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

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Date:   Wed, 23 Mar 2022 06:24:06 +0900
From:   Stafford Horne <shorne@...il.com>
To:     Waiman Long <longman@...hat.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 Tue, Mar 22, 2022 at 05:14:05PM -0400, Waiman Long wrote:
> 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.

Since we use IS_ENABLED which is defined in <linux/kconfig.h> I thought its best
to include it.  However, it seems the requirement for explicit includes differs
from subsystem to subsystem.

Whoever picks up this patch for the series can drop the include.  I confirm it
works even if we drop the include.

-Stafford