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

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Date:   Tue, 22 Mar 2022 12:10:36 +0900
From:   Stafford Horne <shorne@...il.com>
To:     guoren@...nel.org
Cc:     palmer@...belt.com, arnd@...db.de, boqun.feng@...il.com,
        longman@...hat.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

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);
> +}
> +