[PATCH tip/core/rcu 0/4] RCU-protected list updates for 4.2

[PATCH tip/core/rcu 0/4] RCU-protected list updates for 4.2

[Paul E. McKenney]

Hello!

This series contains a few updates to RCU-protected lists.

1.	Add RCU annotation to suppress sparse warnings in
	hlist_for_each_entry_from_rcu(), courtesy of Ying Xue.

2.	Fix list_entry_rcu to read ptr with rcu_dereference_raw() to
	save a load instruction, courtesy of Patrick Marlier.

3.	Fix list_entry_rcu() usage in next_active_rdev(), courtesy
	of Patrick Marlier.

4.	Fix list_entry_rcu() usage in nf_hook_slow(), courtesy of
	Patrick Marlier.

							Thanx, Paul

------------------------------------------------------------------------

 b/drivers/md/bitmap.c     |    2 +-
 b/include/linux/rculist.h |    9 +++------
 b/net/netfilter/core.c    |    2 +-
 3 files changed, 5 insertions(+), 8 deletions(-)

[PATCH tip/core/rcu 1/4] rculist: Fix another sparse warning

[Paul E. McKenney]

From: Ying Xue <ying.xue@windriver.com>

This fixes the following sparse warnings:

make C=1 CF=-D__CHECK_ENDIAN__ net/tipc/name_table.o
net/tipc/name_table.c:977:17: error: incompatible types in comparison expression (different address spaces)
net/tipc/name_table.c:977:17: error: incompatible types in comparison expression (different address spaces)

To silence these spare complaints, an RCU annotation should be added to
"next" pointer of hlist_node structure through hlist_next_rcu() macro
when iterating over a hlist with hlist_for_each_entry_from_rcu().

Signed-off-by: Ying Xue <ying.xue@windriver.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 include/linux/rculist.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/linux/rculist.h b/include/linux/rculist.h
index 665397247e82..17c6b1f84a77 100644
--- a/include/linux/rculist.h
+++ b/include/linux/rculist.h
@@ -549,8 +549,8 @@ static inline void hlist_add_behind_rcu(struct hlist_node *n,
  */
 #define hlist_for_each_entry_from_rcu(pos, member)			\
 	for (; pos;							\
-	     pos = hlist_entry_safe(rcu_dereference((pos)->member.next),\
-			typeof(*(pos)), member))
+	     pos = hlist_entry_safe(rcu_dereference_raw(hlist_next_rcu(	\
+			&(pos)->member)), typeof(*(pos)), member))
 
 #endif	/* __KERNEL__ */
 #endif
-- 
1.8.1.5

[PATCH tip/core/rcu 2/4] rculist: Fix list_entry_rcu to read ptr with rcu_dereference_raw

[Paul E. McKenney]

From: Patrick Marlier <patrick.marlier@gmail.com>

Change to read effectively ptr with rcu_dereference_raw and not the
__ptr variable on the stack.

Signed-off-by: Patrick Marlier <patrick.marlier@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 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 17c6b1f84a77..c881741fbacd 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()?
-- 
1.8.1.5

[PATCH tip/core/rcu 3/4] md/bitmap: Fix list_entry_rcu usage

[Paul E. McKenney]

From: Patrick Marlier <patrick.marlier@gmail.com>

Signed-off-by: Patrick Marlier <patrick.marlier@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 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 2bc56e2a3526..32901772e4ee 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);
-- 
1.8.1.5

[PATCH tip/core/rcu 4/4] netfilter: Fix list_entry_rcu usage

[Paul E. McKenney]

From: Patrick Marlier <patrick.marlier@gmail.com>

Signed-off-by: Patrick Marlier <patrick.marlier@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 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 e6163017c42d..ad70195929a4 100644
--- a/net/netfilter/core.c
+++ b/net/netfilter/core.c
@@ -166,7 +166,7 @@ int nf_hook_slow(struct sk_buff *skb, struct nf_hook_state *state)
 	/* We may already have this, but read-locks nest anyway */
 	rcu_read_lock();
 
-	elem = list_entry_rcu(&nf_hooks[state->pf][state->hook],
+	elem = list_entry_rcu(nf_hooks[state->pf][state->hook].next,
 			      struct nf_hook_ops, list);
 next_hook:
 	verdict = nf_iterate(&nf_hooks[state->pf][state->hook], skb, state,
-- 
1.8.1.5

Re: [PATCH tip/core/rcu 3/4] md/bitmap: Fix list_entry_rcu usage

[Steven Rostedt]

On Tue, 12 May 2015 15:46:26 -0700
"Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote:

> From: Patrick Marlier <patrick.marlier@gmail.com>
> 
> Signed-off-by: Patrick Marlier <patrick.marlier@gmail.com>
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> ---
>  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 2bc56e2a3526..32901772e4ee 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);

Hmm, this changes the semantics.

The original code looks nasty, I first thought it was broken, but it
seems to work out of sheer luck (or clever hack)

>  	else {
>  		/* release the previous rdev and start from there. */
>  		rdev_dec_pending(rdev, mddev);


What comes after this is:

	list_for_each_entry_continue_rcu(rdev, &mddev->disks, same_set) {
		if (rdev->raid_disk >= 0 &&

Now the original code had:

  rdev = list_entry_rcu(&mddev->disks, struct md_rdev, same_set);

Where &mddev->disks would return the address of the disks field of
mddev which is a list head. Then it would get the 'same_set' offset,
which is 0, and rdev is pointing to a makeshift md_rdev struct. But it
isn't used, as the list_for_each_entry_continue_rcu() has:

#define list_for_each_entry_continue_rcu(pos, head, member) 		\
	for (pos = list_entry_rcu(pos->member.next, typeof(*pos), member); \
	     &pos->member != (head);	\
	     pos = list_entry_rcu(pos->member.next, typeof(*pos), member))

Thus the first use of pos is pos->member.next or:

  mddev->disks.next

But now you converted it to rdev = mddev->disks.next, which means the
first use is:

  pos = mddev->disks.next->next

I think you are skipping the first element here.

-- Steve

Re: [PATCH tip/core/rcu 4/4] netfilter: Fix list_entry_rcu usage

[Steven Rostedt]

On Tue, 12 May 2015 15:46:27 -0700
"Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote:

> From: Patrick Marlier <patrick.marlier@gmail.com>
> 
> Signed-off-by: Patrick Marlier <patrick.marlier@gmail.com>
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> ---
>  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 e6163017c42d..ad70195929a4 100644
> --- a/net/netfilter/core.c
> +++ b/net/netfilter/core.c
> @@ -166,7 +166,7 @@ int nf_hook_slow(struct sk_buff *skb, struct nf_hook_state *state)
>  	/* We may already have this, but read-locks nest anyway */
>  	rcu_read_lock();
>  
> -	elem = list_entry_rcu(&nf_hooks[state->pf][state->hook],
> +	elem = list_entry_rcu(nf_hooks[state->pf][state->hook].next,
>  			      struct nf_hook_ops, list);

Looks to be setting to the next element, not the current one.

