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

[David Vossel]

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Ship it!


Nice!

- David


On Nov. 5, 2011, 4:20 p.m., rmudgett wrote:
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> (Updated Nov. 5, 2011, 4:20 p.m.)
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> Review request for Asterisk Developers, David Vossel and schmidts.
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> Summary
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> Timing between dialog destruction and a MWI event sending a message could result in a deadlock.
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> Order of events causing deadlock:
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> 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
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> This is based off of the original diff posted for review: https://reviewboard.asterisk.org/r/1557/
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> It directly addresses the comment dvossel posted to the original diff of https://reviewboard.asterisk.org/r/1557/
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> This addresses bug ASTERISK-18747.
>     https://issues.asterisk.org/jira/browse/ASTERISK-18747
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> Diffs
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>   /branches/1.8/channels/chan_sip.c 343488 
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> Diff: https://reviewboard.asterisk.org/r/1564/diff
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> Testing
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> It compiles. :)
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> Thanks,
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> rmudgett
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