Re: [Boost-users] boost::shared_locks and boost::upgrade_locks

Subject: Re: [Boost-users] boost::shared_locks and boost::upgrade_locks
From: Brian Budge (brian.budge_at_[hidden])
Date: 2011-12-08 10:22:36

On Wed, Dec 7, 2011 at 9:00 PM, Kelvin Chung <kelvSYC_at_[hidden]> wrote:
> On 2011-12-08 04:27:51 +0000, Brian Budge said:
>>
>>
>> Upgrade ownership is just shared ownership that can be upgraded to
>> exclusive ownership.
>
>
> My understanding of the documentation of the UpgradeLockable concept seems
> to suggest otherwise.  And I quote:
>
> "a single thread may have upgradeable ownership at the same time as others
> have shared ownership"
>
> This seems to imply that shared and upgrade are very different levels.
>  Especially when it later says "upgradeable ownership can be downgraded to
> plain shared ownership".  If upgrade is just shared with a license to
> upgrade, why would you ever need to downgrade?  Why would "downgrade to
> shared" even exist in the first place?
>
> Here's a scenario that proves my point: Suppose cache::query() is just
> implemented with upgrade locks.  Suppose you have two Inputs, foo and bar.
>  Suppose you also have two threads, one calling Cache::query(foo) and the
> other Cache::query(bar).  Both will be trying to get the upgrade_lock, but
> according to the UpgradeLockable concept, only one thread gets it, and the
> other one will be blocked.  So, if foo is not in the cache and bar is in the
> cache, and bar is the one that gets blocked, then bar has to wait for foo to
> finish (which, as compute_output() could be expensive, could take a while) -
> this is no better than doing things serially, when you could just let the
> bar go through (since it only needs to read from the cache, which is cheap)
> as foo is waiting for the exclusive lock upgrade.
>
> Thus, my conception is that you get a shared lock when you don't know that
> you need to write to cache.  When it turns out that you do, you unlock and
> get an upgrade lock, which expresses the intention of writing to the cache,
> while still letting other threads get shared locks for their cache lookups.
>  After checking whether your input is in the cache again (since another
> thread may have written what you needed into the cache while waiting for the
> upgrade lock), you upgrade to exclusive (you have the upgrade lock and so no
> one else could have written to the cache while all the other threads with
> shared locks leave), where you actually write to cache.  Then downgrade from
> exclusive to upgrade to shared when you are done, and return the cached
> value.
>
>
>
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Hmmm, I'm not so sure.  I wonder if Anthony or one of the other boost
thread gurus might chime in?