On Mon, Jan 28, 2013 at 10:27:37AM -0800, Tejun Heo wrote: > Hello, guys. > > On Mon, Jan 28, 2013 at 10:15:28AM -0800, Kent Overstreet wrote: > > > percpu_ref_kill(); > > > put_and_dsetroy(); > > > > > > And this can race with another holder which drops the last reference, > > > its put_and_dsetroy() can see PCPU_REF_DYING and return false. > > > > > > Or I misunderstood the code/interface? > > > > Nope, nailed it :) That should _definitely_ be in the documentation. > > Can we just combine kill initiation and base ref put and make that the > responsibility of the owner? Extra features on basic constructs may > seem good for certain use cases but tend to bring more confusion than > good in the long run. If a user needs to synchronize among multiple > killers, let the user deal with the issue.
Don't follow... Something I forgot to mention in the last mail though is that often the caller will need its own synchronize_rcu()/call_rcu() - percpu_ref_kill() corresponds to when you make the object unavailable (i.e. deleting it from the rcu protected hash table in aio) and you need a synchronize_rcu() before you drop your initial ref. So letting the caller do it means the caller can merge the two synchronize_rcu()s. > > > Actually - I think it'd be better to have the default percpu_ref_kill() > > do the second synchronize_rcu(), and have an unsafe version that skips > > it. > > Note that synchronize_rcu/sched() can be very slow and cause problems > in paths which are frequently traveled and visible to userland. It's > fine for things like module destruction but can be a problem even > during device destruction - blkcg had synchronize_rcu() in > request_queue destruction which led to huge latencies during boot > because SCSI wants to create and then destroy request_queues for all > possible LUNs on certain configurations. So, if you put > synchronize_rcu/sched() in percpu_ref_kill(), that better not be used > from e.g. close(2). Yeah. It'd be really nice if it was doable without synchronize_rcu(), but it'd definitely make get/put heavier. Though, re. close() - considering we only need a synchronize_rcu() if the ref was in percpu mode, I wonder if that would be a dealbreaker. I have no clue myself. Getting rid of synchronize_rcu would basically require turning get and put into cmpxchg() loops - even in the percpu fastpath. However, percpu mode would still be getting rid of the shared cacheline contention, we'd just be adding another branch that can be safely marked unlikely() - and my current version has one of those already, so two branches instead of one in the fast path. I suppose I should give it a shot. As long as I'm going down that route I could probably make the bare non percpu ref 8 bytes instead of 16, too... -- 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/
