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

[schmidts]

> On Nov. 1, 2011, 5:47 p.m., David Vossel wrote:
> > To avoid the alloc and free during each iteration if the do_monitor loop, we could create a container just used to store the items marked for destruction, then do a callback on that other container to unlink the objects from both containers.  You could pass this container and the &t pointer in to the dialog_need_destroy callback as the arg in some new structure you create.
> > 
> > This is essentially the exact same thing you've done here, except we don't have to allocate the new container every iteration, which is what is happens behind the scenes when the iterator is created.

this own container for dialogs which need to be destroyed is allready in 10 and also my 1.8 backport branch unleash-the-beast. i have noticed this kind of deadlock a lot of times with normal 1.8 but its solved when using this own container.

see rev 335783 for the patch to generate an own dialog container for dialogs which has to be destroyed.

best regards


- schmidts


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On Nov. 1, 2011, 5:13 p.m., rmudgett wrote:
> 
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> This is an automatically generated e-mail. To reply, visit:
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> 
> (Updated Nov. 1, 2011, 5:13 p.m.)
> 
> 
> Review request for Asterisk Developers and David Vossel.
> 
> 
> Summary
> -------
> 
> 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
> -----
> 
>   /branches/1.8/channels/chan_sip.c 342973 
> 
> Diff: https://reviewboard.asterisk.org/r/1557/diff
> 
> 
> Testing
> -------
> 
> It compiles. :)
> 
> 
> Thanks,
> 
> rmudgett
> 
>

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