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

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Message-ID: <20150328104712.GA4016@linux.vnet.ibm.com>
Date:	Sat, 28 Mar 2015 03:47:12 -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 Sat, Mar 28, 2015 at 03:42:10AM -0700, Paul E. McKenney wrote:
> 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.

Except that I hit conflicts.  Could you please rebase to rcu/dev at
git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu.git?

							Thanx, Paul

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