rcu alignment warning tripping on m68k

rcu alignment warning tripping on m68k

[Greg Ungerer]

Hi All,

Inside kernel/rcy/tree.c in __call_rcu() it does an alignment check on
the head pointer passed in. This trips on m68k systems, because they only
need alignment of 32bit quantities to 16bit boundaries.

...
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------------[ cut here ]------------
WARNING: CPU: 0 PID: 18 at kernel/rcu/tree.c:2470 0x400577e0()
Modules linked in:
CPU: 0 PID: 18 Comm: kworker/u2:0 Not tainted 3.15.0-rc4-00013-g1c07b64-dirty #14
Stack from 418ada9c:
        418ada9c 401e8336 400258e0 401c69c4 418ac000 418a86f6 4003bc0a 418a8390
        418a86f6 4002590e 401e176b 000009a6 400577e0 00000009 00000000 400577e0
        401e176b 000009a6 401c69c4 418ac000 418a8390 418a8390 418ac000 40057928
        418a86f6 40025bcc 40218fe0 00000000 401ddb56 401e17ad 000002af 400261c2
        418a86f6 40025bcc 00000013 00000030 00000010 00000013 418ac000 418adf88
        418a8390 418adf70 400263f0 418a8000 418adf18 41885950 00000000 00000000
Call Trace:
        [<400258e0>] 0x400258e0
 [<401c69c4>] 0x401c69c4
 [<4003bc0a>] 0x4003bc0a
 [<4002590e>] 0x4002590e
 [<400577e0>] 0x400577e0


What is the best solution to fix?
Make the alignment check be only 16bits?

--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2467,7 +2467,7 @@ __call_rcu(struct rcu_head *head, void (*func)(struct rcu_
        unsigned long flags;
        struct rcu_data *rdp;
 
-       WARN_ON_ONCE((unsigned long)head & 0x3); /* Misaligned rcu_head! */
+       WARN_ON_ONCE((unsigned long)head & 0x1); /* Misaligned rcu_head! */
        if (debug_rcu_head_queue(head)) {
                /* Probable double call_rcu(), so leak the callback. */
                ACCESS_ONCE(head->func) = rcu_leak_callback;


Or should we just get rid of this check altogether?

Regards
Greg

Re: rcu alignment warning tripping on m68k

[One Thousand Gnomes]

On Thu, 29 May 2014 12:08:32 +1000
Greg Ungerer <gerg@uclinux.org> wrote:

> Hi All,
> 
> Inside kernel/rcy/tree.c in __call_rcu() it does an alignment check on
> the head pointer passed in. This trips on m68k systems, because they only
> need alignment of 32bit quantities to 16bit boundaries.

__alignof perhaps ?

or just an ARCH_ define ?

Re: rcu alignment warning tripping on m68k

[Greg Ungerer]

On 29/05/14 23:11, One Thousand Gnomes wrote:
> On Thu, 29 May 2014 12:08:32 +1000
> Greg Ungerer <gerg@uclinux.org> wrote:
> 
>> Hi All,
>>
>> Inside kernel/rcy/tree.c in __call_rcu() it does an alignment check on
>> the head pointer passed in. This trips on m68k systems, because they only
>> need alignment of 32bit quantities to 16bit boundaries.
> 
> __alignof perhaps ?

That might do. Change then becomes something like:

--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2467,7 +2467,7 @@ __call_rcu(struct rcu_head *head, void (*func)(struct rcu_
        unsigned long flags;
        struct rcu_data *rdp;
 
-       WARN_ON_ONCE((unsigned long)head & 0x3); /* Misaligned rcu_head! */
+       WARN_ON_ONCE((unsigned long)head & (__alignof__(head) - 1)); /* Misaligned rcu_head! */
        if (debug_rcu_head_queue(head)) {
                /* Probable double call_rcu(), so leak the callback. */
                ACCESS_ONCE(head->func) = rcu_leak_callback;

Thanks
Greg

Re: rcu alignment warning tripping on m68k

[Paul E. McKenney]

