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
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