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<p class="MsoNormal">Thank you Joshua.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><b>From:</b> asterisk-dev <asterisk-dev-bounces@lists.digium.com>
<b>On Behalf Of </b>Joshua C. Colp<br>
<b>Sent:</b> Wednesday, December 9, 2020 10:27 AM<br>
<b>To:</b> Asterisk Developers Mailing List <asterisk-dev@lists.digium.com><br>
<b>Subject:</b> Re: [asterisk-dev] session channel locks<o:p></o:p></p>
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<p class="MsoNormal">On Wed, Dec 9, 2020 at 12:02 PM Dan Cropp <<a href="mailto:dan@amtelco.com">dan@amtelco.com</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">We are working on a patch for the REFER support and have a question on whether a lock is needed or not.<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">In chan_pjsip.c function xfer_client_on_evsub_state, code is processing the REFER-NOTIFY subscription.<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">It gets an ast_sip_session reference, then it acquires a ast_channel reference.<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Should we make the code bump the lock count for the session’s channel while we use it? Unlocking when done using it?<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Is it safe to assume the channel will not be deleted while this routine is running?<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Obviously, the safest approach is to increment/decrement the reference count while using, but is that the necessary/preferred approach?<o:p></o:p></p>
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<p class="MsoNormal">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.<o:p></o:p></p>
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<p class="MsoNormal">-- <o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Tahoma",sans-serif;color:#073763">Joshua C. Colp</span><span style="font-family:"Tahoma",sans-serif"><o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-family:"Tahoma",sans-serif;color:#073763">Asterisk Technical Lead</span><span style="font-family:"Tahoma",sans-serif"><o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-family:"Tahoma",sans-serif;color:#073763">Sangoma Technologies</span><span style="font-family:"Tahoma",sans-serif"><o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-family:"Tahoma",sans-serif;color:#073763">Check us out at
<a href="http://www.sangoma.com" target="_blank">www.sangoma.com</a> and <a href="http://www.asterisk.org" target="_blank">
www.asterisk.org</a></span><span style="font-family:"Tahoma",sans-serif"><o:p></o:p></span></p>
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