On Fri, May 30, 2014 at 11:29:41AM +1000, Greg Ungerer wrote:
> On 29/05/14 23:11, One Thousand Gnomes wrote:
> > On Thu, 29 May 2014 12:08:32 +1000
> > Greg Ungerer <gerg@uclinux.org> wrote:
> > 
> >> Hi All,
> >>
> >> Inside kernel/rcy/tree.c in __call_rcu() it does an alignment check on
> >> the head pointer passed in. This trips on m68k systems, because they only
> >> need alignment of 32bit quantities to 16bit boundaries.
> > 
> > __alignof perhaps ?
> 
> That might do. Change then becomes something like:
> 
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -2467,7 +2467,7 @@ __call_rcu(struct rcu_head *head, void (*func)(struct rcu_
>         unsigned long flags;
>         struct rcu_data *rdp;
> 
> -       WARN_ON_ONCE((unsigned long)head & 0x3); /* Misaligned rcu_head! */
> +       WARN_ON_ONCE((unsigned long)head & (__alignof__(head) - 1)); /* Misaligned rcu_head! */

Hmmm...  The purpose of the check is to reserve the low-order bits to
allow RCU to classify callbacks as being time-critical or not.  RCU
can probably live with a single bit, but if there is some architecture
out there that simply refuses to do alignment, I need to know about it.

(See "git show 0bb7b59d6e2b8" for more info.)

So how about this instead?

 -       WARN_ON_ONCE((unsigned long)head & 0x1); /* Misaligned rcu_head! */

(Trying to remember if I have seen Linux kernel code that uses both
the lower bits...)

							Thanx, Paul

>         if (debug_rcu_head_queue(head)) {
>                 /* Probable double call_rcu(), so leak the callback. */
>                 ACCESS_ONCE(head->func) = rcu_leak_callback;
> 
> Thanks
> Greg
> 
> 
> --
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> Please read the FAQ at  http://www.tux.org/lkml/
> 

Re: rcu alignment warning tripping on m68k

[Mikael Pettersson]

Paul E. McKenney writes:
 > On Fri, May 30, 2014 at 11:29:41AM +1000, Greg Ungerer wrote:
 > > On 29/05/14 23:11, One Thousand Gnomes wrote:
 > > > On Thu, 29 May 2014 12:08:32 +1000
 > > > Greg Ungerer <gerg@uclinux.org> wrote:
 > > > 
 > > >> Hi All,
 > > >>
 > > >> Inside kernel/rcy/tree.c in __call_rcu() it does an alignment check on
 > > >> the head pointer passed in. This trips on m68k systems, because they only
 > > >> need alignment of 32bit quantities to 16bit boundaries.
 > > > 
 > > > __alignof perhaps ?
 > > 
 > > That might do. Change then becomes something like:
 > > 
 > > --- a/kernel/rcu/tree.c
 > > +++ b/kernel/rcu/tree.c
 > > @@ -2467,7 +2467,7 @@ __call_rcu(struct rcu_head *head, void (*func)(struct rcu_
 > >         unsigned long flags;
 > >         struct rcu_data *rdp;
 > > 
 > > -       WARN_ON_ONCE((unsigned long)head & 0x3); /* Misaligned rcu_head! */
 > > +       WARN_ON_ONCE((unsigned long)head & (__alignof__(head) - 1)); /* Misaligned rcu_head! */
 > 
 > Hmmm...  The purpose of the check is to reserve the low-order bits to
 > allow RCU to classify callbacks as being time-critical or not.  RCU
 > can probably live with a single bit, but if there is some architecture
 > out there that simply refuses to do alignment, I need to know about it.
 > 
 > (See "git show 0bb7b59d6e2b8" for more info.)
 > 
 > So how about this instead?
 > 
 >  -       WARN_ON_ONCE((unsigned long)head & 0x1); /* Misaligned rcu_head! */
 > 
 > (Trying to remember if I have seen Linux kernel code that uses both
 > the lower bits...)

As stated above, m68k-linux aligns to 16-bit boundaries by default, so you'd
get one bit but not necessarily more.  If you want more free low bits, why
not attach an explicit attribute aligned to the rcu_head type declaration?

Re: rcu alignment warning tripping on m68k

[Paul E. McKenney]

