* [PATCH v3.14-rt] netpoll: guard the access to dev->npinfo with rcu_read_lock/unlock_bh() for CONFIG_PREEMPT_RT_FULL=y
@ 2014-12-03 12:05 Kevin Hao
2014-12-03 12:50 ` Nicholas Mc Guire
0 siblings, 1 reply; 5+ messages in thread
From: Kevin Hao @ 2014-12-03 12:05 UTC (permalink / raw
To: linux-rt-users; +Cc: Paul E. McKenney
For vanilla kernel we don't need to invoke rcu_read_lock/unlock_bh
explicitly to mark an RCU-bh critical section in the softirq context
because bh is already disabled in this case. But for a rt kernel,
the commit ("rcu: Merge RCU-bh into RCU-preempt") implements the
RCU-bh in term of RCU-preempt. So we have to use
rcu_read_lock/unlock_bh() to mark an RCU-bh critical section even in
a softirq context. Otherwise we will get a call trace like this:
include/linux/netpoll.h:90 suspicious rcu_dereference_check() usage!
other info that might help us debug this:
rcu_scheduler_active = 1, debug_locks = 0
1 lock held by irq/177-eth0_g0/129:
#0: (&per_cpu(local_softirq_locks[i], __cpu).lock){+.+...}, at: [<8002f544>] do_current_softirqs+0x12c/0x5ec
stack backtrace:
CPU: 0 PID: 129 Comm: irq/177-eth0_g0 Not tainted 3.14.23 #11
[<80018c0c>] (unwind_backtrace) from [<800138b0>] (show_stack+0x20/0x24)
[<800138b0>] (show_stack) from [<8075c3bc>] (dump_stack+0x84/0xd0)
[<8075c3bc>] (dump_stack) from [<8008111c>] (lockdep_rcu_suspicious+0xe8/0x11c)
[<8008111c>] (lockdep_rcu_suspicious) from [<805e94e8>] (dev_gro_receive+0x240/0x724)
[<805e94e8>] (dev_gro_receive) from [<805e9c34>] (napi_gro_receive+0x3c/0x1e8)
[<805e9c34>] (napi_gro_receive) from [<804b01ac>] (gfar_clean_rx_ring+0x2d4/0x624)
[<804b01ac>] (gfar_clean_rx_ring) from [<804b078c>] (gfar_poll_rx_sq+0x58/0xe8)
[<804b078c>] (gfar_poll_rx_sq) from [<805eada8>] (net_rx_action+0x1c8/0x418)
[<805eada8>] (net_rx_action) from [<8002f62c>] (do_current_softirqs+0x214/0x5ec)
[<8002f62c>] (do_current_softirqs) from [<8002fa88>] (__local_bh_enable+0x84/0x9c)
[<8002fa88>] (__local_bh_enable) from [<8002fab8>] (local_bh_enable+0x18/0x1c)
[<8002fab8>] (local_bh_enable) from [<80093924>] (irq_forced_thread_fn+0x50/0x74)
[<80093924>] (irq_forced_thread_fn) from [<80093c30>] (irq_thread+0x158/0x1c4)
[<80093c30>] (irq_thread) from [<800555b8>] (kthread+0xd4/0xe8)
[<800555b8>] (kthread) from [<8000ee88>] (ret_from_fork+0x14/0x20)
Signed-off-by: Kevin Hao <kexin.hao@windriver.com>
---
include/linux/netpoll.h | 16 ++++++++++++++--
1 file changed, 14 insertions(+), 2 deletions(-)
diff --git a/include/linux/netpoll.h b/include/linux/netpoll.h
index fbfdb9d8d3a7..039dd5df0ef7 100644
--- a/include/linux/netpoll.h
+++ b/include/linux/netpoll.h
@@ -87,9 +87,21 @@ static inline void netpoll_send_skb(struct netpoll *np, struct sk_buff *skb)
#ifdef CONFIG_NETPOLL
static inline bool netpoll_rx_on(struct sk_buff *skb)
{
- struct netpoll_info *npinfo = rcu_dereference_bh(skb->dev->npinfo);
+ struct netpoll_info *npinfo;
+ bool ret;
+
+#ifdef CONFIG_PREEMPT_RT_FULL
+ rcu_read_lock_bh();
+#endif
+
+ npinfo = rcu_dereference_bh(skb->dev->npinfo);
+ ret = npinfo && (!list_empty(&npinfo->rx_np) || npinfo->rx_flags);
+
+#ifdef CONFIG_PREEMPT_RT_FULL
+ rcu_read_unlock_bh();
+#endif
- return npinfo && (!list_empty(&npinfo->rx_np) || npinfo->rx_flags);
+ return ret;
}
