linux-kernel - Re: [RFC 3/5] powerpc: atomic: implement atomic{,64}_{add,sub}_return

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Date:	Mon, 14 Sep 2015 14:11:56 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Will Deacon <will.deacon@....com>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Boqun Feng <boqun.feng@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
	Ingo Molnar <mingo@...nel.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>,
	Michael Ellerman <mpe@...erman.id.au>,
	Thomas Gleixner <tglx@...utronix.de>,
	Waiman Long <waiman.long@...com>
Subject: Re: [RFC 3/5] powerpc: atomic: implement
 atomic{,64}_{add,sub}_return_* variants

On Mon, Sep 14, 2015 at 02:01:53PM +0200, Peter Zijlstra wrote:
> The scenario is:
> 
> 	CPU0			CPU1
> 
> 				unlock(x)
> 				  smp_store_release(&x->lock, 0);
> 
> 	unlock(y)
> 	  smp_store_release(&next->lock, 1); /* next == &y */
> 
> 				lock(y)
> 				  while (!(smp_load_acquire(&y->lock))
> 					cpu_relax();
> 
> 
> Where the lock does _NOT_ issue a store to acquire the lock at all. Now
> I don't think any of our current primitives manage this, so we should be
> good, but it might just be possible.

So with a bit more through this seems fundamentally impossible, you
always needs some stores in a lock() implementation, the above for
instance needs to queue itself, otherwise CPU0 will not be able to find
it etc..
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