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

[Patrick Marlier]

Strange I don't get any conflict.
Maybe due to my email client so I attached the patches to this email.

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