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.marl...@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).
--
Patrick Marlier
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