static inline bool netpoll_rx(struct sk_buff *skb)
--
1.9.3
^ permalink raw reply related [flat|nested] 5+ messages in thread
* Re: [PATCH v3.14-rt] netpoll: guard the access to dev->npinfo with rcu_read_lock/unlock_bh() for CONFIG_PREEMPT_RT_FULL=y
2014-12-03 12:05 [PATCH v3.14-rt] netpoll: guard the access to dev->npinfo with rcu_read_lock/unlock_bh() for CONFIG_PREEMPT_RT_FULL=y Kevin Hao
@ 2014-12-03 12:50 ` Nicholas Mc Guire
2014-12-03 21:19 ` Paul E. McKenney
0 siblings, 1 reply; 5+ messages in thread
From: Nicholas Mc Guire @ 2014-12-03 12:50 UTC (permalink / raw
To: Kevin Hao; +Cc: linux-rt-users, Paul E. McKenney
On Wed, 03 Dec 2014, Kevin Hao wrote:
> For vanilla kernel we don't need to invoke rcu_read_lock/unlock_bh
> explicitly to mark an RCU-bh critical section in the softirq context
> because bh is already disabled in this case. But for a rt kernel,
> the commit ("rcu: Merge RCU-bh into RCU-preempt") implements the
> RCU-bh in term of RCU-preempt. So we have to use
> rcu_read_lock/unlock_bh() to mark an RCU-bh critical section even in
> +
> +#ifdef CONFIG_PREEMPT_RT_FULL
> + rcu_read_lock_bh();
> +#endif
> +
> + npinfo = rcu_dereference_bh(skb->dev->npinfo);
> + ret = npinfo && (!list_empty(&npinfo->rx_np) || npinfo->rx_flags);
> +
> +#ifdef CONFIG_PREEMPT_RT_FULL
> + rcu_read_unlock_bh();
> +#endif
Is that not actually a bug indepedent of RT ?
without the rcu_read_lock/unlock who says that the
rcu_dereference is still valid at this point ?
I though that if bh are already disabled you still
need the read_lock. disabled bh would allow to "downgrad"
the rcu_read_lock_bh to rcu_read_lock but you still need it.
thx!
hofrat
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: [PATCH v3.14-rt] netpoll: guard the access to dev->npinfo with rcu_read_lock/unlock_bh() for CONFIG_PREEMPT_RT_FULL=y
2014-12-03 12:50 ` Nicholas Mc Guire
@ 2014-12-03 21:19 ` Paul E. McKenney
2015-02-17 14:29 ` Sebastian Andrzej Siewior
0 siblings, 1 reply; 5+ messages in thread
From: Paul E. McKenney @ 2014-12-03 21:19 UTC (permalink / raw
To: Nicholas Mc Guire; +Cc: Kevin Hao, linux-rt-users
On Wed, Dec 03, 2014 at 01:50:20PM +0100, Nicholas Mc Guire wrote:
> On Wed, 03 Dec 2014, Kevin Hao wrote:
>
> > For vanilla kernel we don't need to invoke rcu_read_lock/unlock_bh
> > explicitly to mark an RCU-bh critical section in the softirq context
> > because bh is already disabled in this case. But for a rt kernel,
> > the commit ("rcu: Merge RCU-bh into RCU-preempt") implements the
> > RCU-bh in term of RCU-preempt. So we have to use
> > rcu_read_lock/unlock_bh() to mark an RCU-bh critical section even in
> > +
> > +#ifdef CONFIG_PREEMPT_RT_FULL
> > + rcu_read_lock_bh();
> > +#endif
> > +
> > + npinfo = rcu_dereference_bh(skb->dev->npinfo);
> > + ret = npinfo && (!list_empty(&npinfo->rx_np) || npinfo->rx_flags);
> > +
> > +#ifdef CONFIG_PREEMPT_RT_FULL
> > + rcu_read_unlock_bh();
> > +#endif
>
> Is that not actually a bug indepedent of RT ?
> without the rcu_read_lock/unlock who says that the
> rcu_dereference is still valid at this point ?
> I though that if bh are already disabled you still
> need the read_lock. disabled bh would allow to "downgrad"
> the rcu_read_lock_bh to rcu_read_lock but you still need it.
