The problem about main thread and worker thread processing the same event
nginx-forum at nginx.us
Mon Mar 12 05:23:30 UTC 2012
In the case of work process is configured to support multi-threading in
the following scene, there is an event at the same time will be handled
by the main thread and worker thread.
step1: in ngx_process_events_and_timers function, the main thread get
the accept lock and set the ngx_accept_mutex_held to 1, at this time,
the flags | = NGX_POST_EVENTS. And then enter ngx_process_events (epoll,
under for ngx_epoll_process_events function) function, all received
non-accept epoll events will be added to the the queue
step2: in the main thread of the work process handler function
ngx_worker_process_cycle to, inform all the worker threads to process
in the queue ngx_posted_events .
step3: the main thread to re-enter the event loop, try to grab the
accept lock, no grab, ngx_accept_mutex_held be set to 0, then, the flags
0. Then enter ngx_process_events, received all non-accept epoll events.
The flag is not NGX_POST_EVENTS, Therefore, directly calling the event
handler to handle the event.
The problem lies here:
If in step2 a worker thread is processing a connection event ev1
(c-> read or c-> write), while in step3 connection produced a similar
event ev1 (c-> the read or c-> write), the main thread and worker thread
handle the same event conflictly.
Posted at Nginx Forum: http://forum.nginx.org/read.php?2,223741,223741#msg-223741
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