From: Peter Zijlstra <peterz@infradead•org>
To: Torsten Duwe <duwe@lst•de>
Cc: Tom Musta <tommusta@gmail•com>,
linux-kernel@vger•kernel.org, Paul Mackerras <paulus@samba•org>,
Anton Blanchard <anton@samba•org>,
Scott Wood <scottwood@freescale•com>,
"Paul E. McKenney" <paulmck@linux•vnet.ibm.com>,
linuxppc-dev@lists•ozlabs.org, Ingo Molnar <mingo@kernel•org>
Subject: Re: [PATCH] Convert powerpc simple spinlocks into ticket locks
Date: Fri, 7 Feb 2014 18:19:00 +0100 [thread overview]
Message-ID: <20140207171900.GS5002@laptop.programming.kicks-ass.net> (raw)
In-Reply-To: <20140207170845.GD2107@lst.de>
On Fri, Feb 07, 2014 at 06:08:45PM +0100, Torsten Duwe wrote:
> > static inline unsigned int xadd(unsigned int *v, unsigned int i)
> > {
> > int t, ret;
> >
> > __asm__ __volatile__ (
> > "1: lwarx %0, 0, %4\n"
> > " mr %1, %0\n"
> > " add %0, %3, %0\n"
> > " stwcx. %0, %0, %4\n"
> > " bne- 1b\n"
> > : "=&r" (t), "=&r" (ret), "+m" (*v)
> > : "r" (i), "r" (v)
> > : "cc");
> >
> > return ret;
> > }
> >
> I don't like this xadd thing -- it's so x86 ;)
> x86 has its LOCK prefix, ppc has ll/sc.
> That should be reflected somehow IMHO.
Its the operational semantics I care about; this version is actually
nicer in that it doesn't actually imply all sorts of barriers :-)
> Maybe if xadd became mandatory for some kernel library.
call it fetch_add() its not an uncommon operation and many people
understand the semantics.
But you can simply include the asm bits in ticket_lock() and be done
with it. In that case you can also replace the add with an addi which
might be a little more efficient.
> > void ticket_unlock(tickets_t *lock)
> > {
> > ticket_t tail = lock->tail + 1;
> >
> > /*
> > * The store is save against the xadd for it will make the ll/sc fail
> > * and try again. Aside from that PowerISA guarantees single-copy
> > * atomicy for half-word writes.
> > *
> > * And since only the lock owner will ever write the tail, we're good.
> > */
> > smp_store_release(&lock->tail, tail);
> > }
>
> Yeah, let's try that on top of v2 (just posted).
> First, I want to see v2 work as nicely as v1 --
> compiling a debug kernel takes a while...
Use a faster machine... it can be done < 1 minute :-)
next prev parent reply other threads:[~2014-02-07 17:19 UTC|newest]
Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-02-06 10:37 [PATCH] Convert powerpc simple spinlocks into ticket locks Torsten Duwe
2014-02-06 15:53 ` Benjamin Herrenschmidt
2014-02-06 16:38 ` Peter Zijlstra
2014-02-06 17:37 ` Torsten Duwe
2014-02-06 18:08 ` Peter Zijlstra
2014-02-06 19:28 ` Tom Musta
2014-02-10 2:54 ` Benjamin Herrenschmidt
2014-02-07 8:24 ` Torsten Duwe
2014-02-06 20:19 ` Scott Wood
2014-02-07 9:02 ` Torsten Duwe
2014-02-07 10:31 ` Peter Zijlstra
2014-02-07 10:36 ` Peter Zijlstra
2014-02-07 10:45 ` Peter Zijlstra
2014-02-07 11:49 ` Torsten Duwe
2014-02-07 12:28 ` Peter Zijlstra
2014-02-07 15:18 ` Peter Zijlstra
2014-02-07 15:43 ` Peter Zijlstra
2014-02-07 17:08 ` Torsten Duwe
2014-02-07 17:19 ` Peter Zijlstra [this message]
2014-02-07 15:51 ` Kumar Gala
2014-02-07 16:10 ` Peter Zijlstra
2014-02-10 3:05 ` Benjamin Herrenschmidt
2014-02-10 3:02 ` Benjamin Herrenschmidt
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