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Subject: Re: [PATCH v2] powerpc ticket locks
From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
To: Torsten Duwe <duwe@lst.de>
Date: Mon, 10 Feb 2014 14:10:23 +1100
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Cc: Tom Musta <tommusta@gmail.com>, Peter Zijlstra <peterz@infradead.org>,
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 "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>, linuxppc-dev@lists.ozlabs.org,
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On Fri, 2014-02-07 at 17:58 +0100, Torsten Duwe wrote:
>  typedef struct {
> -       volatile unsigned int slock;
> -} arch_spinlock_t;
> +       union {
> +               __ticketpair_t head_tail;
> +               struct __raw_tickets {
> +#ifdef __BIG_ENDIAN__          /* The "tail" part should be in the MSBs */
> +                       __ticket_t tail, head;
> +#else
> +                       __ticket_t head, tail;
> +#endif
> +               } tickets;
> +       };
> +#if defined(CONFIG_PPC_SPLPAR)
> +       u32 holder;
> +#endif
> +} arch_spinlock_t __aligned(4);

That's still broken with lockref (which we just merged).

We must have the arch_spinlock_t and the ref in the same 64-bit word
otherwise it will break.

We can make it work in theory since the holder doesn't have to be
accessed atomically, but the practicals are a complete mess ...
lockref would essentially have to re-implement the holder handling
of the spinlocks and use lower level ticket stuff.

Unless you can find a sneaky trick ... :-(

Ben.