-- Steve



>  next_hook:
>  	verdict = nf_iterate(&nf_hooks[state->pf][state->hook], skb, state,

Re: [PATCH tip/core/rcu 3/4] md/bitmap: Fix list_entry_rcu usage

[NeilBrown]

[-- Attachment #1: Type: text/plain, Size: 2934 bytes --]

On Tue, 12 May 2015 22:38:53 -0400 Steven Rostedt <rostedt@goodmis.org> wrote:

> On Tue, 12 May 2015 15:46:26 -0700
> "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote:
> 
> > From: Patrick Marlier <patrick.marlier@gmail.com>
> > 
> > Signed-off-by: Patrick Marlier <patrick.marlier@gmail.com>
> > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> > ---
> >  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 2bc56e2a3526..32901772e4ee 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);
> 
> Hmm, this changes the semantics.
> 
> The original code looks nasty, I first thought it was broken, but it
> seems to work out of sheer luck (or clever hack)

Definitely a clever hack - no question of "luck" here :-)

It might makes sense to change it to use list_for_each_entry_from_rcu()

  if (rdev == NULL)
     rdev = list_entry_rcu(mddev->disks.next, struct md_rdev, same_set);
  else {
     rdev_dec_pending(rdev, mddev);
     rdev = list_next_entry_rcu(rdev->same_set.next, struct md_rdev, same_set);
  }
  list_for_each_entry_from_rcu(rdev, ....)

but there isn't a "list_next_entry_rcu"....


Also, it would have been polity to at least 'cc' them Maintainer of this code
in the original patch - no?

Thanks,
NeilBrown

> 
> >  	else {
> >  		/* release the previous rdev and start from there. */
> >  		rdev_dec_pending(rdev, mddev);
> 
> 
> What comes after this is:
> 
> 	list_for_each_entry_continue_rcu(rdev, &mddev->disks, same_set) {
> 		if (rdev->raid_disk >= 0 &&
> 
> Now the original code had:
> 
>   rdev = list_entry_rcu(&mddev->disks, struct md_rdev, same_set);
> 
> Where &mddev->disks would return the address of the disks field of
> mddev which is a list head. Then it would get the 'same_set' offset,
> which is 0, and rdev is pointing to a makeshift md_rdev struct. But it
> isn't used, as the list_for_each_entry_continue_rcu() has:
> 
> #define list_for_each_entry_continue_rcu(pos, head, member) 		\
> 	for (pos = list_entry_rcu(pos->member.next, typeof(*pos), member); \
> 	     &pos->member != (head);	\
> 	     pos = list_entry_rcu(pos->member.next, typeof(*pos), member))
> 
> Thus the first use of pos is pos->member.next or:
> 
>   mddev->disks.next
> 
> But now you converted it to rdev = mddev->disks.next, which means the
> first use is:
> 
>   pos = mddev->disks.next->next
> 
> I think you are skipping the first element here.
> 
> -- Steve


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Re: [PATCH tip/core/rcu 3/4] md/bitmap: Fix list_entry_rcu usage

[Paul E. McKenney]

On Wed, May 13, 2015 at 12:58:39PM +1000, NeilBrown wrote:
> On Tue, 12 May 2015 22:38:53 -0400 Steven Rostedt <rostedt@goodmis.org> wrote:
> 
> > On Tue, 12 May 2015 15:46:26 -0700
> > "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote:
> > 
> > > From: Patrick Marlier <patrick.marlier@gmail.com>
> > > 
> > > Signed-off-by: Patrick Marlier <patrick.marlier@gmail.com>
> > > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> > > ---
> > >  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 2bc56e2a3526..32901772e4ee 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);
> > 
> > Hmm, this changes the semantics.
> > 
> > The original code looks nasty, I first thought it was broken, but it
> > seems to work out of sheer luck (or clever hack)
> 
> Definitely a clever hack - no question of "luck" here :-)
> 
> It might makes sense to change it to use list_for_each_entry_from_rcu()
> 
>   if (rdev == NULL)
>      rdev = list_entry_rcu(mddev->disks.next, struct md_rdev, same_set);
>   else {
>      rdev_dec_pending(rdev, mddev);
>      rdev = list_next_entry_rcu(rdev->same_set.next, struct md_rdev, same_set);
>   }
>   list_for_each_entry_from_rcu(rdev, ....)
> 
> but there isn't a "list_next_entry_rcu"....

Patrick, this one is for you.  As are all of the questions from Steven.

> Also, it would have been polity to at least 'cc' them Maintainer of this code
> in the original patch - no?

Rest assured that this set is not going to -tip without at least an
Acked-by from at least one of the maintainers.  In some cases, I will
interpret silence as assent, but this is not one of those cases.  ;-)

							Thanx, Paul

> Thanks,
> NeilBrown
> 
> > 
> > >  	else {
> > >  		/* release the previous rdev and start from there. */
> > >  		rdev_dec_pending(rdev, mddev);
> > 
> > 
> > What comes after this is:
> > 
> > 	list_for_each_entry_continue_rcu(rdev, &mddev->disks, same_set) {
> > 		if (rdev->raid_disk >= 0 &&
> > 
> > Now the original code had:
> > 
> >   rdev = list_entry_rcu(&mddev->disks, struct md_rdev, same_set);
> > 
> > Where &mddev->disks would return the address of the disks field of
> > mddev which is a list head. Then it would get the 'same_set' offset,
> > which is 0, and rdev is pointing to a makeshift md_rdev struct. But it
> > isn't used, as the list_for_each_entry_continue_rcu() has:
> > 
> > #define list_for_each_entry_continue_rcu(pos, head, member) 		\
> > 	for (pos = list_entry_rcu(pos->member.next, typeof(*pos), member); \
> > 	     &pos->member != (head);	\
> > 	     pos = list_entry_rcu(pos->member.next, typeof(*pos), member))
> > 
> > Thus the first use of pos is pos->member.next or:
> > 
> >   mddev->disks.next
> > 
> > But now you converted it to rdev = mddev->disks.next, which means the
> > first use is:
> > 
> >   pos = mddev->disks.next->next
> > 
> > I think you are skipping the first element here.
> > 
> > -- Steve
> 

Re: [PATCH tip/core/rcu 3/4] md/bitmap: Fix list_entry_rcu usage

[Patrick Marlier]

On 05/13/2015 04:58 AM, NeilBrown wrote:
> On Tue, 12 May 2015 22:38:53 -0400 Steven Rostedt <rostedt@goodmis.org> wrote:
>
>> On Tue, 12 May 2015 15:46:26 -0700
>> "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote:
>>
>>> From: Patrick Marlier <patrick.marlier@gmail.com>
>>>
>>> Signed-off-by: Patrick Marlier <patrick.marlier@gmail.com>
>>> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
>>> ---
>>>   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 2bc56e2a3526..32901772e4ee 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);
>>
>> Hmm, this changes the semantics.
>>
>> The original code looks nasty, I first thought it was broken, but it
>> seems to work out of sheer luck (or clever hack)
>
> Definitely a clever hack - no question of "luck" here :-)
>
> It might makes sense to change it to use list_for_each_entry_from_rcu()
>
>    if (rdev == NULL)
>       rdev = list_entry_rcu(mddev->disks.next, struct md_rdev, same_set);
>    else {
>       rdev_dec_pending(rdev, mddev);
>       rdev = list_next_entry_rcu(rdev->same_set.next, struct md_rdev, same_set);
>    }
>    list_for_each_entry_from_rcu(rdev, ....)
>
> but there isn't a "list_next_entry_rcu"....
>
>
> Also, it would have been polity to at least 'cc' them Maintainer of this code
> in the original patch - no?

