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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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