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

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Message-ID: <54d6221d-0c4f-9329-042d-4f74c4ea288f@redhat.com>
Date:   Tue, 22 Mar 2022 11:54:37 -0400
From:   Waiman Long <longman@...hat.com>
To:     Stafford Horne <shorne@...il.com>, guoren@...nel.org
Cc:     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 3/21/22 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.

Cheers,
Longman