On Thu, Nov 29, 2012 at 07:59:53PM +0100, Andi Kleen wrote: > On Thu, Nov 29, 2012 at 10:57:20AM -0800, Kent Overstreet wrote: > > On Thu, Nov 29, 2012 at 10:45:04AM -0800, Andi Kleen wrote: > > > Kent Overstreet <koverstr...@google.com> writes: > > > > > > > This implements a refcount with similar semantics to > > > > atomic_get()/atomic_dec_and_test(), that starts out as just an atomic_t > > > > but dynamically switches to per cpu refcounting when the rate of > > > > gets/puts becomes too high. > > > > > > This will only work if you put on the same CPU as you get, right? > > > > Nope, no such restriction. > > I don't see how you ensure you're doing the __this_cpu_dec on the same > CPU as you did the get
I'm not. (I probably should've documented this a bit more before I sent it out...) The trick is that we don't watch for the refcount hitting 0 until we're shutting down - so this only works if you keep track of your initial refcount. As long as we're not shutting down, we know the refcount can't hit 0 because we haven't released the initial refcount. When we do want to shutdown, the user calls percpu_ref_kill() which converts the percpu ref back to a single atomic ref, calls synchronize_rcu(), then sets the ref's state to PCPU_REF_DEAD. Only then does the caller drop the initial ref, and percpu_ref_put() only does atomic_dec_and_test() when the ref is dead - otherwise it's just doing a decrement. Also, with the percpu refs - you can have all your gets happening on one cpu, and all your puts happening on another - the percpu refs are unsigned ints so overflow isn't undefined, and if they wrap they'll still sum to the right value when we go to shut things down in percpu_ref_kill(). -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
