[asterisk-dev] session channel locks

[Joshua C. Colp]

On Wed, Dec 9, 2020 at 12:02 PM Dan Cropp <dan at amtelco.com> wrote:

> We are working on a patch for the REFER support and have a question on
> whether a lock is needed or not.
>
>
>
> In chan_pjsip.c function xfer_client_on_evsub_state, code is processing
> the REFER-NOTIFY subscription.
>
>
>
> It gets an ast_sip_session reference, then it acquires a ast_channel
> reference.
>
> Should we make the code bump the lock count for the session’s channel
> while we use it?  Unlocking when done using it?
>
>
>
> Is it safe to assume the channel will not be deleted while this routine is
> running?
>
> Obviously, the safest approach is to increment/decrement the reference
> count while using, but is that the necessary/preferred approach?
>

The PJSIP implementation in Asterisk doesn't heavily use locks. Instead it
approaches things from a serializer perspective - that is ensuring that
things related to a dialog/session can't be done in multiple threads at the
same time. To that end the xfer_client_on_evsub_state function should
allocate a structure with the appropriate information and queue it up using
ast_sip_push_task. You can find this being done in the chan_pjsip_transfer
function. This will give you a guarantee that session won't go away, the
channel pointer won't go away, and that other threads aren't doing things
at the same time.

-- 
Joshua C. Colp
Asterisk Technical Lead
Sangoma Technologies
Check us out at www.sangoma.com and www.asterisk.org
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