Sure my bad. I hesitated to CC maintainers. I was almost sure that it 
will be rejected so I wanted to avoid noise.


>
> Thanks,
> NeilBrown
>
>>
>>>   	else {
>>>   		/* release the previous rdev and start from there. */
>>>   		rdev_dec_pending(rdev, mddev);
>>
>>
>> What comes after this is:
>>
>> 	list_for_each_entry_continue_rcu(rdev, &mddev->disks, same_set) {
>> 		if (rdev->raid_disk >= 0 &&
>>
>> Now the original code had:
>>
>>    rdev = list_entry_rcu(&mddev->disks, struct md_rdev, same_set);
>>
>> Where &mddev->disks would return the address of the disks field of
>> mddev which is a list head. Then it would get the 'same_set' offset,
>> which is 0, and rdev is pointing to a makeshift md_rdev struct. But it
>> isn't used, as the list_for_each_entry_continue_rcu() has:
>>
>> #define list_for_each_entry_continue_rcu(pos, head, member) 		\
>> 	for (pos = list_entry_rcu(pos->member.next, typeof(*pos), member); \
>> 	     &pos->member != (head);	\
>> 	     pos = list_entry_rcu(pos->member.next, typeof(*pos), member))
>>
>> Thus the first use of pos is pos->member.next or:
>>
>>    mddev->disks.next
>>
>> But now you converted it to rdev = mddev->disks.next, which means the
>> first use is:
>>
>>    pos = mddev->disks.next->next
>>
>> I think you are skipping the first element here.


struct mddev {
...
	struct list_head                disks;
...}

struct list_head {
         struct list_head *next, *prev;
};

The tricky thing is that "list_entry_rcu" before and after the patch is 
reading the same thing.

However in your case, the change I proposed is probably wrong I trust 
you on this side. :) What's your proposal to fix it with the rculist patch?

PS: In the rculist patch I proposed, I avoid the store and the atomic 
reload in the stack variable __ptr. (yeap, the 
rcu_dereference_raw/ACCESS_ONCE is a bit confusing because it implicitly 
do & on the parameter).

Thanks.
--
Pat

Re: [PATCH tip/core/rcu 3/4] md/bitmap: Fix list_entry_rcu usage

[NeilBrown]

[-- Attachment #1: Type: text/plain, Size: 4964 bytes --]

On Sat, 16 May 2015 19:42:54 +0200 Patrick Marlier
<patrick.marlier@gmail.com> wrote:

> 
> 
> On 05/13/2015 04:58 AM, NeilBrown wrote:
> > On Tue, 12 May 2015 22:38:53 -0400 Steven Rostedt <rostedt@goodmis.org> wrote:
> >
> >> On Tue, 12 May 2015 15:46:26 -0700
> >> "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote:
> >>
> >>> From: Patrick Marlier <patrick.marlier@gmail.com>
> >>>
> >>> Signed-off-by: Patrick Marlier <patrick.marlier@gmail.com>
> >>> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> >>> ---
> >>>   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 2bc56e2a3526..32901772e4ee 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);
> >>
> >> Hmm, this changes the semantics.
> >>
> >> The original code looks nasty, I first thought it was broken, but it
> >> seems to work out of sheer luck (or clever hack)
> >
> > Definitely a clever hack - no question of "luck" here :-)
> >
> > It might makes sense to change it to use list_for_each_entry_from_rcu()
> >
> >    if (rdev == NULL)
> >       rdev = list_entry_rcu(mddev->disks.next, struct md_rdev, same_set);
> >    else {
> >       rdev_dec_pending(rdev, mddev);
> >       rdev = list_next_entry_rcu(rdev->same_set.next, struct md_rdev, same_set);
> >    }
> >    list_for_each_entry_from_rcu(rdev, ....)
> >
> > but there isn't a "list_next_entry_rcu"....
> >
> >
> > Also, it would have been polity to at least 'cc' them Maintainer of this code
> > in the original patch - no?
> 
> Sure my bad. I hesitated to CC maintainers. I was almost sure that it 
> will be rejected so I wanted to avoid noise.

Well... If the subject has contained the magic string "RFC" I might have been
less concerned.
But there have been enough times that people have changed md without telling
me, and thereby broken it, that I'd much rather  see the patch than not.


> 
> 
> >
> > Thanks,
> > NeilBrown
> >
> >>
> >>>   	else {
> >>>   		/* release the previous rdev and start from there. */
> >>>   		rdev_dec_pending(rdev, mddev);
> >>
> >>
> >> What comes after this is:
> >>
> >> 	list_for_each_entry_continue_rcu(rdev, &mddev->disks, same_set) {
> >> 		if (rdev->raid_disk >= 0 &&
> >>
> >> Now the original code had:
> >>
> >>    rdev = list_entry_rcu(&mddev->disks, struct md_rdev, same_set);
> >>
> >> Where &mddev->disks would return the address of the disks field of
> >> mddev which is a list head. Then it would get the 'same_set' offset,
> >> which is 0, and rdev is pointing to a makeshift md_rdev struct. But it
> >> isn't used, as the list_for_each_entry_continue_rcu() has:
> >>
> >> #define list_for_each_entry_continue_rcu(pos, head, member) 		\
> >> 	for (pos = list_entry_rcu(pos->member.next, typeof(*pos), member); \
> >> 	     &pos->member != (head);	\
> >> 	     pos = list_entry_rcu(pos->member.next, typeof(*pos), member))
> >>
> >> Thus the first use of pos is pos->member.next or:
> >>
> >>    mddev->disks.next
> >>
> >> But now you converted it to rdev = mddev->disks.next, which means the
> >> first use is:
> >>
> >>    pos = mddev->disks.next->next
> >>
> >> I think you are skipping the first element here.
> 
> 
> struct mddev {
> ...
> 	struct list_head                disks;
> ...}
> 
> struct list_head {
>          struct list_head *next, *prev;
> };
> 
> The tricky thing is that "list_entry_rcu" before and after the patch is 
> reading the same thing.

No it isn't.
Before the patch it is passed the address of the 'next' field.  After the
patch it is passed the contents of the 'next' field.


> 
> However in your case, the change I proposed is probably wrong I trust 
> you on this side. :) What's your proposal to fix it with the rculist patch?

What needs fixing?  I don't see anything broken.

Maybe there is something in this "rculist patch" that I'm missing.  Can you
point me at it?

Thanks,
NeilBrown


> 
> PS: In the rculist patch I proposed, I avoid the store and the atomic 
> reload in the stack variable __ptr. (yeap, the 
> rcu_dereference_raw/ACCESS_ONCE is a bit confusing because it implicitly 
> do & on the parameter).
> 
> Thanks.
> --
> Pat
> --
> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
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> More majordomo info at  http://vger.kernel.org/majordomo-info.html
> Please read the FAQ at  http://www.tux.org/lkml/


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Re: [PATCH tip/core/rcu 3/4] md/bitmap: Fix list_entry_rcu usage

[Steven Rostedt]

On Mon, 18 May 2015 12:06:47 +1000
NeilBrown <neilb@suse.de> wrote:


> > struct mddev {
> > ...
> > 	struct list_head                disks;
> > ...}
> > 
> > struct list_head {
> >          struct list_head *next, *prev;
> > };
> > 
> > The tricky thing is that "list_entry_rcu" before and after the patch is 
> > reading the same thing.
> 
> No it isn't.
> Before the patch it is passed the address of the 'next' field.  After the
> patch it is passed the contents of the 'next' field.

