Re: [RFC][PATCH] identify in_dev_get rcu read-side critical sections

On Fri, Sep 30, 2005 at 11:56:41PM -0700, Suzanne Wood wrote:

But it is interesting to have discarded what was developed yesterday
to minimize rcu_dereference impact:

quoted

quoted

----- Original message  -----
From: Herbert Xu <herbert@gondor.apana.org.au>
Date: Fri, 30 Sep 2005 11:19:07 +1000

On Thu, Sep 29, 2005 at 06:16:03PM -0700, Paul E. McKenney wrote:

quoted

OK, how about this instead?

rcu_read_lock();
in_dev = dev->ip_ptr;
if (in_dev) {
atomic_inc(&rcu_dereference(in_dev)->refcnt);
}
rcu_read_unlock();
return in_dev;

Looks great. 

while adding a function call level by wrapping  __in_dev_get_rcu
with in_dev_get as suggested here.

It might look different, but it should compile to the same result.
GCC should be smart enough to combine the two branches and produce a
memory barrier only when in_dev is not NULL.
 

The other thing I'd hoped to address in pktgen.c was 
removing the __in_dev_put() which decrements refcnt 
while __in_dev_get_rcu() does not increment.

Well spotted.

Here is a patch on top of the last one to fix this bogus decrement.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

Thanks,
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