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

Subject: Re: [Boost-users] [Thread] why isn't it possible to acquire 2 different upgrade_lock from 2 different threads?
From: Lindley French (lindleyf_at_[hidden])
Date: 2014-03-27 12:59:41

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 <eladm26_at_[hidden]> wrote:

> > 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 <watanabesj_at_[hidden]>wrote:
>
>> AMDG
>>
>> On 03/27/2014 01:21 AM, Elad Moshe wrote:
>> > 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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>
>
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