[Freeswitch-dev] upgrade a rdlock to wrlock

[Anthony Minessale]

you want to use a regular mutex around the finding the pointer part
(hash look up etc).
while the hash's mutex is locked you read lock the rwlock on the
individual item.

There are some functions in switch_core_hash now to specify a mutex
andor if you want to fetch items read or write locked (see mod_rtmp's
usage)



On Thu, Aug 18, 2011 at 5:16 PM, Seven Du <dujinfang at gmail.com> wrote:
>
> On Friday, August 19, 2011 at 1:35 AM, Christopher Rienzo wrote:
>
> Even if that works, what happens when two threads that have a read lock
> attempt to switch to a write lock simultaneously?  How does the thread know
> it got the write lock first and the data is unchanged?
>
>
> maybe try_get_write_lock and let the latter one failed ? no idea. see
> following comments.
>
>
> On Thu, Aug 18, 2011 at 12:20 PM, Anthony Minessale
> <anthony.minessale at gmail.com> wrote:
>
> can't you just write lock it right away?
>
>
> I think there's a potential race when two threads try to find and launch new
> listeners respectively. So I try to add a find_or_create_listener() to avoid
> race.  It usually only need to create once unless 1) at the first time, 2)
> the node is broken so need a reconnect
>
> http://pastebin.freeswitch.org/17091 ( I moved some codes, so just look for
> find_or_create_listener() )
>
> So my concern is - isn't it expensive for a write lock than a read lock? Do
> you have other suggestions on this? or should I just get a write lock on not
> found and simply find again?
> I'm not sure if this is related
> to http://jira.freeswitch.org/browse/FS-3432  but want to fix the potential
> race first.
> let me know if you want to attach the patch to jira but it's not the final
> patch still need some further work.
> Thanks.
>
> On Thu, Aug 18, 2011 at 7:19 AM, Christopher Rienzo <cmrienzo at gmail.com>
> wrote:
>> I forgot to mention that you'd have to repeat the find operation once you
>> get the write lock since there is a small window between the unlock and
>> lock
>> where another thread may have created the item.
>>
>>
>> On Thu, Aug 18, 2011 at 6:42 AM, Christopher Rienzo <cmrienzo at gmail.com>
>> wrote:
>>>
>>> Unlock the read lock and then lock as a write lock.
>>>
>>> On Aug 17, 2011 8:20 PM, "Seven Du" <dujinfang at gmail.com> wrote:
>>>
>>> Hi,
>>>
>>>
>>> Just wondering how to upgrade a read lock to a write lock, say I do a
>>> find_or_create_item_in_queue()
>>> switch_thread_rwlock_rdlock(lock);
>>> find an item from queue, if not found, create one
>>> switch_thread_rwlock_upgrade_to_wrlock(lock);
>>> write
>>> switch_thread_rwlock_unlock(lock);
>>> Thanks
>>> --
>>> Seven Du
>>> About: http://about.me/dujinfang
>>> Blog: http://www.dujinfang.com
>>> Proj:  http://www.freeswitch.org.cn
>>> Sent with Sparrow
>>>
>>>
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>>
>>
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>
>
>
> --
> Anthony Minessale II
>
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-- 
Anthony Minessale II

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