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

[rmudgett]

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https://reviewboard.asterisk.org/r/1557/
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Review request for Asterisk Developers and David Vossel.


Summary
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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.
    https://issues.asterisk.org/jira/browse/ASTERISK-18747


Diffs
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  /branches/1.8/channels/chan_sip.c 342973 

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


Testing
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It compiles. :)


Thanks,

rmudgett

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