linux-kernel - Re: [PATCH] Convert powerpc simple spinlocks into ticket locks

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Message-ID: <20140207103613.GF5126@laptop.programming.kicks-ass.net>
Date:	Fri, 7 Feb 2014 11:36:13 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Torsten Duwe <duwe@....de>
Cc:	Scott Wood <scottwood@...escale.com>, linux-kernel@...r.kernel.org,
	Paul Mackerras <paulus@...ba.org>,
	Anton Blanchard <anton@...ba.org>,
	Tom Musta <tommusta@...il.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	linuxppc-dev@...ts.ozlabs.org, Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH] Convert powerpc simple spinlocks into ticket locks

> So if you have ll/sc on the whole word concurrent with the half-word
> store, you can loose the half-word store like:
> 
>   lwarx &tickets
>   ...			sth &tail
>   stwcd &tickets
> 
> 
> The stwcd will over-write the tail store.

Oh wait, that's stupid, it will invalidate the lock and fail the store
and make it try again, so you could try and combine the load, but you'd
need an extra shift instruction instead of an extra load.

Not sure that's a valid trade-off..
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