linux-kernel - Re: [PATCH tip/locking/core v4 1/6] powerpc: atomic: Make *xchg and *cmpxchg a full barrier

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Date:	Wed, 21 Oct 2015 12:36:38 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Boqun Feng <boqun.feng@...il.com>, linux-kernel@...r.kernel.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>,
	Will Deacon <will.deacon@....com>,
	Waiman Long <waiman.long@...com>,
	Davidlohr Bueso <dave@...olabs.net>, stable@...r.kernel.org
Subject: Re: [PATCH tip/locking/core v4 1/6] powerpc: atomic: Make *xchg and
 *cmpxchg a full barrier

On Wed, Oct 21, 2015 at 10:18:33AM +0200, Peter Zijlstra wrote:
> On Tue, Oct 20, 2015 at 02:28:35PM -0700, Paul E. McKenney wrote:
> > I am not seeing a sync there, but I really have to defer to the
> > maintainers on this one.  I could easily have missed one.
> 
> So x86 implies a full barrier for everything that changes the CPL; and
> some form of implied ordering seems a must if you change the privilege
> level unless you tag every single load/store with the priv level at that
> time, which seems the more expensive option.

And it is entirely possible that there is some similar operation
somewhere in the powerpc entry/exit code.  I would not trust myself
to recognize it, though.

> So I suspect the typical implementation will flush all load/stores,
> change the effective priv level and continue.
> 
> This can of course be implemented at a pure per CPU ordering (RCpc),
> which would be in line with the rest of Power, in which case you do
> indeed need an explicit sync to make it visible to other CPUs.
> 
> But yes, if Michael or Ben could clarify this it would be good.
> 
> Back then I talked to Ralf about what MIPS says on this, and MIPS arch
> spec is entirely quiet on this, it allows implementations full freedom
> IIRC.

:-) ;-) ;-)

> </ramble>

							Thanx, Paul

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