On Fri, Jan 21, 2011 at 05:31:53AM +1100, Nick Piggin wrote: > On Thu, Jan 20, 2011 at 3:03 PM, Paul E. McKenney > <paulmck@xxxxxxxxxxxxxxxxxx> wrote: > > On Thu, Jan 20, 2011 at 08:20:00AM +1100, Nick Piggin wrote: > >> On Thu, Jan 20, 2011 at 8:03 AM, Jeff Moyer <jmoyer@xxxxxxxxxx> wrote: > >> >> I don't know exactly how all programs use io_destroy -- of the small > >> >> number that do, probably an even smaller number would care here. But I > >> >> don't think it simplifies things enough to use synchronize_rcu for it. > >> > > >> > Above it sounded like you didn't think AIO should be using RCU at all. > >> > >> synchronize_rcu of course, not RCU (typo). > > > > I think that Nick is suggesting that call_rcu() be used instead. > > Perhaps also very sparing use of synchronize_rcu_expedited(), which > > is faster than synchronize_rcu(), but which which uses more CPU time. > > call_rcu() is the obvious alternative, yes. > > Basically, once we give in to synchronize_rcu() we're basically giving > up. That's certainly a very good tradeoff for something like filesystem > unregistration or module unload, it buys big simplifications in real > fastpaths. But I just don't think it should be taken lightly. Makes sense to me! BTW, on your earlier usage classification: > I think synchronize_rcu should firstly not be used unless it gives a good > simplification, or speedup in fastpath. > > When that is satified, then it is a question of exactly what kind of slow > path it should be used in. I don't think it should be used in process- > synchronous code (eg syscalls) except for error cases, resource > exhaustion, management syscalls (like module unload). I don't have any feedback either way on your guidance to where synchronize_rcu() should be used, as I believe that it depends a lot on the details of usage, and would vary from one part of the kernel to another, and possibly also over time. But I am very glad to see that you have been thinking about it and that you are putting forth some clear guidelines! Thanx, Paul -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
