Subject: Re: reproducable panic eviction work queue

From: Nikolay Aleksandrov <[email-masked]>
Date: Sat, 18 Jul 2015 15:31:34 +0200

On 07/18/2015 12:02 PM, Nikolay Aleksandrov wrote:
> On 07/18/2015 11:01 AM, Johan Schuijt wrote:
>> Yes, we already found these and are included in our kernel, but even with these patches we still receive the panic.
>>
>> - Johan
>>
>>
>>> On 18 Jul 2015, at 10:56, Eric Dumazet <[email-masked]> wrote:
>>>
>>> On Fri, 2015-07-17 at 21:18 +0000, Johan Schuijt wrote:
>>>> Hey guys, 
>>>>
>>>>
>>>> We’re currently running into a reproducible panic in the eviction work
>>>> queue code when we pin al our eth* IRQ to different CPU cores (in
>>>> order to scale our networking performance for our virtual servers).
>>>> This only occurs in kernels >= 3.17 and is a result of the following
>>>> change:
>>>> https://git.kernel.org/cgit/linux/kernel/git/stable/linux-stable.git/commit/?h=linux-3.18.y&id=b13d3cbfb8e8a8f53930af67d1ebf05149f32c24
>>>>
>>>>
>>>> The race/panic we see seems to be the same as, or similar to:
>>>> https://git.kernel.org/cgit/linux/kernel/git/stable/linux-stable.git/commit/?h=linux-3.18.y&id=65ba1f1ec0eff1c25933468e1d238201c0c2cb29
>>>>
>>>>
>>>> We can confirm that this is directly exposed by the IRQ pinning since
>>>> disabling this stops us from being able to reproduce this case :)
>>>>
>>>>
>>>> How te reproduce: in our test-setup we have 4 machines generating UDP
>>>> packets which are send to the vulnerable host. These all have a MTU of
>>>> 100 (for test purposes) and send UDP packets of a size of 256 bytes.
>>>> Within half an hour you will see the following panic:
>>>>
>>>>
>>>> crash> bt
>>>> PID: 56     TASK: ffff885f3d9fc210  CPU: 9   COMMAND: "kworker/9:0"
>>>> #0 [ffff885f3da03b60] machine_kexec at ffffffff8104a1f7
>>>> #1 [ffff885f3da03bb0] crash_kexec at ffffffff810db187
>>>> #2 [ffff885f3da03c80] oops_end at ffffffff81015140
>>>> #3 [ffff885f3da03ca0] general_protection at ffffffff814f6c88
>>>>    [exception RIP: inet_evict_bucket+281]
>>>>    RIP: ffffffff81480699  RSP: ffff885f3da03d58  RFLAGS: 00010292
>>>>    RAX: ffff885f3da03d08  RBX: dead0000001000a8  RCX:
>>>> ffff885f3da03d08
>>>>    RDX: 0000000000000006  RSI: ffff885f3da03ce8  RDI:
>>>> dead0000001000a8
>>>>    RBP: 0000000000000002   R8: 0000000000000286   R9:
>>>> ffff88302f401640
>>>>    R10: 0000000080000000  R11: ffff88602ec0c138  R12:
>>>> ffffffff81a8d8c0
>>>>    R13: ffff885f3da03d70  R14: 0000000000000000  R15:
>>>> ffff881d6efe1a00
>>>>    ORIG_RAX: ffffffffffffffff  CS: 0010  SS: 0018
>>>> #4 [ffff885f3da03db0] inet_frag_worker at ffffffff8148075a
>>>> #5 [ffff885f3da03e10] process_one_work at ffffffff8107be19
>>>> #6 [ffff885f3da03e60] worker_thread at ffffffff8107c6e3
>>>> #7 [ffff885f3da03ed0] kthread at ffffffff8108103e
>>>> #8 [ffff885f3da03f50] ret_from_fork at ffffffff814f4d7c
>>>>
>>>>
>>>> We would love to receive your input on this matter.
>>>>
>>>>
>>>> Thx in advance,
>>>>
>>>>
>>>> - Johan
>>>
>>> Check commits 65ba1f1ec0eff1c25933468e1d238201c0c2cb29 &
>>> d70127e8a942364de8dd140fe73893efda363293
>>>
>>> Also please send your mails in text format, not html, and CC netdev ( I
>>> did here)
>>>
>>>>
>>>>
>>>
>>>
>>
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>>
> 
> Thank you for the report, I will try to reproduce this locally
> Could you please post the full crash log ? Also could you test
> with a clean current kernel from Linus' tree or Dave's -net ?
> These are available at:
> git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
> git://git.kernel.org/pub/scm/linux/kernel/git/davem/net
> respectively.
> 
> One last question how many IRQs do you pin i.e. how many cores
> do you actively use for receive ?
> 

Flags seems to be modified while still linked and we may get the
following (theoretical) situation:
CPU 1						CPU 2
inet_frag_evictor (wait for chainlock)		spin_lock(chainlock)
						unlock(chainlock)
get lock, set EVICT flag, hlist_del etc.
						change flags again while
						qp is in the evict list

So could you please try the following patch which sets the flag while
holding the chain lock:

diff --git a/net/ipv4/inet_fragment.c b/net/ipv4/inet_fragment.c
index 5e346a082e5f..2521ed9c1b52 100644
--- a/net/ipv4/inet_fragment.c
+++ b/net/ipv4/inet_fragment.c
@@ -354,8 +354,8 @@ static struct inet_frag_queue *inet_frag_intern(struct netns_frags *nf,
 	hlist_for_each_entry(qp, &hb->chain, list) {
 		if (qp->net == nf && f->match(qp, arg)) {
 			atomic_inc(&qp->refcnt);
-			spin_unlock(&hb->chain_lock);
 			qp_in->flags |= INET_FRAG_COMPLETE;
+			spin_unlock(&hb->chain_lock);
 			inet_frag_put(qp_in, f);
 			return qp;
 		}