Re: [patch] fs: aio fix rcu lookup

[Eric Dumazet]

Le jeudi 20 janvier 2011 Ã 12:02 -0800, Paul E. McKenney a Ãcrit :
> 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())



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