[asterisk-dev] [Code Review] Fix deadlock between subscription event RWLOCK and dialogs container lock in chan_sip.

rmudgett reviewboard at asterisk.org
Thu Nov 3 13:12:16 CDT 2011

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(Updated Nov. 3, 2011, 1:12 p.m.)

Review request for Asterisk Developers and David Vossel.


* Made hold the container lock when unlink/link cycle is done on the dialogs, dialogs_rtpcheck, and dialogs_needdestroy containers.
* Made use the ao2_callback OBJ_UNLINK flag for the dialogs_rtpcheck dialog_checkrtp_cb().  Unlinking by using the OBJ_UNLINK flag is faster than calling ao2_unlink in the dialog_checkrtp_cb callback.


Timing between dialog destruction and a MWI event sending a message could result in a deadlock.

Order of events causing deadlock:

1a) The event subscription system calls the registered callbacks with its list RWLOCK held.
1b) The SIP monitor checks for dialogs needing destruction.  It does an ao2_callback that holds the dialogs container lock while searching for dialogs to destroy.
2a) The event subscription SIP callback needs to create a temporary dialog to send out the MWI notification.  That temporary dialog needs to be inserted in the dialogs container so it must wait.
2b) The dialog search finds a dialog to destroy and as a result releases the last reference for a peer.  The peer destructor attempts to get the subscription RWLOCK but must wait.
3) deadlock

This addresses bug ASTERISK-18747.

Diffs (updated)

  /branches/1.8/channels/chan_sip.c 343335 

Diff: https://reviewboard.asterisk.org/r/1557/diff


It compiles. :)



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