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Date: Thu, 06 Feb 2014 13:28:21 -0600
From: Tom Musta <tommusta@gmail.com>
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To: Peter Zijlstra <peterz@infradead.org>, Torsten Duwe <duwe@lst.de>
Subject: Re: [PATCH] Convert powerpc simple spinlocks into ticket locks
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Cc: linux-kernel@vger.kernel.org, Paul Mackerras <paulus@samba.org>,
 Anton Blanchard <anton@samba.org>,
 "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>, linuxppc-dev@lists.ozlabs.org,
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On 2/6/2014 12:08 PM, Peter Zijlstra wrote:
>>> Can you pair lwarx with sthcx ? I couldn't immediately find the answer
>>> > > in the PowerISA doc. If so I think you can do better by being able to
>>> > > atomically load both tickets but only storing the head without affecting
>>> > > the tail.
>> > 
>> > V2.06b, Book II, Chapter 3, "sthcx" says:
>> > | If a reservation exists and the length associated [...] is not 2 bytes,
>> > | it is undefined whether (RS)_48:63 are stored [...]
>> > 
>> > That doesn't make me feel comfortable :(
> That's on page 692, right? The way I read that is of the lharx/sthcx
> don't have the exact same address, storage is undefined. But I can't
> find mention of non-matching load and store size, although I can imagine
> it being the same undefined.

My read is consistent with Torsten's ... this looks like a bad idea.

Look at the RTL for sthcx. on page 692 (Power ISA V2.06) and you will see this:

if RESERVE then
  if RESERVE_LENGTH = 2 then
     ...
  else
     undefined_case <- 1
else
  ...

A legal implementation might never perform the store.