Messages in this thread | ![/](/images/icornerl.gif) | | Date | Thu, 29 Sep 2005 17:23:46 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [RFC][PATCH] identify in_dev_get rcu read-side critical sections |
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On Thu, Sep 29, 2005 at 04:30:28PM -0700, Suzanne Wood wrote: > > Date: Fri, 30 Sep 2005 07:28:36 +1000 > > From: Herbert Xu <herbert@gondor.apana.org.au> > > > On Thu, Sep 29, 2005 at 09:02:29AM -0700, Suzanne Wood wrote: > > > > > > The exchange below suggests that it is equally important > > > to have the rcu_dereference() in __in_dev_get(), so the > > > idea of the only difference between in_dev_get and > > > __in_dev_get being the refcnt may be accepted. > > > With __in_dev_get() it's the caller's responsibility to ensure > > that RCU works correctly. Therefore if any rcu_dereference is > > needed it should be done by the caller. > > This sounds reasonable to me. Does everyone agree?
Is there any case where __in_dev_get() might be called without needing to be wrapped with rcu_dereference()? If so, then I agree (FWIW, given my meagre knowledge of Linux networking).
If all __in_dev_get() invocations need to be wrapped in rcu_dereference(), then it seems to me that there would be motivation to bury rcu_dereference() in __in_dev_get().
> > Some callers of __in_dev_get() don't need rcu_dereference at all > > because they're protected by the rtnl. > > > BTW, could you please move the rcu_dereference in in_dev_get() > > into the if clause? The barrier is not needed when ip_ptr is > > NULL. > > The trouble with that may be that there are three events, the > dereference, the assignment, and the conditional test. The > rcu_dereference() is meant to assure deferred destruction > throughout.
One only needs an rcu_dereference() once on the data-flow path from fetching the RCU-protected pointer to dereferencing that pointer. If the pointer is NULL, there is no way you can dereference it, so, technically, Herbert is quite correct.
However, rcu_dereference() only generates a memory barrier on DEC Alpha, so there is normally no penalty for using it in the NULL-pointer case. So, when using rcu_dereference() unconditionally simplifies the code, it may make sense to "just do it".
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