Re: ipc-msg broken again on 3.11-rc7?

[Vineet Gupta]

On 09/02/2013 09:59 PM, Manfred Spraul wrote:
> Hi,
>
> [forgot to cc everyone, thus I'll summarize some mails...]
> On 09/02/2013 06:58 AM, Vineet Gupta wrote:
>> On 08/31/2013 11:20 PM, Linus Torvalds wrote:
>>> Vineet, actual patch for what Davidlohr suggests attached. Can you try it?
>>>
>>>               Linus
>> Apologies for late in getting back to this - I was away from my computer for a bit.
>>
>> Unfortunately, with a quick test, this patch doesn't help.
>> FWIW, this is latest mainline (.config attached).
>>
>> Let me know what diagnostics I can add to help with this.
> msgctl08 is a bulk message send/receive test. I had to look at it once 
> before, then it was a broken hardware:
> https://lkml.org/lkml/2008/6/12/365
> This can be ruled out, because it works with 3.10.
>
> msgctl08 uses pairs of threads: one thread does msgsnd(), the other one 
> msgrcv().
> There is no synchronization, i.e. the msgsnd() can race ahead until the 
> kernel buffer is full and then a block with msgrcv() follows or it could 
> be pairs of alternating msgsnd()/msgrcv() operations.
> No special features are used: each pair of threads has it's own message 
> queues, all messages have type=1.
>
> Vineet ran strace - and just before the signal from killing msgctl08, 
> there are only msgsnd()/msgrcv() calls.
> Vineet:
> a) could you run strace tomorrow again, with '-ttt' as an additional 
> option? I don't see where exactly it hangs.

Yet to do this.

> b) Could you check that it is not just a performance regression?
>      Does ./msgctl08 1000 16 hang, too?

Nope that doesn't hang. The minimal configuration that hangs reliably is msgctl
50000 2

With this config there are 3 processes.
...
  555   554 root     S     1208  0.4   0  0.0 ./msgctl08 50000 2
  554   551 root     S     1208  0.4   0  0.0 ./msgctl08 50000 2
  551   496 root     S     1208  0.4   0  0.0 ./msgctl08 50000 2
...

[ARCLinux]$ cat /proc/551/stack
[<80aec3c6>] do_wait+0xa02/0xc94
[<80aecad2>] SyS_wait4+0x52/0xa4
[<80ae24fc>] ret_from_system_call+0x0/0x4

[ARCLinux]$ cat /proc/555/stack
[<80c2950e>] SyS_msgrcv+0x252/0x420
[<80ae24fc>] ret_from_system_call+0x0/0x4

[ARCLinux]$ cat /proc/554/stack
[<80c28c82>] do_msgsnd+0x116/0x35c
[<80ae24fc>] ret_from_system_call+0x0/0x4

Is this a case of lost wakeup or some such. I'm running with some more diagnostics
and will report soon ...

-Vineet
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