Re: net: suspicious RCU usage in nf_hook

[Eric Dumazet]

On Wed, 2017-02-01 at 15:48 -0800, Eric Dumazet wrote:
> On Wed, Feb 1, 2017 at 3:29 PM, Cong Wang <xiyou.wangcong@xxxxxxxxx> wrote:
> 
> > Not sure if it is better. The difference is caught up in net_enable_timestamp(),
> > which is called setsockopt() path and sk_clone() path, so we could be
> > in netstamp_needed state for a long time too until user-space exercises
> > these paths.
> >
> > I am feeling we probably need to get rid of netstamp_needed_deferred,
> > and simply defer the whole static_key_slow_dec(), like the attached patch
> > (compile only).
> >
> > What do you think?
> 
> I think we need to keep the atomic.
> 
> If two cpus call net_disable_timestamp() roughly at the same time, the
> work will be scheduled once.

Updated patch (but not tested yet)

diff --git a/net/core/dev.c b/net/core/dev.c
index 7f218e095361520d11c243d650e053321ea7274f..29101c98399f40b6b8e42c31a255d8f1fb6bd7a1 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -1695,24 +1695,19 @@ EXPORT_SYMBOL_GPL(net_dec_egress_queue);
 
 static struct static_key netstamp_needed __read_mostly;
 #ifdef HAVE_JUMP_LABEL
-/* We are not allowed to call static_key_slow_dec() from irq context
- * If net_disable_timestamp() is called from irq context, defer the
- * static_key_slow_dec() calls.
- */
 static atomic_t netstamp_needed_deferred;
-#endif
-
-void net_enable_timestamp(void)
+static void netstamp_clear(struct work_struct *work)
 {
-#ifdef HAVE_JUMP_LABEL
 	int deferred = atomic_xchg(&netstamp_needed_deferred, 0);
 
-	if (deferred) {
-		while (--deferred)
-			static_key_slow_dec(&netstamp_needed);
-		return;
-	}
+	while (deferred--)
+		static_key_slow_dec(&netstamp_needed);
+}
+static DECLARE_WORK(netstamp_work, netstamp_clear);
 #endif
+
+void net_enable_timestamp(void)
+{
 	static_key_slow_inc(&netstamp_needed);
 }
 EXPORT_SYMBOL(net_enable_timestamp);
@@ -1720,12 +1715,12 @@ EXPORT_SYMBOL(net_enable_timestamp);
 void net_disable_timestamp(void)
 {
 #ifdef HAVE_JUMP_LABEL
-	if (in_interrupt()) {
-		atomic_inc(&netstamp_needed_deferred);
-		return;
-	}
-#endif
+	/* net_disable_timestamp() can be called from non process context */
+	atomic_inc(&netstamp_needed_deferred);
+	schedule_work(&netstamp_work);
+#else
 	static_key_slow_dec(&netstamp_needed);
+#endif
 }
 EXPORT_SYMBOL(net_disable_timestamp);