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For the explanation, here is a copy-paste of the review board explanation: Initially, it was discovered that performing an attended transfer of a multiparty bridge with a PJSIP channel would cause a deadlock. A PBX thread started a masquerade and reached the point where it was calling the fixup() callback on the "original" channel. For chan_pjsip, this involves pushing a synchronous task to the session's serializer. The problem was that a task ahead of the fixup task was also attempting to perform a channel masquerade. However, since masquerades are designed in a way to only allow for one to occur at a time, the task ahead of the fixup could not continue until the masquerade already in progress had completed. And of course, the masquerade in progress could not complete until the task ahead of the fixup task had completed. Deadlock. The initial fix was to change the fixup task to be asynchronous. While this prevented the deadlock from occurring, it had the frightful side effect of potentially allowing for tasks in the session's serializer to operate on a zombie channel. Taking a step back from this particular deadlock, it became clear that the problem was not really this one particular issue but that masquerades themselves needed to be addressed. A PJSIP attended transfer operation calls ast_channel_move(), which attempts to both set up and execute a masquerade. The problem was that after it had set up the masquerade, the PBX thread had swooped in and tried to actually perform the masquerade. Looking at changes that had been made to Asterisk 12, it became clear that there never is any time now that anyone ever wants to set up a masquerade and allow for the channel thread to actually perform the masquerade. Everyone always is calling ast_channel_move(), performs the masquerade itself before returning. In this patch, I have removed all blocks of code from channel.c that will attempt to perform a masquerade if ast_channel_masq() returns true. Now, there is no distinction between setting up a masquerade and performing the masquerade. It is one operation. The only remaining checks for ast_channel_masq() and ast_channel_masqr() are in ast_hangup() since we do not want to interrupt a masquerade by hanging up the channel. Instead, now ast_hangup() will wait for a masquerade to complete before moving forward with its operation. The ast_channel_move() function has been modified to basically in-line the logic that used to be in ast_channel_masquerade(). ast_channel_masquerade() has been killed off for real. ast_channel_move() now has a lock associated with it that is used to prevent any simultaneous moves from occurring at once. This means there is no need to make sure that ast_channel_masq() or ast_channel_masqr() are already set on a channel when ast_channel_move() is called. It also means the channel container lock is not pulling double duty by both keeping the container locked and preventing multiple masquerades from occurring simultaneously. The ast_do_masquerade() function has been renamed to do_channel_masquerade() and is now internal to channel.c. The function now takes explicit arguments of which channels are involved in the masquerade instead of a single channel. While it probably is possible to do some further refactoring of this method, I feel that I would be treading dangerously. Instead, all I did was change some comments that no longer are true after this changeset. The other more minor change introduced in this patch is to res_pjsip.c to make ast_sip_push_task_synchronous() run the task in-place if we are already a SIP servant thread. This is related to this patch because even when we isolate the channel masquerade to only running in the SIP servant thread, we would still deadlock when the fixup() callback is reached since we would essentially be waiting forever for ourselves to finish before actually running the fixup. This makes it so the fixup is run without having to push a task into a serializer at all. (closes issue ASTERISK-22936) Reported by Jonathan Rose Review: https://reviewboard.asterisk.org/r/3069 ........ Merged revisions 404356 from http://svn.asterisk.org/svn/asterisk/branches/12 git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@404368 65c4cc65-6c06-0410-ace0-fbb531ad65f3
114 lines
3.7 KiB
C
114 lines
3.7 KiB
C
/*
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* Asterisk -- An open source telephony toolkit.
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*
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* Copyright (C) 2009, Digium, Inc.
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*
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* Mark Michelson <mmichelson@digium.com>
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*
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* See http://www.asterisk.org for more information about
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* the Asterisk project. Please do not directly contact
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* any of the maintainers of this project for assistance;
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* the project provides a web site, mailing lists and IRC
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* channels for your use.
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*
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* This program is free software, distributed under the terms of
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* the GNU General Public License Version 2. See the LICENSE file
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* at the top of the source tree.
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*/
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/*!
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* \file
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* \brief "smart" channels that update automatically if a channel is masqueraded
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*
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* \author Mark Michelson <mmichelson@digium.com>
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*/
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#include "asterisk.h"
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#include "asterisk/linkedlists.h"
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#ifndef _ASTERISK_AUTOCHAN_H
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#define _ASTERISK_AUTOCHAN_H
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struct ast_autochan {
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struct ast_channel *chan;
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AST_LIST_ENTRY(ast_autochan) list;
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};
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/*!
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* \par Just what the $!@# is an autochan?
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*
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* An ast_autochan is a structure which contains an ast_channel. The pointer
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* inside an autochan has the ability to update itself if the channel it points
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* to is masqueraded into a different channel.
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*
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* This has a great benefit for any application or service which creates a thread
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* outside of the channel's main operating thread which keeps a pointer to said
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* channel. when a masquerade occurs on the channel, the autochan's chan pointer
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* will automatically update to point to the new channel.
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*
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* Some rules for autochans
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*
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* 1. If you are going to use an autochan, then be sure to always refer to the
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* channel using the chan pointer inside the autochan if possible, since this is
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* the pointer that will be updated during a masquerade.
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*
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* 2. If you are going to save off a pointer to the autochan's chan, then be sure
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* to save off the pointer using ast_channel_ref and to unref the channel when you
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* are finished with the pointer. If you do not do this and a masquerade occurs on
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* the channel, then it is possible that your saved pointer will become invalid.
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*/
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/*!
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* \brief set up a new ast_autochan structure
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*
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* \details
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* Allocates and initializes an ast_autochan, sets the
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* autochan's chan pointer to point to the chan parameter, and
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* adds the autochan to the global list of autochans. The newly-
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* created autochan is returned to the caller. This function will
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* cause the refcount of chan to increase by 1.
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*
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* \param chan The channel our new autochan will point to
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*
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* \note autochans must be freed using ast_autochan_destroy
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*
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* \retval NULL Failure
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* \retval non-NULL success
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*/
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struct ast_autochan *ast_autochan_setup(struct ast_channel *chan);
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/*!
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* \brief destroy an ast_autochan structure
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*
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* \details
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* Removes the passed-in autochan from the list of autochans and
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* unrefs the channel that is pointed to. Also frees the autochan
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* struct itself. This function will unref the channel reference
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* which was made in ast_autochan_setup
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*
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* \param autochan The autochan that you wish to destroy
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*
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* \retval void
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*/
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void ast_autochan_destroy(struct ast_autochan *autochan);
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/*!
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* \brief Switch what channel autochans point to
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*
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* \details
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* Traverses the list of autochans. All autochans which point to
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* old_chan will be updated to point to new_chan instead. Currently
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* this is only called during an ast_channel_move() operation in
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* channel.c.
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*
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* \pre Both channels must be locked before calling this function.
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*
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* \param old_chan The channel that autochans may currently point to
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* \param new_chan The channel that we want to point those autochans to now
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*
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* \retval void
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*/
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void ast_autochan_new_channel(struct ast_channel *old_chan, struct ast_channel *new_chan);
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#endif /* _ASTERISK_AUTOCHAN_H */
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