Messages in this thread | | | Date | Wed, 29 May 2013 14:09:29 +0400 | From | Roman Gushchin <> | Subject | Re: [PATCH v2] rcu: fix a race in hlist_nulls_for_each_entry_rcu macro |
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On 29.05.2013 09:08, Eric Dumazet wrote: > On Tue, 2013-05-28 at 18:31 -0700, Paul E. McKenney wrote: >> On Tue, May 28, 2013 at 05:34:53PM -0700, Eric Dumazet wrote: >>> On Tue, 2013-05-28 at 13:10 +0400, Roman Gushchin wrote: >>>> On 28.05.2013 04:12, Eric Dumazet wrote:
> About your earlier question, I really don't know why compiler would > cache a memory read if we explicitly use barrier() to prevent this from > happening.
I agree.
> BTW Roman patch generates a double load as in : > > 2cb1: 49 8b 07 mov (%r15),%rax > 2cb4: 49 8b 07 mov (%r15),%rax > > > ... > 2ea2: e8 f9 dc ff ff callq ba0 <sock_put> > 2ea7: 8b 0c 24 mov (%rsp),%ecx > 2eaa: e9 02 fe ff ff jmpq 2cb1 <udp4_lib_lookup2+0x91> > > because of ACCESS_ONCE() used twice, once explicitly in > hlist_nulls_for_each_entry_rcu(), and once in rcu_dereference_raw() > > While barrier();ptr = rcu_dereference(X); does generate a single load.
It's true.
Unfortunately, using barrier() can also prevent gcc from making some (acceptable) code optimizations, because barrier() has a global effect, and everything we want to reload is the (head->first) pointer. So, to be absolutely precise, we have to introduce and use the ACCESS_FIELD_ONCE() macro.
In any case, it doesn't look like a big problem. In my mind, the best solution is to use the ACCESS_FIELD_ONCE() macro, but using barrier() is also an acceptable solution.
Regards, Roman
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