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

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Message-ID: <CAC4Lta0=DojhnvPta5Rj3z61UzoEXO4H78iApa4nhNqmZcU+3g@mail.gmail.com>
Date:	Tue, 11 Feb 2014 15:09:23 +0530
From:	Raghavendra KT <raghavendra.kt.linux@...il.com>
To:	Torsten Duwe <duwe@....de>
Cc:	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	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 Fri, Feb 7, 2014 at 10:28 PM, Torsten Duwe <duwe@....de> wrote:
> Ticket locks for ppc, version 2. Changes since v1:
> * The atomically exchanged entity is always 32 bits.
> * asm inline string variations thus removed.
> * Carry the additional holder hint only #if defined(CONFIG_PPC_SPLPAR)
>
> Signed-off-by: Torsten Duwe <duwe@...e.de>
> --
[...]
> +static __always_inline void arch_spin_lock(arch_spinlock_t *lock)
>  {
> +       register struct __raw_tickets old, tmp,
> +               inc = { .tail = TICKET_LOCK_INC };
> +
>         CLEAR_IO_SYNC;
> -       while (1) {
> -               if (likely(__arch_spin_trylock(lock) == 0))
> -                       break;
> +       __asm__ __volatile__(
> +"1:    lwarx   %0,0,%4         # arch_spin_lock\n"
> +"      add     %1,%3,%0\n"
> +       PPC405_ERR77(0, "%4")
> +"      stwcx.  %1,0,%4\n"
> +"      bne-    1b"
> +       : "=&r" (old), "=&r" (tmp), "+m" (lock->tickets)
> +       : "r" (inc), "r" (&lock->tickets)
> +       : "cc");
> +
> +       if (likely(old.head == old.tail))
> +               goto out;
> +
> +       for (;;) {
> +               unsigned count = 100;

I am sure you wanted to tune the total loops to typical lock holding time ...
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