[asterisk-dev] Locking, coding guidelines addition

[Hans Petter Selasky]

Hi,

I think this same issue was discussed some years ago.

Extract from my solution:

/*
 * LOCKING RULES
 * =============
 *
 * This channel driver uses several locks. One must be 
 * careful not to reverse the locking order, which will
 * lead to a so called deadlock. Here is the locking order
 * that must be followed:
 *
 * struct call_desc *cd;
 * struct cc_capi_application *p_app;
 *
...
 *
 * 2. cc_mutex_lock(&modlock); (See Asterisk source code)
 *
 * 3. cc_mutex_lock(&chlock); (See Asterisk source code)
 *
 * 4. cc_mutex_lock(&cd->pbx_chan->lock); **
 *
 * 5. cc_mutex_lock(&p_app->lock);
 *
 * 6. cc_mutex_lock(&capi_global_lock);

...

 *
 *  ** the PBX will call the callback functions with 
 *     this lock locked. This lock protects the 
 *     structure pointed to by 'cd->pbx_chan'. Also note
 *     that calling some PBX functions will lock
 *     this lock!
 */

Rule #1: You cannot call any "ast_xxx()" functions with any private channel 
driver locks locked. You need to handle any races when you pickup your locks 
again.

Rule #2: In all Asterisk callbacks you have to lookup your softc in a list 
from the ast_channel pointer! Never access any private channel structures 
before you know that the channel is still there!

If you follow these two simple rules you will be safe from trouble!

--HPS

Reference:
http://www.selasky.org/hans_petter/capi4pbx/sources/chan_capi.c