[Boost-users] [Thread] why isn't it possible to acquire 2 different upgrade_lock from 2 different threads?

Subject: [Boost-users] [Thread] why isn't it possible to acquire 2 different upgrade_lock from 2 different threads?
From: Elad Moshe (eladm26_at_[hidden])
Date: 2014-03-27 04:21:48

Hi,
this is my first post here so please bare with me :) .
I recently used upgrade_lock to try access a shared resource from 2
different threads.
I noticed that once the first thread acquired its upgrade_lock the second
thread was blacked
until the first thread unlock it.
can someone please explain me the logic behind this behavior?
isn't an upgrade lock just a shared_lock that can be upgraded to
unique_lock?
why not let both threads acquire it just like shared_lock until, and if,
they decide to upgrade it to unique_lock?

Thanks a lot,
Elad.