On Thu, Jan 20, 2011 at 09:15:55PM +0100, Eric Dumazet wrote: > Le jeudi 20 janvier 2011 à 12:02 -0800, Paul E. McKenney a écrit : > > On Fri, Jan 21, 2011 at 05:31:53AM +1100, Nick Piggin wrote: > > > 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. > > > > Sometime, a mixture of call_rcu() and synchronize_rcu() is used, to have > a limit on pending callbacks (eating too much memory) > > net/ipv4/fib_trie.c for example issues call_rcu() most of the time, but > is able to trigger one synchronize_rcu() if more than XXX (128) pages of > memory were queued in rcu queues. > > For details, check commit c3059477fce2d956 > (ipv4: Use synchronize_rcu() during trie_rebalance()) Good point! 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
