Date:   Fri, 16 Jun 2017 15:33:18 -0500
From:   Corey Minyard <minyard@....org>
To:     Tony Camuso <tcamuso@...hat.com>,
        openipmi-developer@...ts.sourceforge.net
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ipmi: use rcu lock around call to
 intf->handlers->sender()

On 06/16/2017 08:11 AM, Tony Camuso wrote:
> On 06/16/2017 08:15 AM, Corey Minyard wrote:
>> On 06/15/2017 10:54 AM, Corey Minyard wrote:
>>> On 06/13/2017 09:54 AM, Tony Camuso wrote:
>>>> A vendor with a system having more than 128 CPUs occasionally 
>>>> encounters a
>>>> crash during shutdown. This is not an easily reproduceable event, 
>>>> but the
>>>> vendor was able to provide the following analysis of the crash, which
>>>> exhibits the same footprint each time.
>>>>
>>>> crash> bt
>>>> PID: 0      TASK: ffff88017c70ce70  CPU: 5   COMMAND: "swapper/5"
>>>>   #0 [ffff88085c143ac8] machine_kexec at ffffffff81059c8b
>>>>   #1 [ffff88085c143b28] __crash_kexec at ffffffff811052e2
>>>>   #2 [ffff88085c143bf8] crash_kexec at ffffffff811053d0
>>>>   #3 [ffff88085c143c10] oops_end at ffffffff8168ef88
>>>>   #4 [ffff88085c143c38] no_context at ffffffff8167ebb3
>>>>   #5 [ffff88085c143c88] __bad_area_nosemaphore at ffffffff8167ec49
>>>>   #6 [ffff88085c143cd0] bad_area_nosemaphore at ffffffff8167edb3
>>>>   #7 [ffff88085c143ce0] __do_page_fault at ffffffff81691d1e
>>>>   #8 [ffff88085c143d40] do_page_fault at ffffffff81691ec5
>>>>   #9 [ffff88085c143d70] page_fault at ffffffff8168e188
>>>>      [exception RIP: unknown or invalid address]
>>>>      RIP: ffffffffa053c800  RSP: ffff88085c143e28  RFLAGS: 00010206
>>>>      RAX: ffff88017c72bfd8  RBX: ffff88017a8dc000  RCX: 
>>>> ffff8810588b5ac8
>>>>      RDX: ffff8810588b5a00  RSI: ffffffffa053c800  RDI: 
>>>> ffff8810588b5a00
>>>>      RBP: ffff88085c143e58   R8: ffff88017c70d408   R9: 
>>>> ffff88017a8dc000
>>>>      R10: 0000000000000002  R11: ffff88085c143da0  R12: 
>>>> ffff8810588b5ac8
>>>>      R13: 0000000000000100  R14: ffffffffa053c800  R15: 
>>>> ffff8810588b5a00
>>>>      ORIG_RAX: ffffffffffffffff  CS: 0010  SS: 0018
>>>> --- <IRQ stack> ---
>>>>      [exception RIP: cpuidle_enter_state+82]
>>>>      RIP: ffffffff81514192  RSP: ffff88017c72be50  RFLAGS: 00000202
>>>>      RAX: 0000001e4c3c6f16  RBX: 000000000000f8a0  RCX: 
>>>> 0000000000000018
>>>>      RDX: 0000000225c17d03  RSI: ffff88017c72bfd8  RDI: 
>>>> 0000001e4c3c6f16
>>>>      RBP: ffff88017c72be78   R8: 000000000000237e   R9: 
>>>> 0000000000000018
>>>>      R10: 0000000000002494  R11: 0000000000000001  R12: 
>>>> ffff88017c72be20
>>>>      R13: ffff88085c14f8e0  R14: 0000000000000082  R15: 
>>>> 0000001e4c3bb400
>>>>      ORIG_RAX: ffffffffffffff10  CS: 0010  SS: 0018
>>>>
>>>> This is the corresponding stack trace
>>>>
>>>> It has crashed because the area pointed with RIP extracted from timer
>>>> element is already removed during a shutdown process.
>>>>
>>>> The function is smi_timeout().
>>>>
>>>> And we think ffff8810588b5a00 in RDX is a parameter struct smi_info
>>>>
>>>> crash> rd ffff8810588b5a00 20
>>>> ffff8810588b5a00:  ffff8810588b6000 0000000000000000 .`.X............
>>>> ffff8810588b5a10:  ffff880853264400 ffffffffa05417e0 .D&S......T.....
>>>> ffff8810588b5a20:  24a024a000000000 0000000000000000 .....$.$........
>>>> ffff8810588b5a30:  0000000000000000 0000000000000000 ................
>>>> ffff8810588b5a40:  ffffffffa053a040 ffffffffa053a060 @.S.....`.S.....
>>>> ffff8810588b5a50:  0000000000000000 0000000100000001 ................
>>>> ffff8810588b5a60:  0000000000000000 0000000000000e00 ................
>>>> ffff8810588b5a70:  ffffffffa053a580 ffffffffa053a6e0 ..S.......S.....
>>>> ffff8810588b5a80:  ffffffffa053a4a0 ffffffffa053a250 ..S.....P.S.....
>>>> ffff8810588b5a90:  0000000500000002 0000000000000000 ................
>>>>
>>>> Unfortunately the top of this area is already detroyed by someone.
>>>> But because of two reasonns we think this is struct smi_info
>>>>   1) The address included in between  ffff8810588b5a70 and 
>>>> ffff8810588b5a80:
>>>>    are inside of ipmi_si_intf.c  see crash> module ffff88085779d2c0
>>>>
>>>>   2) We've found the area which point this.
>>>>    It is offset 0x68 of  ffff880859df4000
>>>>
>>>> crash> rd  ffff880859df4000 100
>>>> ffff880859df4000:  0000000000000000 0000000000000001 ................
>>>> ffff880859df4010:  ffffffffa0535290 dead000000000200 .RS.............
>>>> ffff880859df4020:  ffff880859df4020 ffff880859df4020 @.Y.... @.Y....
>>>> ffff880859df4030:  0000000000000002 0000000000100010 ................
>>>> ffff880859df4040:  ffff880859df4040 ffff880859df4040 @@.Y....@@.Y....
>>>> ffff880859df4050:  0000000000000000 0000000000000000 ................
>>>> ffff880859df4060:  0000000000000000 ffff8810588b5a00 .........Z.X....
>>>> ffff880859df4070:  0000000000000001 ffff880859df4078 ........x@......
>>>>
>>>>   If we regards it as struct ipmi_smi in shutdown process
>>>>   it looks consistent.
>>>>
>>>> The remedy for this apparent race is affixed below.
>>>
>>> I think you are right about this problem, but in_shutdown is checked 
>>> already
>>> a bit before when newmsg is extracted from the list.  Wouldn't it be 
>>> better
>>> to add the rcu_read_lock() region starting right before the previous
>>> in_shutdown check to after the send?  That would avoid a leak in this
>>> case.
>>
>> While lying awake unable to sleep, I realized that you can't call the
>> sender function while holding rcu_read_lock().  That will break RT,
>> because you can't claim a mutex while holding rcu_read_lock(),
>> and the sender function will claim normal spinlocks.
>>
>> So I need to think about this a bit.
>>
>> -corey
>>
>>>
>>> Thanks,
>>>
>>> -corey
>>>
>
> Would this be adequate to prevent the race?
> Is the sender's mutex/spinlock sufficient to limit access?
>
> diff --git a/drivers/char/ipmi/ipmi_msghandler.c 
> b/drivers/char/ipmi/ipmi_msghandler.c
> index 9f69995..ebce6b3 100644
> --- a/drivers/char/ipmi/ipmi_msghandler.c
> +++ b/drivers/char/ipmi/ipmi_msghandler.c
> @@ -3880,6 +3880,7 @@ static void smi_recv_tasklet(unsigned long val)
>          */
>         if (!run_to_completion)
>                 spin_lock_irqsave(&intf->xmit_msgs_lock, flags);
> +
>         if (intf->curr_msg == NULL && !intf->in_shutdown) {
>                 struct list_head *entry = NULL;
>
> @@ -3894,11 +3895,13 @@ static void smi_recv_tasklet(unsigned long val)
>                         newmsg = list_entry(entry, struct 
> ipmi_smi_msg, link);
>                         intf->curr_msg = newmsg;
>                 }
> +
> +               if (newmsg)
> + intf->handlers->sender(intf->send_info, newmsg);
>         }
> +
>         if (!run_to_completion)
> spin_unlock_irqrestore(&intf->xmit_msgs_lock, flags);
> -       if (newmsg)
> -               intf->handlers->sender(intf->send_info, newmsg);
>
>         handle_new_recv_msgs(intf);
>  }
>

