Messages in this thread | | | Date | Sat, 28 Mar 2015 03:42:10 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH 1/3] rculist: Fix list_entry_rcu to read ptr with rcu_dereference_raw |
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On Wed, Mar 25, 2015 at 04:01:24PM +0100, Patrick Marlier wrote: > On 03/25/2015 03:30 PM, Paul E. McKenney wrote: > >On Tue, Mar 24, 2015 at 11:31:38AM +0100, Patrick Marlier wrote: > >>Change to read effectively ptr with rcu_dereference_raw and not the > >>__ptr variable on the stack. > >> > >>Signed-off-by: Patrick Marlier <patrick.marlier@gmail.com> > >Avoiding an extra load could be worthwhile in a number of situations, > >agreed. > Not only a load. It adds a store and a load on the stack and I think > this creates a dependency in the processor pipeline. > > >However, won't this change cause sparse to complain if invoked on a > >non-RCU-protected pointer? The ability to use list-RCU API > >members on both RCU and non-RCU pointers was one of the points > >of the previous commit, right? > Probably we can put back the cast but I am not familiar enough with > the RCU API. > > Also, the problem here is that you probably want ACCESS_ONCE to > happen on the content of 'ptr' and not on the stack variable > '__ptr'. > > (you have to follow this chain: rcu_dereference_raw -> > rcu_dereference_check -> __rcu_dereference_check -> > lockless_dereference -> ACCESS_ONCE) > > #define lockless_dereference(p) \ > ({ \ > typeof(p) _________p1 = ACCESS_ONCE(p); \ > smp_read_barrier_depends(); /* Dependency order vs. p above. */ \ > (_________p1); \ > }) > > #define __ACCESS_ONCE(x) ({ \ > __maybe_unused typeof(x) __var = (__force typeof(x)) 0; \ > (volatile typeof(x) *)&(x); }) > #define ACCESS_ONCE(x) (*__ACCESS_ONCE(x)) > > Note that ACCESS_ONCE is doing "&" on x. > > IMHO, I would prefer saving some useless instructions for better > performance rather than giving too much flexibility on the API (also > pretty sure the cast can be still done).
OK, what I am going to do is to apply your patches for testing purposes. If there are no complaints, they will likely go into v4.3 or thereabouts.
Thanx, Paul
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