On 09/03/2013 02:53 PM, Manfred Spraul wrote:
> On 09/03/2013 11:16 AM, Vineet Gupta wrote:
>> On 09/03/2013 02:27 PM, Manfred Spraul wrote:
>>> On 09/03/2013 10:44 AM, Vineet Gupta wrote:
>>>>> 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 ...
>>> What is the output of ipcs -q? Is the queue full or empty when it hangs?
>>> I.e. do we forget to wake up a receiver or forget to wake up a sender?
>> / # ipcs -q
>>
>> ------ Message Queues --------
>> key msqid owner perms used-bytes messages
>> 0x72d83160 163841 root 600 0 0
>>
>>
> Ok, a sender is sleeping - even though there are no messages in the queue.
> Perhaps it is the race that I mentioned in a previous mail:
>> 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.
>
> Vineet, could you try to move the test for free space after ipc_lock?
> I.e. the lock must not get dropped between testing for free space and
> enqueueing the messages.
Hmm, the code movement is not trivial. I broke even the simplest of cases (patch
attached). This includes the additional change which Linus/Davidlohr had asked
for.
-Vineet
diff --git a/ipc/msg.c b/ipc/msg.c
index 9f29d9e..a512829 100644
--- a/ipc/msg.c
+++ b/ipc/msg.c
@@ -687,14 +687,6 @@ long do_msgsnd(int msqid, long mtype, void __user *mtext,
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;
- }
/* queue full, wait: */
if (msgflg & IPC_NOWAIT) {
@@ -703,6 +695,10 @@ long do_msgsnd(int msqid, long mtype, void __user *mtext,
}
ipc_lock_object(&msq->q_perm);
+ err = security_msg_queue_msgsnd(msq, msg, msgflg);
+ if (err)
+ goto out_unlock0;
+
ss_add(msq, &s);
if (!ipc_rcu_getref(msq)) {
@@ -734,6 +730,12 @@ long do_msgsnd(int msqid, long mtype, void __user *mtext,
}
ipc_lock_object(&msq->q_perm);
+
+ if (!(msgsz + msq->q_cbytes <= msq->q_qbytes &&
+ 1 + msq->q_qnum <= msq->q_qbytes)) {
+ goto out_unlock0;
+ }
+
msq->q_lspid = task_tgid_vnr(current);
msq->q_stime = get_seconds();