Right.

> 
> 
> > 
> > However in your case, the change I proposed is probably wrong I trust 
> > you on this side. :) What's your proposal to fix it with the rculist patch?
> 
> What needs fixing?  I don't see anything broken.
> 
> Maybe there is something in this "rculist patch" that I'm missing.  Can you
> point me at it?
> 

Probably some debugging tool like sparse notices that the assignment
isn't a true list entry and complains about it. In other words, I think
the real fix is to fix the debugging tool to ignore this, because the
code is correct, and this is a false positive failure, and is causing
more harm than good, because people are sending out broken patches due
to it.

-- Steve

Re: [PATCH tip/core/rcu 3/4] md/bitmap: Fix list_entry_rcu usage

[Patrick Marlier]

On Mon, May 18, 2015 at 4:06 AM, NeilBrown <neilb@suse.de> wrote:
> On Sat, 16 May 2015 19:42:54 +0200 Patrick Marlier
> <patrick.marlier@gmail.com> wrote:
>> On 05/13/2015 04:58 AM, NeilBrown wrote:
>> > On Tue, 12 May 2015 22:38:53 -0400 Steven Rostedt <rostedt@goodmis.org> wrote:
>> >> On Tue, 12 May 2015 15:46:26 -0700
>> >> "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote:
>> >> What comes after this is:
>> >>
>> >>    list_for_each_entry_continue_rcu(rdev, &mddev->disks, same_set) {
>> >>            if (rdev->raid_disk >= 0 &&
>> >>
>> >> Now the original code had:
>> >>
>> >>    rdev = list_entry_rcu(&mddev->disks, struct md_rdev, same_set);
>> >>
>> >> Where &mddev->disks would return the address of the disks field of
>> >> mddev which is a list head. Then it would get the 'same_set' offset,
>> >> which is 0, and rdev is pointing to a makeshift md_rdev struct. But it
>> >> isn't used, as the list_for_each_entry_continue_rcu() has:
>> >>
>> >> #define list_for_each_entry_continue_rcu(pos, head, member)                \
>> >>    for (pos = list_entry_rcu(pos->member.next, typeof(*pos), member); \
>> >>         &pos->member != (head);    \
>> >>         pos = list_entry_rcu(pos->member.next, typeof(*pos), member))
>> >>
>> >> Thus the first use of pos is pos->member.next or:
>> >>
>> >>    mddev->disks.next
>> >>
>> >> But now you converted it to rdev = mddev->disks.next, which means the
>> >> first use is:
>> >>
>> >>    pos = mddev->disks.next->next
>> >>
>> >> I think you are skipping the first element here.
>>
>>
>> struct mddev {
>> ...
>>       struct list_head                disks;
>> ...}
>>
>> struct list_head {
>>          struct list_head *next, *prev;
>> };
>>
>> The tricky thing is that "list_entry_rcu" before and after the patch is
>> reading the same thing.
>
> No it isn't.
> Before the patch it is passed the address of the 'next' field.  After the
> patch it is passed the contents of the 'next' field.

Here I meant "list_entry_rcu" (in include/linux/rculist.h) not the
change to drivers/md/bitmap.c.


>> However in your case, the change I proposed is probably wrong I trust
>> you on this side. :) What's your proposal to fix it with the rculist patch?
>
> What needs fixing?  I don't see anything broken.
>
> Maybe there is something in this "rculist patch" that I'm missing.  Can you
> point me at it?

Do not apply the patch on drivers/md/bitmap.c but only on
include/linux/rculist.h and you will see that the compilation fails.

--
Pat

Re: [PATCH tip/core/rcu 3/4] md/bitmap: Fix list_entry_rcu usage

[Patrick Marlier]

On 05/18/2015 03:53 PM, Patrick Marlier wrote:
> On Mon, May 18, 2015 at 4:06 AM, NeilBrown <neilb@suse.de> wrote:
>> On Sat, 16 May 2015 19:42:54 +0200 Patrick Marlier
>> <patrick.marlier@gmail.com> wrote:
>>> On 05/13/2015 04:58 AM, NeilBrown wrote:
>>>> On Tue, 12 May 2015 22:38:53 -0400 Steven Rostedt <rostedt@goodmis.org> wrote:
>>>>> On Tue, 12 May 2015 15:46:26 -0700
>>>>> "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote:
>>>>> What comes after this is:
>>>>>
>>>>>     list_for_each_entry_continue_rcu(rdev, &mddev->disks, same_set) {
>>>>>             if (rdev->raid_disk >= 0 &&
>>>>>
>>>>> Now the original code had:
>>>>>
>>>>>     rdev = list_entry_rcu(&mddev->disks, struct md_rdev, same_set);
>>>>>
>>>>> Where &mddev->disks would return the address of the disks field of
>>>>> mddev which is a list head. Then it would get the 'same_set' offset,
>>>>> which is 0, and rdev is pointing to a makeshift md_rdev struct. But it
>>>>> isn't used, as the list_for_each_entry_continue_rcu() has:
>>>>>
>>>>> #define list_for_each_entry_continue_rcu(pos, head, member)                \
>>>>>     for (pos = list_entry_rcu(pos->member.next, typeof(*pos), member); \
>>>>>          &pos->member != (head);    \
>>>>>          pos = list_entry_rcu(pos->member.next, typeof(*pos), member))
>>>>>
>>>>> Thus the first use of pos is pos->member.next or:
>>>>>
>>>>>     mddev->disks.next
>>>>>
>>>>> But now you converted it to rdev = mddev->disks.next, which means the
>>>>> first use is:
>>>>>
>>>>>     pos = mddev->disks.next->next
>>>>>
>>>>> I think you are skipping the first element here.
>>>
>>>
>>> struct mddev {
>>> ...
>>>        struct list_head                disks;
>>> ...}
>>>
>>> struct list_head {
>>>           struct list_head *next, *prev;
>>> };
>>>
>>> The tricky thing is that "list_entry_rcu" before and after the patch is
>>> reading the same thing.
>>
>> No it isn't.
>> Before the patch it is passed the address of the 'next' field.  After the
>> patch it is passed the contents of the 'next' field.
>
> Here I meant "list_entry_rcu" (in include/linux/rculist.h) not the
> change to drivers/md/bitmap.c.
>
>
>>> However in your case, the change I proposed is probably wrong I trust
>>> you on this side. :) What's your proposal to fix it with the rculist patch?
>>
>> What needs fixing?  I don't see anything broken.
>>
>> Maybe there is something in this "rculist patch" that I'm missing.  Can you
>> point me at it?
>
> Do not apply the patch on drivers/md/bitmap.c but only on
> include/linux/rculist.h and you will see that the compilation fails.

