[jira] [Work started] (TS-4735) Possible deadlock on traffic_server startup

["Shrihari (JIRA)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/TS-4735?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Work on TS-4735 started by Shrihari.
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> Possible deadlock on traffic_server startup
> -------------------------------------------
>
>                 Key: TS-4735
>                 URL: https://issues.apache.org/jira/browse/TS-4735
>             Project: Traffic Server
>          Issue Type: Bug
>          Components: Core
>    Affects Versions: 6.2.0
>            Reporter: Shrihari
>            Assignee: Shrihari
>             Fix For: 7.0.0
>
>
> As part of startup, traffic_server creates two threads (to begin with).
> 1. (main) Thread (1) blocks till its signaled by another thread
> 1. Thread 2 polls for messages from traffic_manager
> It is waiting for a message from traffic_manager which contains all the configuration required for it to go ahead with initialization. Hence, it is critical that the main Thread (1) wait till it gets the configuration.
> Thread 2 which polls for message from traffic_manager works like this:
> for(;;) {
>   if (pmgmt->require_lm) {     <--- Always True (when using traffic_cop)
>     pmgmt->pollLMConnection();  <--- | for (count = 0; count < 10000; count ++) 
>                                                            |   num = mgmt_read_timeout(...) <---- Blocking call. returns 0 if nothing was received for 1 second
>                                                            |   if !num: break <--- Break out of the loop and return from function 
>                                                            |   else: read(fd), add_to_event_queue, continue the loop, 
>                                                            | Back to fetching another message
>   }
>   pmgmt->processEventQueue();  <--  Process the messages received in pollLMConnection()
>   pmgmt->processSignalQueue();
>   mgmt_sleep_sec(pmgmt->timeout); 
> }
> RCA:
> There are two problems here:
> 1. If we look into the above code, we should observe that the pollLMConnection might not return back for a very long time if it keeps getting messages. As a result, we may not call processEventQueue() which processes the received messages. And unless we process the messages, we cannot signal the main Thread (1) to continue, which is still blocked. Hence we see the issue where traffic_server won't complete initialization for a very long time.
> 2. The second problem is that why is traffic_server receiving so many messages at boot-up? The problem lies in the configuration. In 6.2.x, we replaced 
> 'proxy.process.ssl.total_success_handshake_count' with 
> 'proxy.process.ssl.total_success_handshake_count_in'. 
> In order to provide backwards compatibility, we defined the old stat in stats.config.xml. The caveat here is that, since this statconfig is defined in stats.config.xml, traffic_manager assumes the responsibility of updating this stat. According to the code:
> if (i_am_not_owner_of(stat)) : send traffic_server a notify message.
> Ideally, this code should not be triggered because, traffic_manager does own the stat. However, the ownership in the code is determined solely based on the 'string name'. If the name contains 'process', it is owned by traffic_server. This leads to an interesting scenario where traffic_manger keeps updating its own stat and sends unnecessary events to traffic_server. These updates happen every 1 second (Thanks James for helping me understand this period) which is the same as our timeout in traffic_server.  Due to 'Problem 1' we can prevent traffic_server from processing any messages for up to 10,000 seconds! (Just imagine the case where the message is received just before the timout of 1 second happens)
> I saw this happening with 100% on a VM but 0% on a physical box. I don't have any other results as of now though.



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