In vanilla kernels, anything that disables BH acts as rcu_read_lock_bh().
So yes, you can have cases where rcu_read_lock_bh() is needed only in
the -rt kernel.
Thanx, Paul
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: [PATCH v3.14-rt] netpoll: guard the access to dev->npinfo with rcu_read_lock/unlock_bh() for CONFIG_PREEMPT_RT_FULL=y
2014-12-03 21:19 ` Paul E. McKenney
@ 2015-02-17 14:29 ` Sebastian Andrzej Siewior
2015-02-17 16:16 ` Paul E. McKenney
0 siblings, 1 reply; 5+ messages in thread
From: Sebastian Andrzej Siewior @ 2015-02-17 14:29 UTC (permalink / raw
To: Paul E. McKenney; +Cc: Nicholas Mc Guire, Kevin Hao, linux-rt-users, rostedt
* Paul E. McKenney | 2014-12-03 13:19:11 [-0800]:
>> Is that not actually a bug indepedent of RT ?
>> without the rcu_read_lock/unlock who says that the
>> rcu_dereference is still valid at this point ?
>> I though that if bh are already disabled you still
>> need the read_lock. disabled bh would allow to "downgrad"
>> the rcu_read_lock_bh to rcu_read_lock but you still need it.
>
>In vanilla kernels, anything that disables BH acts as rcu_read_lock_bh().
>So yes, you can have cases where rcu_read_lock_bh() is needed only in
>the -rt kernel.
But it won't hurt mainline using rcu_read_lock_bh() around
rcu_dereference_bh() right?
I am not going to apply this because that code is gone shortly after
v3.14 was released. The fm10k however does the same thing so atleast RCU
knows when to scream :)
>
> Thanx, Paul
Sebastian
^ permalink raw reply [flat|nested] 5+ messages in thread
* Re: [PATCH v3.14-rt] netpoll: guard the access to dev->npinfo with rcu_read_lock/unlock_bh() for CONFIG_PREEMPT_RT_FULL=y
2015-02-17 14:29 ` Sebastian Andrzej Siewior
@ 2015-02-17 16:16 ` Paul E. McKenney
0 siblings, 0 replies; 5+ messages in thread
From: Paul E. McKenney @ 2015-02-17 16:16 UTC (permalink / raw
To: Sebastian Andrzej Siewior
Cc: Nicholas Mc Guire, Kevin Hao, linux-rt-users, rostedt
On Tue, Feb 17, 2015 at 03:29:53PM +0100, Sebastian Andrzej Siewior wrote:
> * Paul E. McKenney | 2014-12-03 13:19:11 [-0800]:
>
> >> Is that not actually a bug indepedent of RT ?
> >> without the rcu_read_lock/unlock who says that the
> >> rcu_dereference is still valid at this point ?
> >> I though that if bh are already disabled you still
> >> need the read_lock. disabled bh would allow to "downgrad"
> >> the rcu_read_lock_bh to rcu_read_lock but you still need it.
> >
> >In vanilla kernels, anything that disables BH acts as rcu_read_lock_bh().
> >So yes, you can have cases where rcu_read_lock_bh() is needed only in
> >the -rt kernel.
>
> But it won't hurt mainline using rcu_read_lock_bh() around
> rcu_dereference_bh() right?
Using rcu_read_lock_bh() around rcu_dereference_bh() is completely
legal, yes.
> I am not going to apply this because that code is gone shortly after
> v3.14 was released. The fm10k however does the same thing so atleast RCU
> knows when to scream :)
That is good, because it has been a good long time since I could
reasonably review all code in the kernel that uses RCU. ;-)
Thanx, Paul
> Sebastian
>
^ permalink raw reply [flat|nested] 5+ messages in thread
end of thread, other threads:[~2015-02-17 16:16 UTC | newest]
Thread overview: 5+ messages (download: mbox.gz follow: Atom feed
-- links below jump to the message on this page --
2014-12-03 12:05 [PATCH v3.14-rt] netpoll: guard the access to dev->npinfo with rcu_read_lock/unlock_bh() for CONFIG_PREEMPT_RT_FULL=y Kevin Hao
2014-12-03 12:50 ` Nicholas Mc Guire
2014-12-03 21:19 ` Paul E. McKenney
2015-02-17 14:29 ` Sebastian Andrzej Siewior
2015-02-17 16:16 ` Paul E. McKenney
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