linux-kernel - Re: [OpenRISC] [PATCH V2 1/5] asm-generic: ticket-lock: New generic ticket-based spinlock

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Message-ID: <cbf542d9-d3d4-9de8-5a96-4d5fcea69f6a@redhat.com>
Date:   Tue, 22 Mar 2022 17:14:05 -0400
From:   Waiman Long <longman@...hat.com>
To:     Stafford Horne <shorne@...il.com>
Cc:     guoren@...nel.org, palmer@...belt.com, arnd@...db.de,
        boqun.feng@...il.com, peterz@...radead.org,
        linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-csky@...r.kernel.org, openrisc@...ts.librecores.org,
        Palmer Dabbelt <palmer@...osinc.com>,
        linux-riscv@...ts.infradead.org
Subject: Re: [OpenRISC] [PATCH V2 1/5] asm-generic: ticket-lock: New generic
 ticket-based spinlock

On 3/22/22 17:06, Stafford Horne wrote:
> On Tue, Mar 22, 2022 at 11:54:37AM -0400, Waiman Long wrote:
>> On 3/21/22 23:10, Stafford Horne wrote:
>>> Hello,
>>>
>>> There is a problem with this patch on Big Endian machines, see below.
>>>
>>> On Sat, Mar 19, 2022 at 11:54:53AM +0800, guoren@...nel.org wrote:
>>>> From: Peter Zijlstra <peterz@...radead.org>
>>>>
>>>> This is a simple, fair spinlock.  Specifically it doesn't have all the
>>>> subtle memory model dependencies that qspinlock has, which makes it more
>>>> suitable for simple systems as it is more likely to be correct.
>>>>
>>>> [Palmer: commit text]
>>>> Signed-off-by: Palmer Dabbelt <palmer@...osinc.com>
>>>>
>>>> --
>>>>
>>>> I have specifically not included Peter's SOB on this, as he sent his
>>>> original patch
>>>> <https://lore.kernel.org/lkml/YHbBBuVFNnI4kjj3@hirez.programming.kicks-ass.net/>
>>>> without one.
>>>> ---
>>>>    include/asm-generic/spinlock.h          | 11 +++-
>>>>    include/asm-generic/spinlock_types.h    | 15 +++++
>>>>    include/asm-generic/ticket-lock-types.h | 11 ++++
>>>>    include/asm-generic/ticket-lock.h       | 86 +++++++++++++++++++++++++
>>>>    4 files changed, 120 insertions(+), 3 deletions(-)
>>>>    create mode 100644 include/asm-generic/spinlock_types.h
>>>>    create mode 100644 include/asm-generic/ticket-lock-types.h
>>>>    create mode 100644 include/asm-generic/ticket-lock.h
>>>>
>>>> diff --git a/include/asm-generic/ticket-lock.h b/include/asm-generic/ticket-lock.h
>>>> new file mode 100644
>>>> index 000000000000..59373de3e32a
>>>> --- /dev/null
>>>> +++ b/include/asm-generic/ticket-lock.h
>>> ...
>>>
>>>> +static __always_inline void ticket_unlock(arch_spinlock_t *lock)
>>>> +{
>>>> +	u16 *ptr = (u16 *)lock + __is_defined(__BIG_ENDIAN);
>>> As mentioned, this patch series breaks SMP on OpenRISC.  I traced it to this
>>> line.  The above `__is_defined(__BIG_ENDIAN)`  does not return 1 as expected
>>> even on BIG_ENDIAN machines.  This works:
>>>
>>>
>>> diff --git a/include/asm-generic/ticket-lock.h b/include/asm-generic/ticket-lock.h
>>> index 59373de3e32a..52b5dc9ffdba 100644
>>> --- a/include/asm-generic/ticket-lock.h
>>> +++ b/include/asm-generic/ticket-lock.h
>>> @@ -26,6 +26,7 @@
>>>    #define __ASM_GENERIC_TICKET_LOCK_H
>>>    #include <linux/atomic.h>
>>> +#include <linux/kconfig.h>
>>>    #include <asm-generic/ticket-lock-types.h>
>>>    static __always_inline void ticket_lock(arch_spinlock_t *lock)
>>> @@ -51,7 +52,7 @@ static __always_inline bool ticket_trylock(arch_spinlock_t *lock)
>>>    static __always_inline void ticket_unlock(arch_spinlock_t *lock)
>>>    {
>>> -       u16 *ptr = (u16 *)lock + __is_defined(__BIG_ENDIAN);
>>> +       u16 *ptr = (u16 *)lock + IS_ENABLED(CONFIG_CPU_BIG_ENDIAN);
>>>           u32 val = atomic_read(lock);
>>>           smp_store_release(ptr, (u16)val + 1);
>>>
>>>
>>>> +	u32 val = atomic_read(lock);
>>>> +
>>>> +	smp_store_release(ptr, (u16)val + 1);
>>>> +}
>>>> +
>> __BIG_ENDIAN is defined in <linux/kconfig.h>. I believe that if you include
>> <linux/kconfig.h>, the second hunk is not really needed and vice versa.
> I thought so too, but it doesn't seem to work.  I think __is_defined is not
> doing what we think in this context.  It looks like __is_defined works when a
> macro is defined as 1, in this case we have __BIG_ENDIAN 4321.

You are right. __is_defined() only for 1 or not 1. So it can't be used 
for __BIG_ENDIAN.

I was not aware of that. Anyway, the <linux/kconfig.h> include is not 
really needed then.

Cheers,
Longman