IIRC, that will currently cause a deadlock.  Let me look at this some more.

-corey

>
>>>> Signed-off-by: Tony Camuso <tcamuso@...hat.com>
>>>> ---
>>>>   drivers/char/ipmi/ipmi_msghandler.c | 9 +++++++--
>>>>   1 file changed, 7 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/drivers/char/ipmi/ipmi_msghandler.c 
>>>> b/drivers/char/ipmi/ipmi_msghandler.c
>>>> index 9f69995..577509f 100644
>>>> --- a/drivers/char/ipmi/ipmi_msghandler.c
>>>> +++ b/drivers/char/ipmi/ipmi_msghandler.c
>>>> @@ -3897,8 +3897,13 @@ static void smi_recv_tasklet(unsigned long val)
>>>>       }
>>>>       if (!run_to_completion)
>>>> spin_unlock_irqrestore(&intf->xmit_msgs_lock, flags);
>>>> -    if (newmsg)
>>>> -        intf->handlers->sender(intf->send_info, newmsg);
>>>> +
>>>> +    if (newmsg) {
>>>> +        rcu_read_lock();
>>>> +        if (!intf->in_shutdown)
>>>> + intf->handlers->sender(intf->send_info, newmsg);
>>>> +        rcu_read_unlock();
>>>> +    }
>>>>         handle_new_recv_msgs(intf);
>>>>   }
>>>
>>>
>>
>

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