lkml.org 
[lkml]   [2013]   [Sep]   [2]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: ipc-msg broken again on 3.11-rc7?
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.
b) Could you check that it is not just a performance regression?
Does ./msgctl08 1000 16 hang, too?

In ipc/msg.c, I haven't seen any obvious reason why it should hang.
The only race I spotted so far is this one:
> for (;;) {
> struct msg_sender s;
>
> err = -EACCES;
> if (ipcperms(ns, &msq->q_perm, S_IWUGO))
> goto out_unlock1;
>
> err = security_msg_queue_msgsnd(msq, msg, msgflg);
> if (err)
> goto out_unlock1;
>
> if (msgsz + msq->q_cbytes <= msq->q_qbytes &&
> 1 + msq->q_qnum <= msq->q_qbytes) {
> break;
> }
>
[snip]
> if (!pipelined_send(msq, msg)) {
> /* no one is waiting for this message, enqueue it */
> list_add_tail(&msg->m_list, &msq->q_messages);
> msq->q_cbytes += msgsz;
> msq->q_qnum++;
> atomic_add(msgsz, &ns->msg_bytes);

The access to msq->q_cbytes is not protected. Thus two parallel msgsnd()
calls could succeed, even if both together brings the queue length above
the limit.
But it can't explain why 3.11-rc7 hangs: As explained above, msgctl08
uses one queue for each thread pair.

--
Manfred



\
 
 \ /
  Last update: 2013-09-02 19:01    [W:0.077 / U:0.584 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site