On Mon, Jan 28, 2013 at 01:18:32PM -0800, Tejun Heo wrote: > Hello, Kent. > > On Mon, Jan 28, 2013 at 12:55:40PM -0800, Kent Overstreet wrote: > > > I don't understand why we need two stages. What prevents the killing > > > thread from fetching percpu counters after dying passes one > > > synchronize_sched()? > > > > It does. The second synchronize_sched() is needed after we set state := > > dead, and before we drop the initial ref. Otherwise the ref could hit 0 > > before percpu_ref_put knows to check for it. > > Still a bit confused. Why do we need to make the two steps separate? > What prevents us from doing the following? > > set dying; > synchronize_sched(); > collect percpu refs into global atomic_t; > put the base ref;
After you set state := dying, percpu_ref_put() decrements the atomic_t, but it can't check if it's 0 yet because the thread that's collecting the percpu refs might not be done yet. So percpu_ref_put can't check for ref == 0 until after state == dead. But the put in your example might have made ref 0. When did you set state to dead? -- 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/
