linux/hardirq.h
+++ b/include/linux/hardirq.h
@@ -62,8 +62,8 @@ extern void irq_exit(void);
 
 #define nmi_enter()						\
 	do {							\
-		printk_nmi_enter();				\
 		lockdep_off();					\
+		printk_nmi_enter();				\
 		ftrace_nmi_enter();				\
 		BUG_ON(in_nmi());				\
 		preempt_count_add(NMI_OFFSET + HARDIRQ_OFFSET);	\
@@ -78,8 +78,8 @@ extern void irq_exit(void);
 		BUG_ON(!in_nmi());				\
 		preempt_count_sub(NMI_OFFSET + HARDIRQ_OFFSET);	\
 		ftrace_nmi_exit();				\
-		lockdep_on();					\
 		printk_nmi_exit();				\
+		lockdep_on();					\
 	} while (0)
 
 #endif /* LINUX_HARDIRQ_H */
diff --git a/include/linux/lockdep.h b/include/linux/lockdep.h
index b0d0b51c4d85..70bb9e8fc8f9 100644
--- a/include/linux/lockdep.h
+++ b/include/linux/lockdep.h
@@ -272,8 +272,15 @@ extern void lockdep_reset_lock(struct lockdep_map *lock);
 extern void lockdep_free_key_range(void *start, unsigned long size);
 extern asmlinkage void lockdep_sys_exit(void);
 
-extern void lockdep_off(void);
-extern void lockdep_on(void);
+static inline void lockdep_off(void)
+{
+	current->lockdep_recursion++;
+}
+
+static inline void lockdep_on(void)
+{
+	current->lockdep_recursion--;
+}
 
 /*
  * These methods are used by specific locking variants (spinlocks,
diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index e406c5fdb41e..da51ed1c0c21 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -317,18 +317,6 @@ static inline u64 iterate_chain_key(u64 key, u32 idx)
 	return k0 | (u64)k1 << 32;
 }
 
-void lockdep_off(void)
-{
-	current->lockdep_recursion++;
-}
-EXPORT_SYMBOL(lockdep_off);
-
-void lockdep_on(void)
-{
-	current->lockdep_recursion--;
-}
-EXPORT_SYMBOL(lockdep_on);
-
 /*
  * Debugging switches:
  */