Here an example of the compilation error in drivers/md/bitmap.c

In file included from include/linux/sched.h:17:0,
                  from include/linux/blkdev.h:4,
                  from drivers/md/bitmap.c:18:
drivers/md/bitmap.c: In function ‘next_active_rdev’:
include/linux/compiler.h:469:24: error: lvalue required as unary ‘&’ operand
   (volatile typeof(x) *)&(x); })
                         ^
include/linux/kernel.h:800:49: note: in definition of macro ‘container_of’
   const typeof( ((type *)0)->member ) *__mptr = (ptr); \
                                                  ^
include/linux/compiler.h:470:26: note: in expansion of macro ‘__ACCESS_ONCE’
  #define ACCESS_ONCE(x) (*__ACCESS_ONCE(x))
                           ^
include/linux/rcupdate.h:630:26: note: in expansion of macro ‘ACCESS_ONCE’
   typeof(p) _________p1 = ACCESS_ONCE(p); \
                           ^
include/linux/rcupdate.h:587:48: note: in expansion of macro 
‘lockless_dereference’
   typeof(*p) *________p1 = (typeof(*p) *__force)lockless_dereference(p); \
                                                 ^
include/linux/rcupdate.h:723:2: note: in expansion of macro 
‘__rcu_dereference_check’
   __rcu_dereference_check((p), rcu_read_lock_held() || (c), __rcu)
   ^
include/linux/rcupdate.h:746:32: note: in expansion of macro 
‘rcu_dereference_check’
  #define rcu_dereference_raw(p) rcu_dereference_check(p, 1) /*@@@ 
needed? @@@*/
                                 ^
include/linux/rculist.h:251:28: note: in expansion of macro 
‘rcu_dereference_raw’
   container_of((typeof(ptr))rcu_dereference_raw(ptr), type, member); \
                             ^
drivers/md/bitmap.c:184:10: note: in expansion of macro ‘list_entry_rcu’
    rdev = list_entry_rcu(&mddev->disks, struct md_rdev, same_set);
           ^

Thanks.
--
Pat

Re: [PATCH tip/core/rcu 3/4] md/bitmap: Fix list_entry_rcu usage

[Paul E. McKenney]

On Mon, May 18, 2015 at 09:43:21AM -0400, Steven Rostedt wrote:
> On Mon, 18 May 2015 12:06:47 +1000
> NeilBrown <neilb@suse.de> wrote:
> 
> 
> > > struct mddev {
> > > ...
> > > 	struct list_head                disks;
> > > ...}
> > > 
> > > struct list_head {
> > >          struct list_head *next, *prev;
> > > };
> > > 
> > > The tricky thing is that "list_entry_rcu" before and after the patch is 
> > > reading the same thing.
> > 
> > No it isn't.
> > Before the patch it is passed the address of the 'next' field.  After the
> > patch it is passed the contents of the 'next' field.
> 
> Right.
> 
> > 
> > 
> > > 
> > > However in your case, the change I proposed is probably wrong I trust 
> > > you on this side. :) What's your proposal to fix it with the rculist patch?
> > 
> > What needs fixing?  I don't see anything broken.
> > 
> > Maybe there is something in this "rculist patch" that I'm missing.  Can you
> > point me at it?
> > 
> 
> Probably some debugging tool like sparse notices that the assignment
> isn't a true list entry and complains about it. In other words, I think
> the real fix is to fix the debugging tool to ignore this, because the
> code is correct, and this is a false positive failure, and is causing
> more harm than good, because people are sending out broken patches due
> to it.

OK, finally did the history trawling that I should have done to begin with.

Back in 2010, Arnd added the __rcu pointer checking in sparse.
But the RCU list primitives were used on non-RCU-protected lists, so
some casting pain was required to avoid sparse complaints.  (Keep in
mind that the list_head structure does not mark ->next with __rcu.)
Arnd's workaround was to copy the pointer to the local stack, casting
it to an __rcu pointer, then use rcu_dereference_raw() to do the needed
traversal of an RCU-protected pointer.

This of course resulted in an extraneous load from the stack, which
Patrick noticed in his performance work, and which motivated him to send
the patches.

Perhaps what I should do is create an rcu_dereference_nocheck() for use
in list traversals, that omits the sparse checking.  That should get rid
of both the sparse warnings and the strange casts.

The code in md probably needs to change in any case, as otherwise we are
invoking rcu_dereference_whatever() on a full struct list_head rather
than on a single pointer.  Or am I missing something here?

							Thanx, Paul

Re: [PATCH tip/core/rcu 3/4] md/bitmap: Fix list_entry_rcu usage

[NeilBrown]

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On Tue, 19 May 2015 15:07:25 -0700 "Paul E. McKenney"
<paulmck@linux.vnet.ibm.com> wrote:


> The code in md probably needs to change in any case, as otherwise we are
> invoking rcu_dereference_whatever() on a full struct list_head rather
> than on a single pointer.  Or am I missing something here?

I think it would be
   rcu_dereference_whatever(&mddev->disks)

I don't know what you mean by "on a full struct list_head", but there is
nothing actually being dereferenced here - right?  Just pointer arithmetic on
'mddev'.

I should probably just

diff --git a/drivers/md/bitmap.c b/drivers/md/bitmap.c
index 2bc56e2a3526..b1d237bf8b3b 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(&mddev->disks, struct md_rdev, same_set);
 	else {
 		/* release the previous rdev and start from there. */
 		rdev_dec_pending(rdev, mddev);

as there really are no RCU issues with getting that address.  Maybe I should
move it outside the rcu_read_lock() just to be blatant.... but that would
make the code a lot more clumsy as the rdev_dec_pending must be inside the
rcu_read_lock..

So this.

Thanks,
NeilBrown

From: NeilBrown <neilb@suse.de>
Date: Wed, 20 May 2015 15:05:09 +1000
Subject: [PATCH] md/bitmap: remove rcu annotation from pointer arithmetic.

Evaluating  "&mddev->disks" is simple pointer arithmetic, so
it does not need 'rcu' annotations - no dereferencing is happening.

Also enhance the comment to explain that 'rdev' in that case
is not actually a pointer to an rdev.

Reported-by: Patrick Marlier <patrick.marlier@gmail.com>
Signed-off-by: NeilBrown <neilb@suse.de>

diff --git a/drivers/md/bitmap.c b/drivers/md/bitmap.c
index 2bc56e2a3526..135a0907e9de 100644
--- a/drivers/md/bitmap.c
+++ b/drivers/md/bitmap.c
@@ -177,11 +177,16 @@ static struct md_rdev *next_active_rdev(struct md_rdev *rdev, struct mddev *mdde
 	 * nr_pending is 0 and In_sync is clear, the entries we return will
 	 * still be in the same position on the list when we re-enter
 	 * list_for_each_entry_continue_rcu.
+	 *
+	 * Note that if entered with 'rdev == NULL' to start at the
+	 * beginning, we temporarily assign 'rdev' to an address which
+	 * isn't really an rdev, but which can be used by
+	 * list_for_each_entry_continue_rcu() to find the first entry.
 	 */
 	rcu_read_lock();
 	if (rdev == NULL)
 		/* start at the beginning */
-		rdev = list_entry_rcu(&mddev->disks, struct md_rdev, same_set);
+		rdev = list_entry(&mddev->disks, struct md_rdev, same_set);
 	else {
 		/* release the previous rdev and start from there. */
 		rdev_dec_pending(rdev, mddev);

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Re: [PATCH tip/core/rcu 3/4] md/bitmap: Fix list_entry_rcu usage

[Paul E. McKenney]

On Wed, May 20, 2015 at 03:09:19PM +1000, NeilBrown wrote:
> On Tue, 19 May 2015 15:07:25 -0700 "Paul E. McKenney"
> <paulmck@linux.vnet.ibm.com> wrote:
> 
> 
> > The code in md probably needs to change in any case, as otherwise we are
> > invoking rcu_dereference_whatever() on a full struct list_head rather
> > than on a single pointer.  Or am I missing something here?
> 
> I think it would be
>    rcu_dereference_whatever(&mddev->disks)
> 
> I don't know what you mean by "on a full struct list_head", but there is
> nothing actually being dereferenced here - right?  Just pointer arithmetic on
> 'mddev'.

