On Mon, Jan 28, 2013 at 01:28:14PM -0800, Tejun Heo wrote: > On Mon, Jan 28, 2013 at 01:24:07PM -0800, Kent Overstreet wrote: > > > 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? > > But at that point, the operation is already global, so there gotta be > a lighter way to synchronize stuff than going through full grace > period. ie. You can add a bias value before marking dead so that the > counter never reaches zero before all percpu counters are collected > and then unbias it right before putting the base ref, that way the > only way you can hit zero ref is all refs are actually zero.
Ahh. Bias value sounds... hacky (i.e. harder to convince myself it's correct) but I see what you're getting at. Something to consider is wrapping; after we set state to dying but before we've collected the percpu counters, the atomic counter may be negative. But since the atomic counter is 64 bits, we can use 1 << 32 for the bias value (and just include that when we first initialize it). Which makes me feel like it's less of a hack too. I'll have to think about it some more but seems like it ought t owork... -- 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/
