Re: [Boost-users] shared_ptr and weak_ptr concurrency

Subject: Re: [Boost-users] shared_ptr and weak_ptr concurrency
From: John Dlugosz (JDlugosz_at_[hidden])
Date: 2009-09-04 15:36:53

> can you point to anything that the boost shared_ptr does that is unsafe
> in the one-writer-multiple-reader case?

Just looking at the header, I see two direct members, px and pn.
So straight assignment isn't going to copy the struct in one atomic operation.

Looking at operator=, I see two separate assignments.  First the assignment to the contained pointer takes place.  Now, a reader on another thread will see the object to have the wrong shared_count structure.  Then, the assigning thread continues, taking its time to crank through the shared_count assignment which changes two reference counts (lengthy pipeline stalls for atomic operations, CPU stays busy for a while) before finally writing the new pn value.

It could be made safe by implementing it like this:

      make a temp shared_ptr object, initialized from the RHS.
      issue a double-wide atomic swap instruction, swapping the LHS for the temp.
      let the temp destruct.

That's machine architecture specific, and compiler specific on how to make it emit the correct swap instruction.  The x86 and x64 have an atomic exchange that's twice the size of a normal pointer, so it will swap two pointers in one struct.  That's what it's there for -- implementing things like this.

You could easily change the function in shared_ptr.hpp if you wanted it to work that way.

--John

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