[ovs-dev] [PATCH 02/12] ovs-atomic: Use memory_order_relaxed for ovs_refcount.

Jarno Rajahalme jrajahalme at nicira.com
Thu Jul 3 22:35:13 UTC 2014


It seems I was too invested in the combined refcount/RCU case here. I still think that with RCU postponed destruction relaxed is the proper memory model. So maybe we should add a relaxed variant of the unref function to be used with RCU objects and make the normal unref use release to guarantee writes to the protected object are done before the reference is dropped.

I do not yet fully understand the need for acquire before the delete. Is the concern that the current thread might immediately recycle the memory and writes to it (e.g., initialization) might be reordered to happen before the atomic sub while other threads might still be using the object?

  Jarno

> On Jul 3, 2014, at 11:52 PM, Ben Pfaff <blp at nicira.com> wrote:
> 
>> On Mon, Jun 30, 2014 at 08:17:18AM -0700, Jarno Rajahalme wrote:
>> Updating the reference count only requires atomicity, but no memory
>> ordering with respect to any other loads or stores.  Avoiding the
>> overhead of the default memory_order_seq_cst can make these more
>> efficient.
>> 
>> Signed-off-by: Jarno Rajahalme <jrajahalme at nicira.com>
> 
> This website makes pretty different claims:
>        http://www.chaoticmind.net/~hcb/projects/boost.atomic/doc/atomic/usage_examples.html
> 
> Relevant excerpts:
> 
>    #include <boost/intrusive_ptr.hpp>
>    #include <boost/atomic.hpp>
> 
>    class X {
>    public:
>      typedef boost::intrusive_ptr<X> pointer;
>      X() : refcount_(0) {}
> 
>    private:
>      mutable boost::atomic<int> refcount_;
>      friend void intrusive_ptr_add_ref(const X * x)
>      {
>        x->refcount_.fetch_add(1, boost::memory_order_relaxed);
>      }
>      friend void intrusive_ptr_release(const X * x)
>      {
>        if (x->refcount_.fetch_sub(1, boost::memory_order_release) == 1) {
>          boost::atomic_thread_fence(boost::memory_order_acquire);
>          delete x;
>        }
>      }
>    };
> 
> ...
> 
>    Increasing the reference counter can always be done with
>    memory_order_relaxed: New references to an object can only be formed
>    from an existing reference, and passing an existing reference from one
>    thread to another must already provide any required synchronization.
> 
>    It is important to enforce any possible access to the object in one
>    thread (through an existing reference) to happen before deleting the
>    object in a different thread. This is achieved by a "release"
>    operation after dropping a reference (any access to the object through
>    this reference must obviously happened before), and an "acquire"
>    operation before deleting the object.
> 
>    It would be possible to use memory_order_acq_rel for the fetch_sub
>    operation, but this results in unneeded "acquire" operations when the
>    reference counter does not yet reach zero and may impose a performance
>    penalty.



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