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

If two threads hold an upgrade lock, and then both of them try to upgrade: -The first thread cannot upgrade to a unique lock until the second thread releases its upgrade lock. -The second thread cannot upgrade to a unique lock until the first thread releases its upgrade lock. Hence, deadlock. There are two ways to do read/write locks with upgradeability. The more common method used by most other libraries is to make the upgrade a non-blocking tryUpgrade() method or similar. The approach used by boost::threads is, IMO, smarter, but more confusing at first. Here is the key: An upgrade lock is *not* a special type of read lock that can be upgraded. Don't think of it that way. Instead, think of it as a special type of write lock that can allow simultaneous read locks. To use boost::thread terminology, multiple upgrade_locks cannot be simultaneously taken on the same mutex; however, while a single upgrade_lock is taken, an arbitrary number of shared_locks can also still be taken on the same mutex. So: unique_lock: No other locks may be taken at the same time. upgrade_lock: Other shared_locks may be taken at the same time, but no other upgrade locks. shared_lock: Other shared_locks may be taken at the same time. You need to keep this in mind when coding, and keep your upgrade_lock usage restricted to those places where you *really* need it. On Thu, Mar 27, 2014 at 12:09 PM, Elad Moshe wrote:

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If both threads tried to upgrade, you would have a deadlock.

But how is it possible? isn't the upgrade operation atomic? if the 2 threads try to upgrade, one will succeed and the other one will block until the first thread will unlock.

On Thu, Mar 27, 2014 at 5:39 PM, Steven Watanabe wrote:

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AMDG

On 03/27/2014 01:21 AM, Elad Moshe wrote:

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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?

If both threads tried to upgrade, you would have a deadlock.

In Christ, Steven Watanabe

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