It really does dereference.  Strange but true.

> I should probably just
> 
> diff --git a/drivers/md/bitmap.c b/drivers/md/bitmap.c
> index 2bc56e2a3526..b1d237bf8b3b 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(&mddev->disks, struct md_rdev, same_set);
>  	else {
>  		/* release the previous rdev and start from there. */
>  		rdev_dec_pending(rdev, mddev);
> 
> as there really are no RCU issues with getting that address.  Maybe I should
> move it outside the rcu_read_lock() just to be blatant.... but that would
> make the code a lot more clumsy as the rdev_dec_pending must be inside the
> rcu_read_lock..
> 
> So this.

Fair enough -- if you aren't using RCU, there is really no point in using
the RCU API.  I will drop this patch from my tree.  You are pushing yours,
I am guessing?

							Thanx, Paul

> Thanks,
> NeilBrown
> 
> From: NeilBrown <neilb@suse.de>
> Date: Wed, 20 May 2015 15:05:09 +1000
> Subject: [PATCH] md/bitmap: remove rcu annotation from pointer arithmetic.
> 
> Evaluating  "&mddev->disks" is simple pointer arithmetic, so
> it does not need 'rcu' annotations - no dereferencing is happening.
> 
> Also enhance the comment to explain that 'rdev' in that case
> is not actually a pointer to an rdev.
> 
> Reported-by: Patrick Marlier <patrick.marlier@gmail.com>
> Signed-off-by: NeilBrown <neilb@suse.de>
> 
> diff --git a/drivers/md/bitmap.c b/drivers/md/bitmap.c
> index 2bc56e2a3526..135a0907e9de 100644
> --- a/drivers/md/bitmap.c
> +++ b/drivers/md/bitmap.c
> @@ -177,11 +177,16 @@ static struct md_rdev *next_active_rdev(struct md_rdev *rdev, struct mddev *mdde
>  	 * nr_pending is 0 and In_sync is clear, the entries we return will
>  	 * still be in the same position on the list when we re-enter
>  	 * list_for_each_entry_continue_rcu.
> +	 *
> +	 * Note that if entered with 'rdev == NULL' to start at the
> +	 * beginning, we temporarily assign 'rdev' to an address which
> +	 * isn't really an rdev, but which can be used by
> +	 * list_for_each_entry_continue_rcu() to find the first entry.
>  	 */
>  	rcu_read_lock();
>  	if (rdev == NULL)
>  		/* start at the beginning */
> -		rdev = list_entry_rcu(&mddev->disks, struct md_rdev, same_set);
> +		rdev = list_entry(&mddev->disks, struct md_rdev, same_set);
>  	else {
>  		/* release the previous rdev and start from there. */
>  		rdev_dec_pending(rdev, mddev);

Re: [PATCH tip/core/rcu 3/4] md/bitmap: Fix list_entry_rcu usage

[NeilBrown]

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On Wed, 20 May 2015 06:28:35 -0700 "Paul E. McKenney"
<paulmck@linux.vnet.ibm.com> wrote:

> On Wed, May 20, 2015 at 03:09:19PM +1000, NeilBrown wrote:
> > On Tue, 19 May 2015 15:07:25 -0700 "Paul E. McKenney"
> > <paulmck@linux.vnet.ibm.com> wrote:
> > 
> > 
> > > The code in md probably needs to change in any case, as otherwise we are
> > > invoking rcu_dereference_whatever() on a full struct list_head rather
> > > than on a single pointer.  Or am I missing something here?
> > 
> > I think it would be
> >    rcu_dereference_whatever(&mddev->disks)
> > 
> > I don't know what you mean by "on a full struct list_head", but there is
> > nothing actually being dereferenced here - right?  Just pointer arithmetic on
> > 'mddev'.
> 
> It really does dereference.  Strange but true.

Well... your the expert.  But without an lvalue, I can't see it.


> 
> > I should probably just
> > 
> > diff --git a/drivers/md/bitmap.c b/drivers/md/bitmap.c
> > index 2bc56e2a3526..b1d237bf8b3b 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(&mddev->disks, struct md_rdev, same_set);
> >  	else {
> >  		/* release the previous rdev and start from there. */
> >  		rdev_dec_pending(rdev, mddev);
> > 
> > as there really are no RCU issues with getting that address.  Maybe I should
> > move it outside the rcu_read_lock() just to be blatant.... but that would
> > make the code a lot more clumsy as the rdev_dec_pending must be inside the
> > rcu_read_lock..
> > 
> > So this.
> 
> Fair enough -- if you aren't using RCU, there is really no point in using
> the RCU API.  I will drop this patch from my tree.  You are pushing yours,
> I am guessing?

Excellent guess :-)
Hopefully for the next -rc.

Thanks,
NeilBrown


> 
> 							Thanx, Paul
> 
> > Thanks,
> > NeilBrown
> > 
> > From: NeilBrown <neilb@suse.de>
> > Date: Wed, 20 May 2015 15:05:09 +1000
> > Subject: [PATCH] md/bitmap: remove rcu annotation from pointer arithmetic.
> > 
> > Evaluating  "&mddev->disks" is simple pointer arithmetic, so
> > it does not need 'rcu' annotations - no dereferencing is happening.
> > 
> > Also enhance the comment to explain that 'rdev' in that case
> > is not actually a pointer to an rdev.
> > 
> > Reported-by: Patrick Marlier <patrick.marlier@gmail.com>
> > Signed-off-by: NeilBrown <neilb@suse.de>
> > 
> > diff --git a/drivers/md/bitmap.c b/drivers/md/bitmap.c
> > index 2bc56e2a3526..135a0907e9de 100644
> > --- a/drivers/md/bitmap.c
> > +++ b/drivers/md/bitmap.c
> > @@ -177,11 +177,16 @@ static struct md_rdev *next_active_rdev(struct md_rdev *rdev, struct mddev *mdde
> >  	 * nr_pending is 0 and In_sync is clear, the entries we return will
> >  	 * still be in the same position on the list when we re-enter
> >  	 * list_for_each_entry_continue_rcu.
> > +	 *
> > +	 * Note that if entered with 'rdev == NULL' to start at the
> > +	 * beginning, we temporarily assign 'rdev' to an address which
> > +	 * isn't really an rdev, but which can be used by
> > +	 * list_for_each_entry_continue_rcu() to find the first entry.
> >  	 */
> >  	rcu_read_lock();
> >  	if (rdev == NULL)
> >  		/* start at the beginning */
> > -		rdev = list_entry_rcu(&mddev->disks, struct md_rdev, same_set);
> > +		rdev = list_entry(&mddev->disks, struct md_rdev, same_set);
> >  	else {
> >  		/* release the previous rdev and start from there. */
> >  		rdev_dec_pending(rdev, mddev);
> 


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Re: [PATCH tip/core/rcu 3/4] md/bitmap: Fix list_entry_rcu usage

[Paul E. McKenney]

