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

[Waiman Long]

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@xxxxxxxxxx wrote:
> > > > From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > > >
> > > > 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@xxxxxxxxxxxx>
> > > >
> > > > --
> > > >
> > > > I have specifically not included Peter's SOB on this, as he sent his
> > > > original patch
> > > > <https://lore.kernel.org/lkml/YHbBBuVFNnI4kjj3@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/>
> > > > 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