linux-kernel - Re: [PATCH 1/3] rculist: Fix list_entry_rcu to read ptr with rcu_dereference

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Date:	Sat, 28 Mar 2015 03:42:10 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Patrick Marlier <patrick.marlier@...il.com>
Cc:	linux-kernel@...r.kernel.org, Josh Triplett <josh@...htriplett.org>
Subject: Re: [PATCH 1/3] rculist: Fix list_entry_rcu to read ptr with
 rcu_dereference_raw

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@...il.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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