On Tue, May 19, 2015 at 03:07:25PM -0700, Paul E. McKenney wrote:
> On Mon, May 18, 2015 at 09:43:21AM -0400, Steven Rostedt wrote:
> > On Mon, 18 May 2015 12:06:47 +1000
> > NeilBrown <neilb@suse.de> wrote:
> > 
> > 
> > > > struct mddev {
> > > > ...
> > > > 	struct list_head                disks;
> > > > ...}
> > > > 
> > > > struct list_head {
> > > >          struct list_head *next, *prev;
> > > > };
> > > > 
> > > > The tricky thing is that "list_entry_rcu" before and after the patch is 
> > > > reading the same thing.
> > > 
> > > No it isn't.
> > > Before the patch it is passed the address of the 'next' field.  After the
> > > patch it is passed the contents of the 'next' field.
> > 
> > Right.
> > 
> > > 
> > > 
> > > > 
> > > > However in your case, the change I proposed is probably wrong I trust 
> > > > you on this side. :) What's your proposal to fix it with the rculist patch?
> > > 
> > > What needs fixing?  I don't see anything broken.
> > > 
> > > Maybe there is something in this "rculist patch" that I'm missing.  Can you
> > > point me at it?
> > > 
> > 
> > Probably some debugging tool like sparse notices that the assignment
> > isn't a true list entry and complains about it. In other words, I think
> > the real fix is to fix the debugging tool to ignore this, because the
> > code is correct, and this is a false positive failure, and is causing
> > more harm than good, because people are sending out broken patches due
> > to it.
> 
> OK, finally did the history trawling that I should have done to begin with.
> 
> Back in 2010, Arnd added the __rcu pointer checking in sparse.
> But the RCU list primitives were used on non-RCU-protected lists, so
> some casting pain was required to avoid sparse complaints.  (Keep in
> mind that the list_head structure does not mark ->next with __rcu.)
> Arnd's workaround was to copy the pointer to the local stack, casting
> it to an __rcu pointer, then use rcu_dereference_raw() to do the needed
> traversal of an RCU-protected pointer.
> 
> This of course resulted in an extraneous load from the stack, which
> Patrick noticed in his performance work, and which motivated him to send
> the patches.
> 
> Perhaps what I should do is create an rcu_dereference_nocheck() for use
> in list traversals, that omits the sparse checking.  That should get rid
> of both the sparse warnings and the strange casts.
> 
> The code in md probably needs to change in any case, as otherwise we are
> invoking rcu_dereference_whatever() on a full struct list_head rather
> than on a single pointer.  Or am I missing something here?

Finally getting back to this one...

I switched to lockless_dereference() instead of rcu_dereference_raw(),
and am running it through the testing gamut.  Patrick, are you OK with
this change?

							Thanx, Paul

------------------------------------------------------------------------

    rculist: Make list_entry_rcu() use lockless_dereference()
    
    The current list_entry_rcu() implementation copies the pointer to a stack
    variable, then invokes rcu_dereference_raw() on it.  This results in an
    additional store-load pair.  Now, most compilers will emit normal store
    and load instructions, which might seem to be of negligible overhead,
    but this results in a load-hit-store situation that can cause surprisingly
    long pipeline stalls, even on modern microprocessors.  The problem is
    that it takes time for the store to get the store buffer updated, which
    can delay the subsequent load, which immediately follows.
    
    This commit therefore switches to the lockless_dereference() primitive,
    which does not expect the __rcu annotations (that are anyway not present
    in the list_head structure) and which, like rcu_dereference_raw(),
    does not check for an enclosing RCU read-side critical section.
    Most importantly, it does not copy the pointer, thus avoiding the
    load-hit-store overhead.
    
    Signed-off-by: Patrick Marlier <patrick.marlier@gmail.com>
    [ paulmck: Switched to lockless_dereference() to suppress sparse warnings. ]
    Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

diff --git a/include/linux/rculist.h b/include/linux/rculist.h
index 17c6b1f84a77..5ed540986019 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(lockless_dereference(ptr), type, member)
 
 /**
  * Where are list_empty_rcu() and list_first_entry_rcu()?

Re: [PATCH tip/core/rcu 3/4] md/bitmap: Fix list_entry_rcu usage

[Patrick Marlier]

On 09/12/2015 01:05 AM, Paul E. McKenney wrote:
> On Tue, May 19, 2015 at 03:07:25PM -0700, Paul E. McKenney wrote:
>> On Mon, May 18, 2015 at 09:43:21AM -0400, Steven Rostedt wrote:
>>> On Mon, 18 May 2015 12:06:47 +1000
>>> NeilBrown <neilb@suse.de> wrote:
>>>
>>>
>>>>> struct mddev {
>>>>> ...
>>>>> 	struct list_head                disks;
>>>>> ...}
>>>>>
>>>>> struct list_head {
>>>>>           struct list_head *next, *prev;
>>>>> };
>>>>>
>>>>> The tricky thing is that "list_entry_rcu" before and after the patch is
>>>>> reading the same thing.
>>>>
>>>> No it isn't.
>>>> Before the patch it is passed the address of the 'next' field.  After the
>>>> patch it is passed the contents of the 'next' field.
>>>
>>> Right.
>>>
>>>>
>>>>
>>>>>
>>>>> However in your case, the change I proposed is probably wrong I trust
>>>>> you on this side. :) What's your proposal to fix it with the rculist patch?
>>>>
>>>> What needs fixing?  I don't see anything broken.
>>>>
>>>> Maybe there is something in this "rculist patch" that I'm missing.  Can you
>>>> point me at it?
>>>>
>>>
>>> Probably some debugging tool like sparse notices that the assignment
>>> isn't a true list entry and complains about it. In other words, I think
>>> the real fix is to fix the debugging tool to ignore this, because the
>>> code is correct, and this is a false positive failure, and is causing
>>> more harm than good, because people are sending out broken patches due
>>> to it.
>>
>> OK, finally did the history trawling that I should have done to begin with.
>>
>> Back in 2010, Arnd added the __rcu pointer checking in sparse.
>> But the RCU list primitives were used on non-RCU-protected lists, so
>> some casting pain was required to avoid sparse complaints.  (Keep in
>> mind that the list_head structure does not mark ->next with __rcu.)
>> Arnd's workaround was to copy the pointer to the local stack, casting
>> it to an __rcu pointer, then use rcu_dereference_raw() to do the needed
>> traversal of an RCU-protected pointer.
>>
>> This of course resulted in an extraneous load from the stack, which
>> Patrick noticed in his performance work, and which motivated him to send
>> the patches.
>>
>> Perhaps what I should do is create an rcu_dereference_nocheck() for use
>> in list traversals, that omits the sparse checking.  That should get rid
>> of both the sparse warnings and the strange casts.
>>
>> The code in md probably needs to change in any case, as otherwise we are
>> invoking rcu_dereference_whatever() on a full struct list_head rather
>> than on a single pointer.  Or am I missing something here?
>
> Finally getting back to this one...
>
> I switched to lockless_dereference() instead of rcu_dereference_raw(),
> and am running it through the testing gamut.  Patrick, are you OK with
> this change?

Paul,

This sounds good to me. It should fix the performance issue (will check 
with my benchmark).
I think for drivers/md/bitmap.c:next_active_rdev() the problem was fixed 
but do you know if it also fixed for net/netfilter/core.c:nf_hook_slow()?

