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: 2011-12-07 22:17:27

On Wed, Dec 7, 2011 at 4:05 PM, Kelvin Chung <kelvSYC_at_[hidden]> wrote:
> I'm trying to implement a read-write lock, and I was told to use
> boost::upgrade_lock.  However, only one thread can have a
> boost::upgrade_lock, and everyone else gets blocked, so it's not much on the
> read end.  (Correct me if I am wrong, but boost::upgrade_lock only waits if
> someone has exclusive, but someone with boost::upgrade_lock will cause
> everyone else to block)
>
> But reads are cheap in my case - I just need to use locks for a cache.  So,
> suppose I have the following:
>

You don't necessarily want to release your lock before upgrading.
This is more or less how I'd think about it (through example
pseudocode):

struct Example {
    boost::shared_mutex m_mtx;
    ...

    Data doRead() {
        boost::shared_lock<boost::shared_mutex> rlock(m_mtx);
        ...read and return
    }
    void doWrite(OtherData d) {
        boost::unique_lock<boost::shared_mutex> wlock(m_mtx);
        ...write d somewhere protected by m_mtx
    }
    void getCachedOrComputed(Data &d) {
        boost::upgrade_lock<boost::shared_mutex> uplock(m_mtx);
        if(found in cache)
            ...read data from a cache into d
        d = compute_output()
        //upgrade the lock to unique for write
        boost::upgrade_to_unique_lock<SharedMutex> wlock(uplock);
        ... write d into the cache
    }
}

Obviously "getCachedOrComputed" is most akin to what you want to do
(including where I would place compute_output()), but I included the
other functions to illustrate other cases (no need for upgrade).  I
had a difficult time figuring this out myself a while back, so I hope
this helps.

  Brian