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From: Tim Blechmann (tim_at_[hidden])
Date: 2008-07-06 06:33:10

thanks for having a look at my patch ...

> * The error is in flyweight_core::erase(). You have moved the
> invocation to check(h) outside of the write lock, which can cause a
> race condition:
> · Thread A destroys the last flyweight associated to some value x.
> check(h) passes as there are no more references and just when
> factory. erase() is about to be called the thread is interrupted. ·
> Thread B creates a flyweight with value x. · Thread A resumes and
> the entry with x is destroyed leaving a dangling reference in B.

i see your point ... double-checking before erasing value x should work
around this issue ...

> * The design issue has to do with your moving all the knowledge of the
> locks to the factory itself. As I envision the design, all the lock
> stuff should be made by flyweight_core and factories should be kept as
> dumb as possible: see for instance that you've got a fair amount of
> code repetition across the insert() and erase() memfuns of the
> different factories, this is precisely a sign that you can factor this
> out, and the proper place (IMHO) to have it is in flyweight_core.

i see your point, it would be possible to extend the factory api with a
find() memfun ...
i didn't do that for now, since i think, this way certain factories could
implement an more efficient insert() than a combined find()/insert()
pair ...

> Apart from this the patch is nice and I'd like to use it as inspiration
> (with your permission) to enhance the lib in the future when rw locks
> are included. Unfortunately I haven't been able to devote much time to
> the redesign the lib after the thorough and valuable criticisms from
> Alberto Barbati, maybe this summer I'll be able to make progress. As I
> see it, rw locks will be an additional option coexisting with
> traditional lock --this will be so because, to the best of my knowledge,

i didn't know, that you were thinking about some architecture changes,
but sure, go ahead and adapt my patch to your needs ... i would like to
replace my own class with a similar concept as soon as flyweight provides
rw locks :)

> Boost.Thread needs linking and I would like to keep Boost.Flyweight
> header only when possible. Could you please confirm/deny that using rw
> locks requires building and linking the Boost.Threads module?

having a brief look at the new (1.35) boost.threads code, it seems that
mutex/lock types are implemented header-only, but the used exceptions
aren't ...
however it would be quite straight-forward to implement the exceptions
header-only, since the implementation provides just trivial constructors/
destructors and what() functions ...

> My last request is: now that you've got rw locks implemented is a very
> good opportunity to test them agains traditional locks in the context of
> Boost.Flyweight and see how they compare (after all, if the gain is not
> significative maybe rw locks shouldn't be used after all). Do you plan
> to write some performance tests of rw against traditional locks? That
> would be terrific.

sure, i could try to implement a small performance test for
boost.flyweight to compare unique locks with shared locks ...

cheers, tim

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the ones who never yawn or say a commonplace thing, but burn, burn,
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