Thanks.
--
Pat

Re: [PATCH tip/core/rcu 3/4] md/bitmap: Fix list_entry_rcu usage

[Paul E. McKenney]

On Sun, Sep 13, 2015 at 12:06:33PM +0200, Patrick Marlier wrote:
> 
> On 09/12/2015 01:05 AM, Paul E. McKenney wrote:
> >On Tue, May 19, 2015 at 03:07:25PM -0700, Paul E. McKenney wrote:
> >>On Mon, May 18, 2015 at 09:43:21AM -0400, Steven Rostedt wrote:
> >>>On Mon, 18 May 2015 12:06:47 +1000
> >>>NeilBrown <neilb@suse.de> wrote:
> >>>
> >>>
> >>>>>struct mddev {
> >>>>>...
> >>>>>	struct list_head                disks;
> >>>>>...}
> >>>>>
> >>>>>struct list_head {
> >>>>>          struct list_head *next, *prev;
> >>>>>};
> >>>>>
> >>>>>The tricky thing is that "list_entry_rcu" before and after the patch is
> >>>>>reading the same thing.
> >>>>
> >>>>No it isn't.
> >>>>Before the patch it is passed the address of the 'next' field.  After the
> >>>>patch it is passed the contents of the 'next' field.
> >>>
> >>>Right.
> >>>
> >>>>
> >>>>
> >>>>>
> >>>>>However in your case, the change I proposed is probably wrong I trust
> >>>>>you on this side. :) What's your proposal to fix it with the rculist patch?
> >>>>
> >>>>What needs fixing?  I don't see anything broken.
> >>>>
> >>>>Maybe there is something in this "rculist patch" that I'm missing.  Can you
> >>>>point me at it?
> >>>>
> >>>
> >>>Probably some debugging tool like sparse notices that the assignment
> >>>isn't a true list entry and complains about it. In other words, I think
> >>>the real fix is to fix the debugging tool to ignore this, because the
> >>>code is correct, and this is a false positive failure, and is causing
> >>>more harm than good, because people are sending out broken patches due
> >>>to it.
> >>
> >>OK, finally did the history trawling that I should have done to begin with.
> >>
> >>Back in 2010, Arnd added the __rcu pointer checking in sparse.
> >>But the RCU list primitives were used on non-RCU-protected lists, so
> >>some casting pain was required to avoid sparse complaints.  (Keep in
> >>mind that the list_head structure does not mark ->next with __rcu.)
> >>Arnd's workaround was to copy the pointer to the local stack, casting
> >>it to an __rcu pointer, then use rcu_dereference_raw() to do the needed
> >>traversal of an RCU-protected pointer.
> >>
> >>This of course resulted in an extraneous load from the stack, which
> >>Patrick noticed in his performance work, and which motivated him to send
> >>the patches.
> >>
> >>Perhaps what I should do is create an rcu_dereference_nocheck() for use
> >>in list traversals, that omits the sparse checking.  That should get rid
> >>of both the sparse warnings and the strange casts.
> >>
> >>The code in md probably needs to change in any case, as otherwise we are
> >>invoking rcu_dereference_whatever() on a full struct list_head rather
> >>than on a single pointer.  Or am I missing something here?
> >
> >Finally getting back to this one...
> >
> >I switched to lockless_dereference() instead of rcu_dereference_raw(),
> >and am running it through the testing gamut.  Patrick, are you OK with
> >this change?
> 
> Paul,
> 
> This sounds good to me. It should fix the performance issue (will
> check with my benchmark).

Thank you, looking forward to seeing the results!

> I think for drivers/md/bitmap.c:next_active_rdev() the problem was
> fixed but do you know if it also fixed for
> net/netfilter/core.c:nf_hook_slow()?

It does appear so.  The statement now reads:

	elem = list_entry_rcu(state->hook_list, struct nf_hook_ops, list);

And ->hook_list is defined as follows:

	struct list_head *hook_list;

							Thanx, Paul

Re: [PATCH tip/core/rcu 3/4] md/bitmap: Fix list_entry_rcu usage

[Paul E. McKenney]

On Sun, Sep 13, 2015 at 09:10:24AM -0700, Paul E. McKenney wrote:
> On Sun, Sep 13, 2015 at 12:06:33PM +0200, Patrick Marlier wrote:

[ . . . ]

> > Paul,
> > 
> > This sounds good to me. It should fix the performance issue (will
> > check with my benchmark).
> 
> Thank you, looking forward to seeing the results!

Just following up -- how is the benchmarking going?

							Thanx, Paul

Re: [PATCH tip/core/rcu 3/4] md/bitmap: Fix list_entry_rcu usage

[Patrick Marlier]

On 09/22/2015 10:50 PM, Paul E. McKenney wrote:
> On Sun, Sep 13, 2015 at 09:10:24AM -0700, Paul E. McKenney wrote:
>> On Sun, Sep 13, 2015 at 12:06:33PM +0200, Patrick Marlier wrote:
>
> [ . . . ]
>
>>> Paul,
>>>
>>> This sounds good to me. It should fix the performance issue (will
>>> check with my benchmark).
>>
>> Thank you, looking forward to seeing the results!
>
> Just following up -- how is the benchmarking going?

Note that in my module I am using the kernel version 3.16.0-31 (I ported 
your change).
Here the results of my benchmark that tests rculist in the case of read 
only.

# 1st column : The number of threads
# 2nd : ops/s the original version
# 3rd : ops/s your version with lockless_dereference
1 883923 1747554
2 1741441 3543062
3 2462360 5103647
4 3437273 7176608
6 5155803 9812348
8 6718111 13330050
10 8519227 17458294
12 9773632 20298897
14 11555198 23191424
16 11643264 25125712

I get same performance with my patch.
So indeed it fixes the performance problem I was seeing.

Thanks a lot!
--
Pat

Re: [PATCH tip/core/rcu 3/4] md/bitmap: Fix list_entry_rcu usage

[Paul E. McKenney]

On Wed, Sep 23, 2015 at 07:57:53PM +0200, Patrick Marlier wrote:
> 
> On 09/22/2015 10:50 PM, Paul E. McKenney wrote:
> >On Sun, Sep 13, 2015 at 09:10:24AM -0700, Paul E. McKenney wrote:
> >>On Sun, Sep 13, 2015 at 12:06:33PM +0200, Patrick Marlier wrote:
> >
> >[ . . . ]
> >
> >>>Paul,
> >>>
> >>>This sounds good to me. It should fix the performance issue (will
> >>>check with my benchmark).
> >>
> >>Thank you, looking forward to seeing the results!
> >
> >Just following up -- how is the benchmarking going?
> 
> Note that in my module I am using the kernel version 3.16.0-31 (I
> ported your change).
> Here the results of my benchmark that tests rculist in the case of
> read only.
> 
> # 1st column : The number of threads
> # 2nd : ops/s the original version
> # 3rd : ops/s your version with lockless_dereference
> 1 883923 1747554
> 2 1741441 3543062
> 3 2462360 5103647
> 4 3437273 7176608
> 6 5155803 9812348
> 8 6718111 13330050
> 10 8519227 17458294
> 12 9773632 20298897
> 14 11555198 23191424
> 16 11643264 25125712
> 
> I get same performance with my patch.
> So indeed it fixes the performance problem I was seeing.

Nice, thank you!!!

							Thanx, Paul