Re: [Boost-users] shared_ptr and thread safety

From: Peter Dimov (pdimov_at_[hidden])
Date: 2007-08-19 15:06:36

Emil Wojak wrote:

> // pre-thread
> boost::shared_ptr<const A> ptr(new A);
> 
> // reader thread A
> boost::shared_ptr<const A> p1=ptr;   // get the latest value
> p1->getSomeValue();
> p1->getSomeOtherValue();
> p1.reset();  // don't need that anymore
> 
> // reader thread B
> boost::shared_ptr<const A> p2=ptr;   // get the latest value
> p2->getSomeValue();
> p2->getSomeOtherValue();
> p2.reset();  // don't need that anymore
> 
> // writer thread
> ptr.reset(new A);  // look Ma! I've brought you something new!
> 
> So is the substitution thread-safe?
> I think it could be, if the steps necessary to do that were taken in
> the following order:
> - setting new object's counter to 1 (atomicity irrelevant at this
> point) 
> - switching internal pointer to point to the new object (atomic) -
> from now on, reader threads would get the new object
> - decrementing old object's counter (atomic) - the old object gets
> released and eventually deleted

No, this is not safe.

1. reader A reads the pointer value
2. writer thread sets new pointer value
3. writer thread deletes old object and count
4. reader A tries to increment the deleted count, bad things happen

The proposed additions in

http://www.open-std.org/JTC1/SC22/WG21/docs/papers/2007/n2297.html#atomic

aim to support this use case, but I haven't implemented them yet.