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SubjectRe: net: suspicious RCU usage in nf_hook
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On Wed, 2017-02-01 at 12:51 -0800, Cong Wang wrote:
> On Tue, Jan 31, 2017 at 7:44 AM, Eric Dumazet <eric.dumazet@gmail.com> wrote:
> > On Mon, 2017-01-30 at 22:19 -0800, Cong Wang wrote:
> >
> >>
> >> The context is process context (TX path before hitting qdisc), and
> >> BH is not disabled, so in_interrupt() doesn't catch it. Hmm, this
> >> makes me thinking maybe we really need to disable BH in this
> >> case for nf_hook()? But it is called in RX path too, and BH is
> >> already disabled there.
> >
> > ipt_do_table() and similar netfilter entry points disable BH.
> >
> > Maybe it is done too late.
>
> I think we need a fix like the following one for minimum impact.
>
> diff --git a/net/core/dev.c b/net/core/dev.c
> index 727b6fd..eee7d63 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -1720,12 +1720,10 @@ EXPORT_SYMBOL(net_enable_timestamp);
> void net_disable_timestamp(void)
> {
> #ifdef HAVE_JUMP_LABEL
> - if (in_interrupt()) {
> - atomic_inc(&netstamp_needed_deferred);
> - return;
> - }
> -#endif
> + atomic_inc(&netstamp_needed_deferred);
> +#else
> static_key_slow_dec(&netstamp_needed);
> +#endif
> }
> EXPORT_SYMBOL(net_disable_timestamp);

This would permanently leave the kernel in the netstamp_needed state.

I would prefer the patch using a process context to perform the
cleanup ? Note there is a race window, but probably not a big deal.

net/core/dev.c | 30 ++++++++++--------------------
1 file changed, 10 insertions(+), 20 deletions(-)

diff --git a/net/core/dev.c b/net/core/dev.c
index 7f218e095361520d11c243d650e053321ea7274f..1cae681b6cfd1cf2c9bee7072eb8af9cf79cced8 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -1695,37 +1695,27 @@ 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);

void net_disable_timestamp(void)
{
-#ifdef HAVE_JUMP_LABEL
- if (in_interrupt()) {
- atomic_inc(&netstamp_needed_deferred);
- return;
- }
-#endif
- static_key_slow_dec(&netstamp_needed);
+ atomic_inc(&netstamp_needed_deferred);
+ schedule_work(&netstamp_work);
}
EXPORT_SYMBOL(net_disable_timestamp);


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