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.