14 Sep
2009
14 Sep
'09
11:41 a.m.
Hi Jan,
On 09/14/2009 01:48 PM, Jan Janak wrote:
Thanks for the pointer. This is what I was afraid of, it seems that
unref_new_dialog() does not use the unsafe version of shm_free. It
unrefs the dialog that may be freed, so it uses shm_free regardless of
whether or not the shm lock is held.
I do not think that it is related to the cell lock but rather to the
lock of the shm memory allocator. Lots of memory needs to be freed,
hence I assume Andrei decided to lock, free all the stuff, and than
unlock, so the frequent lock-unlock is not needed.
Hi Miklos,
On 10-09 18:42, Miklos Tirpak wrote:
yes, the release function should know about this fact however I would
rather not depend on this. The module writers have to be aware of this,
shall not forget it, the release function may be quite complex, ... so I
think it is just safer to call the release func outside of the locks.
Hi Jan,
when free_cell() frees the memory of a transaction the shm memory lock
is already held:
shm_lock();
...
/* callbacks */
for( cbs=(struct tm_callback*)dead_cell->tmcb_hl.first ; cbs ; ) {
cbs_tmp = cbs;
cbs = cbs->next;
if (cbs_tmp->release) {
cbs_tmp->release(cbs_tmp->param);
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
I think this can cause a dead-lock because the release function is not
aware of the state of the shm mem lock.
I am not sure I understand. The release function assumes that it is called
from within the lock, so it knows about it, right? }
shm_free_unsafe( cbs_tmp );
}
I saw that you have added this function call, but I cannot found any
customer of this function in the repository, so I do not know whether
the cb functions use safe or unsafe shm_free(). Do you know anything
about this?
The release function was added there on request. It was needed by the kamailio
version of dialog module. See modules_k/dialog, function unref_new_dialog is
the handle that is called from the code above.
Unfortunately I do not know much about the dialog module. I recently added another place where this cb is
called from (without
locking), hence I think it would be better to move this outside of the
shm mem lock to be on the safe side.
Why is the shm_lock there? Is this some sort of performance optimization to
ensure that as little as possible is done with the cell lock being?
I think you are right that the release callabacks should not be executed with
the shm_lock being held, because the callbacks can execute arbitrary complex
functions and those functions might not be aware that they need to use the
non-locking version of shm related functions.
Indeed.
So feel free to change it it is safe.
Ok, I will.
Thanks,
Miklos
Jan.