On Sat, Jun 07, 2014 at 03:17:24PM +0200, Mikael Pettersson wrote:
> Paul E. McKenney writes:
>  > On Fri, May 30, 2014 at 11:29:41AM +1000, Greg Ungerer wrote:
>  > > On 29/05/14 23:11, One Thousand Gnomes wrote:
>  > > > On Thu, 29 May 2014 12:08:32 +1000
>  > > > Greg Ungerer <gerg@uclinux.org> wrote:
>  > > > 
>  > > >> Hi All,
>  > > >>
>  > > >> Inside kernel/rcy/tree.c in __call_rcu() it does an alignment check on
>  > > >> the head pointer passed in. This trips on m68k systems, because they only
>  > > >> need alignment of 32bit quantities to 16bit boundaries.
>  > > > 
>  > > > __alignof perhaps ?
>  > > 
>  > > That might do. Change then becomes something like:
>  > > 
>  > > --- a/kernel/rcu/tree.c
>  > > +++ b/kernel/rcu/tree.c
>  > > @@ -2467,7 +2467,7 @@ __call_rcu(struct rcu_head *head, void (*func)(struct rcu_
>  > >         unsigned long flags;
>  > >         struct rcu_data *rdp;
>  > > 
>  > > -       WARN_ON_ONCE((unsigned long)head & 0x3); /* Misaligned rcu_head! */
>  > > +       WARN_ON_ONCE((unsigned long)head & (__alignof__(head) - 1)); /* Misaligned rcu_head! */
>  > 
>  > Hmmm...  The purpose of the check is to reserve the low-order bits to
>  > allow RCU to classify callbacks as being time-critical or not.  RCU
>  > can probably live with a single bit, but if there is some architecture
>  > out there that simply refuses to do alignment, I need to know about it.
>  > 
>  > (See "git show 0bb7b59d6e2b8" for more info.)
>  > 
>  > So how about this instead?
>  > 
>  >  -       WARN_ON_ONCE((unsigned long)head & 0x1); /* Misaligned rcu_head! */
>  > 
>  > (Trying to remember if I have seen Linux kernel code that uses both
>  > the lower bits...)
> 
> As stated above, m68k-linux aligns to 16-bit boundaries by default, so you'd
> get one bit but not necessarily more.  If you want more free low bits, why
> not attach an explicit attribute aligned to the rcu_head type declaration?

One bit should do it for the time being, but yes, if I ever need two bits,
your suggestion of explicitly aligning the rcu_head type declaration
sounds like a very good one.

								Thanx, Paul

Re: rcu alignment warning tripping on m68k

[Greg Ungerer]

Hi Paul,

On 07/06/14 04:46, Paul E. McKenney wrote:
> On Fri, May 30, 2014 at 11:29:41AM +1000, Greg Ungerer wrote:
>> On 29/05/14 23:11, One Thousand Gnomes wrote:
>>> On Thu, 29 May 2014 12:08:32 +1000
>>> Greg Ungerer <gerg@uclinux.org> wrote:
>>>
>>>> Hi All,
>>>>
>>>> Inside kernel/rcy/tree.c in __call_rcu() it does an alignment check on
>>>> the head pointer passed in. This trips on m68k systems, because they only
>>>> need alignment of 32bit quantities to 16bit boundaries.
>>>
>>> __alignof perhaps ?
>>
>> That might do. Change then becomes something like:
>>
>> --- a/kernel/rcu/tree.c
>> +++ b/kernel/rcu/tree.c
>> @@ -2467,7 +2467,7 @@ __call_rcu(struct rcu_head *head, void (*func)(struct rcu_
>>         unsigned long flags;
>>         struct rcu_data *rdp;
>>
>> -       WARN_ON_ONCE((unsigned long)head & 0x3); /* Misaligned rcu_head! */
>> +       WARN_ON_ONCE((unsigned long)head & (__alignof__(head) - 1)); /* Misaligned rcu_head! */
> 
> Hmmm...  The purpose of the check is to reserve the low-order bits to
> allow RCU to classify callbacks as being time-critical or not.  RCU
> can probably live with a single bit, but if there is some architecture
> out there that simply refuses to do alignment, I need to know about it.

This change was prompted by this check tripping, so the alignment
issue is certainly real for m68k.

Regards
Greg


> (See "git show 0bb7b59d6e2b8" for more info.)
> 
> So how about this instead?
> 
>  -       WARN_ON_ONCE((unsigned long)head & 0x1); /* Misaligned rcu_head! */
> 
> (Trying to remember if I have seen Linux kernel code that uses both
> the lower bits...)
> 
> 							Thanx, Paul
> 
>>         if (debug_rcu_head_queue(head)) {
>>                 /* Probable double call_rcu(), so leak the callback. */
>>                 ACCESS_ONCE(head->func) = rcu_leak_callback;
>>
>> Thanks
>> Greg
>>
>>
>> --
>> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
>> the body of a message to majordomo@vger.kernel.org
>> More majordomo info at  http://vger.kernel.org/majordomo-info.html
>> Please read the FAQ at  http://www.tux.org/lkml/
>>
> 
>