Re: [PATCH v2] powerpc ticket locks

[Raghavendra KT]

On Mon, Feb 10, 2014 at 8:40 AM, Benjamin Herrenschmidt
<benh@xxxxxxxxxxxxxxxxxxx> wrote:
> 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.
>

Probably very basic and stupid question from me.
How much important to have holder information for PPC? From my
previous experiment
on x86, it was lock-waiter preemption which is problematic rather than
lock-holder preemption.
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