From: nicolas.pitre@linaro.org (Nicolas Pitre)
To: linux-arm-kernel@lists.infradead.org
Subject: [PATCH] ARM: locks: remove opencoded #16 for ticket shift
Date: Thu, 18 Jul 2013 13:53:06 -0400 (EDT) [thread overview]
Message-ID: <alpine.LFD.2.03.1307181350440.14924@syhkavp.arg> (raw)
In-Reply-To: <20130718172159.GC9837@mudshark.cambridge.arm.com>
On Thu, 18 Jul 2013, Will Deacon wrote:
> On Wed, Jul 17, 2013 at 02:56:59PM +0100, Nicolas Pitre wrote:
> > On Mon, 15 Jul 2013, Will Deacon wrote:
> >
> > > The ticket width of our spinlocks is defined by TICKET_SHIFT, so remove
> > > the opencoded #16 from the trylock implementation.
> > >
> > > Signed-off-by: Will Deacon <will.deacon@arm.com>
> >
> > This is pointless.
>
> I disagree (see below).
>
> > > " ldrex %0, [%3]\n"
> > > " mov %2, #0\n"
> > > - " subs %1, %0, %0, ror #16\n"
> >
> > Any value other than 16 would break that code. You'd rather ensure it
> > never gets defined to anything else.
>
> There are two aspects to the ticket lock:
>
> 1. The size of each ticket
> 2. The location of the next ticket within the 32-bit lock word
>
> TICKET_SHIFT actually defines (2) and could be 0 or 16, hence why I was
> making this small cosmetic change.
Doesn't matter where the location of the next ticket is. Whether it is
0 or 16, for the above code to work, it *must* always perform a ror #16.
Hence tying the ror constant with TICKET_SHIFT is wrong.
Nicolas
next prev parent reply other threads:[~2013-07-18 17:53 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-07-15 13:27 [PATCH] ARM: locks: remove opencoded #16 for ticket shift Will Deacon
2013-07-16 16:35 ` Dave Martin
2013-07-17 13:56 ` Nicolas Pitre
2013-07-18 17:21 ` Will Deacon
2013-07-18 17:53 ` Nicolas Pitre [this message]
2013-07-19 9:05 ` Will Deacon
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