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SubjectRe: [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@kernel.org wrote:
> From: Peter Zijlstra <peterz@infradead.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@rivosinc.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);
> +}
> +


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