linux-kernel - Re: [PATCH] tracing: Do not synchronize freeing of trigger filter on boot up

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Date:   Wed, 14 Dec 2022 12:03:33 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Masami Hiramatsu <mhiramat@...nel.org>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux Trace Kernel <linux-trace-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Subject: Re: [PATCH] tracing: Do not synchronize freeing of trigger filter on
 boot up

On Wed, Dec 14, 2022 at 08:49:54AM +0900, Masami Hiramatsu wrote:
> On Tue, 13 Dec 2022 17:24:29 -0500
> Steven Rostedt <rostedt@...dmis.org> wrote:
> 
> > From: "Steven Rostedt (Google)" <rostedt@...dmis.org>
> > 
> > If a trigger filter on the kernel command line fails to apply (due to
> > syntax error), it will be freed. The freeing will call
> > tracepoint_synchronize_unregister(), but this is not needed during early
> > boot up, and will even trigger a lockdep splat.
> > 
> > Avoid calling the synchronization function when system_state is
> > SYSTEM_BOOTING.
> 
> Shouldn't this be done inside tracepoint_synchronize_unregister()?
> Then, it will prevent similar warnings if we expand boot time feature.

How about the following wide-spectrum fix within RCU_LOCKDEP_WARN()?
Just in case there are ever additional issues of this sort?

							Thanx, Paul

------------------------------------------------------------------------

commit d493ffca2df6c1963bd1d7b8f8c652a172f095ae
Author: Paul E. McKenney <paulmck@...nel.org>
Date:   Wed Dec 14 11:41:44 2022 -0800

    rcu: Make RCU_LOCKDEP_WARN() avoid early lockdep checks
    
    Currently, RCU_LOCKDEP_WARN() checks the condition before checking
    to see if lockdep is still enabled.  This is necessary to avoid the
    false-positive splats fixed by commit 3066820034b5dd ("rcu: Reject
    RCU_LOCKDEP_WARN() false positives").  However, the current state can
    result in false-positive splats during early boot before lockdep is fully
    initialized.  This commit therefore checks debug_lockdep_rcu_enabled()
    both before and after checking the condition, thus avoiding both sets
    of false-positive error reports.
    
    Reported-by: Steven Rostedt <rostedt@...dmis.org>
    Reported-by: Masami Hiramatsu (Google) <mhiramat@...nel.org>
    Reported-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
    Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
    Cc: Boqun Feng <boqun.feng@...il.com>
    Cc: Matthew Wilcox <willy@...radead.org>
    Cc: Thomas Gleixner <tglx@...utronix.de>

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index aa1b1c3546d7a..1aec1d53b0c91 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -364,11 +364,18 @@ static inline int debug_lockdep_rcu_enabled(void)
  * RCU_LOCKDEP_WARN - emit lockdep splat if specified condition is met
  * @c: condition to check
  * @s: informative message
+ *
+ * This checks debug_lockdep_rcu_enabled() before checking (c) to
+ * prevent early boot splats due to lockdep not yet being initialized,
+ * and rechecks it after checking (c) to prevent false-positive splats
+ * due to races with lockdep being disabled.  See commit 3066820034b5dd
+ * ("rcu: Reject RCU_LOCKDEP_WARN() false positives") for more detail.
  */
 #define RCU_LOCKDEP_WARN(c, s)						\
 	do {								\
 		static bool __section(".data.unlikely") __warned;	\
-		if ((c) && debug_lockdep_rcu_enabled() && !__warned) {	\
+		if (debug_lockdep_rcu_enabled() && (c) &&		\
+		    debug_lockdep_rcu_enabled() && !__warned) {		\
 			__warned = true;				\
 			lockdep_rcu_suspicious(__FILE__, __LINE__, s);	\
 		}							\