Re: [PATCH 1/3] rculist: Fix list_entry_rcu to read ptr with rcu_dereference_raw

[Paul E. McKenney]

On Mon, Apr 13, 2015 at 01:39:04PM +0200, Patrick Marlier wrote:
> Strange I don't get any conflict.
> Maybe due to my email client so I attached the patches to this email.

That did the trick, thank you!  I of course pass on the ritual advice
to check your email client settings.

I have queued these for 4.2, and will let you know if any problems
show up during testing.

							Thanx, Paul

> Thanks,
> --
> Patrick
> 
> On Sat, Mar 28, 2015 at 11:47 AM, Paul E. McKenney
> <paulmck@xxxxxxxxxxxxxxxxxx> wrote:
> > 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@xxxxxxxxx>
> >> > >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
> >

> From 8ac818d418068105623e43bbd289d9553c182e6c Mon Sep 17 00:00:00 2001
> From: Patrick Marlier <patrick.marlier@xxxxxxxxx>
> Date: Tue, 24 Mar 2015 11:16:55 +0100
> Subject: [PATCH 1/3] rculist: Fix list_entry_rcu to read ptr with
>  rcu_dereference_raw
> 
> Change to read effectively ptr with rcu_dereference_raw and not the
> __ptr variable on the stack.
> 
> Signed-off-by: Patrick Marlier <patrick.marlier@xxxxxxxxx>
> ---
>  include/linux/rculist.h | 5 +----
>  1 file changed, 1 insertion(+), 4 deletions(-)
> 
> diff --git a/include/linux/rculist.h b/include/linux/rculist.h
> index a18b16f..9d9baea 100644
> --- a/include/linux/rculist.h
> +++ b/include/linux/rculist.h
> @@ -247,10 +247,7 @@ static inline void list_splice_init_rcu(struct list_head *list,
>   * primitives such as list_add_rcu() as long as it's guarded by rcu_read_lock().
>   */
>  #define list_entry_rcu(ptr, type, member) \
> -({ \
> -	typeof(*ptr) __rcu *__ptr = (typeof(*ptr) __rcu __force *)ptr; \
> -	container_of((typeof(ptr))rcu_dereference_raw(__ptr), type, member); \
> -})
> +	container_of((typeof(ptr))rcu_dereference_raw(ptr), type, member)
>  
>  /**
>   * Where are list_empty_rcu() and list_first_entry_rcu()?
> -- 
> 2.1.0
> 

> From 3ab5f342939f768b693708bd32ef3f350af3b5a6 Mon Sep 17 00:00:00 2001
> From: Patrick Marlier <patrick.marlier@xxxxxxxxx>
> Date: Tue, 24 Mar 2015 11:21:05 +0100
> Subject: [PATCH 2/3] netfilter: fix list_entry_rcu usage.
> 
> Signed-off-by: Patrick Marlier <patrick.marlier@xxxxxxxxx>
> ---
>  net/netfilter/core.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/net/netfilter/core.c b/net/netfilter/core.c
> index fea9ef5..05bd311 100644
> --- a/net/netfilter/core.c
> +++ b/net/netfilter/core.c
> @@ -174,7 +174,7 @@ int nf_hook_slow(u_int8_t pf, unsigned int hook, struct sk_buff *skb,
>  	/* We may already have this, but read-locks nest anyway */
>  	rcu_read_lock();
>  
> -	elem = list_entry_rcu(&nf_hooks[pf][hook], struct nf_hook_ops, list);
> +	elem = list_entry_rcu(nf_hooks[pf][hook].next, struct nf_hook_ops, list);
>  next_hook:
>  	verdict = nf_iterate(&nf_hooks[pf][hook], skb, hook, indev,
>  			     outdev, &elem, okfn, hook_thresh);
> -- 
> 2.1.0
> 

> From 84f0428e2c9172692aba727636a643efb6994752 Mon Sep 17 00:00:00 2001
> From: Patrick Marlier <patrick.marlier@xxxxxxxxx>
> Date: Tue, 24 Mar 2015 11:22:10 +0100
> Subject: [PATCH 3/3] md/bitmap: fix list_entry_rcu usage.
> 
> Signed-off-by: Patrick Marlier <patrick.marlier@xxxxxxxxx>
> ---
>  drivers/md/bitmap.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/md/bitmap.c b/drivers/md/bitmap.c
> index 3a57679..ed00e46 100644
> --- a/drivers/md/bitmap.c
> +++ b/drivers/md/bitmap.c
> @@ -181,7 +181,7 @@ static struct md_rdev *next_active_rdev(struct md_rdev *rdev, struct mddev *mdde
>  	rcu_read_lock();
>  	if (rdev == NULL)
>  		/* start at the beginning */
> -		rdev = list_entry_rcu(&mddev->disks, struct md_rdev, same_set);
> +		rdev = list_entry_rcu(mddev->disks.next, struct md_rdev, same_set);
>  	else {
>  		/* release the previous rdev and start from there. */
>  		rdev_dec_pending(rdev, mddev);
> -- 
> 2.1.0
> 

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