Re: [patch] Make std::tr1::shared_ptr thread-safe.

[Jonathan Wakely <cow at compsoc dot man dot ac dot uk>]

On Tue, Mar 29, 2005 at 05:19:49PM +0300, Peter Dimov wrote:

> Jonathan Wakely wrote:
> >These patches make std::tr1::shared_ptr thread-safe and change some
> >tests to check equality, not inequality, since we know what value we
> >expect and it's better to be precise about what we're checking.
> 
> It is, unfortunately, still not thread safe. There are two memory 
> visibility issues with the code.

Ouch.  First of all, thanks for the review, Peter.

> (here I am assuming that __exchange_and_add has unspecified memory 
> synchronization properties.)

I never got a clear answer on that, but I think that's the case in
general.

> I'm inclined to "ignore" this problem from shared_ptr's side and rely on 
> documentation or custom deleters instead to make X responsible for its 
> synchronization, because the barriers on every release() may introduce a 
> significant performance hit, at least in theory. (An empty lock/unlock at 
> the beginning of ~X would be enough.)

That might be consistent with libstdc++'s general thread-safety policy,
which is not to lock everything possible and to rely on the user to do
it where required.

> The second problem is that there must exist a release/acquire pair between 
> the dispose() and destroy() calls, otherwise ~_Deleter will not be able to 
> observe the effects of _M_del( _M_ptr ).

I see.

> Unfortunately, while libstdc++ has _GLIBCXX_READ_MEM_BARRIER and 
> _GLIBCXX_WRITE_MEM_BARRIER (which purpose I don't understand that well), 
> these cannot be used as an acquire/release pair, IIUC. (But I barely 
> understand memory visibility.)

I come in somewhere below "barely"

> The only reliable implementation of these barriers that I see is an empty 
> pthread_mutex_lock/pthread_mutex_unlock pair. This is likely to be a bit 
> more expensive than necessary.

Ouch, again.

I think my patch should be applied anyway, as it's an improvement, and
these other problems can be tackled one by one.  I have a lot of reading
to do so I can get up to speed on these issues!

Thanks again for the comments (I guess you're not keen to see me screw
up your shared_ptr  :-)

jon

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