linux-kernel - Re: [PATCH v2] powerpc ticket locks

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Message-ID: <CAC4Lta2Pssu5QY_QLmy5C3Fm2G3fwpiuc5BNOLBnQTmmRH+W4Q@mail.gmail.com>
Date:	Tue, 11 Feb 2014 15:23:51 +0530
From:	Raghavendra KT <raghavendra.kt.linux@...il.com>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Torsten Duwe <duwe@....de>
Cc:	Paul Mackerras <paulus@...ba.org>,
	Anton Blanchard <anton@...ba.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Scott Wood <scottwood@...escale.com>,
	Tom Musta <tommusta@...il.com>, Ingo Molnar <mingo@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linuxppc-dev@...ts.ozlabs.org,
	Raghavendra KT <raghavendra.kt@...ux.vnet.ibm.com>
Subject: Re: [PATCH v2] powerpc ticket locks

On Mon, Feb 10, 2014 at 8:40 AM, Benjamin Herrenschmidt
<benh@...nel.crashing.org> 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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