sig_pri: Fix deadlock caused by sig_pri_queue_hangup

The change to add setting hangupsource to sig_pri_queue_hangup()
made in https://gerrit.asterisk.org/c/asterisk/+/12857 casued
deadlocks when a hangup request was received from the core at the
same time a hanguprequest was received from the remote end via the
D channel.

Although the PRI's channel private structure was being unlocked
before setting the hangupsource, the PRI's own lock was still being
held during the process.  If channel actions were also coming from
the core, a deadlock on the PRI could result.  This deadlock could
then escalate to the entire DAHDI subsystem via DAHDI's global
interface list lock, especially if someone used the PRI CLI commands.

Fix:

* We now unlock the PRI as well as the PRI's channel private
  structure before setting the hangupsource, then relock both
  afterwards.

ASTERISK-28605
Reported by: Dirk Wendland

Change-Id: Id74aaa5d4e3746063dbe9deed188eb65193cb9c9

diff --git a/channels/sig_pri.c b/channels/sig_pri.c
index 4c70c6beaaf296b506d8bb9c5923ad4f256fa770..4bd5a5191f4cbb1b34b952de2f3286d875ff4450 100644 (file)
--- a/channels/sig_pri.c
+++ b/channels/sig_pri.c
@@ -1387,6 +1387,10 @@ static void pri_queue_control(struct sig_pri_span *pri, int chanpos, int subclas
  * \note Assumes the pri->lock is already obtained.
  * \note Assumes the sig_pri_lock_private(pri->pvts[chanpos]) is already obtained.
  *
+ * \note The unlocking/locking sequence now present has been stress tested
+ *       without deadlocks.  Please don't change it without consulting
+ *       core development team members.
+ *
  * \return Nothing
  */
 static void sig_pri_queue_hangup(struct sig_pri_span *pri, int chanpos)
@@ -1404,9 +1408,11 @@ static void sig_pri_queue_hangup(struct sig_pri_span *pri, int chanpos)
                ast_queue_hangup(owner);
                ast_channel_unlock(owner);
 
-               /* Tell the CDR this DAHDI channel hung up */
                sig_pri_unlock_private(pri->pvts[chanpos]);
+               ast_mutex_unlock(&pri->lock);
+               /* Tell the CDR this DAHDI channel hung up */
                ast_set_hangupsource(owner, ast_channel_name(owner), 0);
+               ast_mutex_lock(&pri->lock);
                sig_pri_lock_private(pri->pvts[chanpos]);
 
                ao2_ref(owner, -1);