Date: Tue, 28 Apr 2015 16:33:57 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Manfred Spraul <manfred@colorfullife.com>, Oleg Nesterov <oleg@redhat.com>,
        Kirill Tkhai <ktkhai@parallels.com>, linux-kernel@vger.kernel.org,
        Ingo Molnar <mingo@redhat.com>, Josh Poimboeuf <jpoimboe@redhat.com>,
        Chris Metcalf <cmetcalf@ezchip.com>
Subject: Re: [PATCH 2/2] [PATCH] sched: Add smp_rmb() in task rq locking
 cycles
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 <54E4E479.4050003@colorfullife.com>
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 <20150219141905.GA11018@redhat.com>
 <54E77CC0.5030401@colorfullife.com>
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On Sun, Apr 26, 2015 at 03:52:13AM -0700, Paul E. McKenney wrote:

> And then an smp_read_barrier_depends() would be needed either here
> or embedded in apin_unlock_wait().  But we also need to check the
> spin_unlock_wait() implementations to see if any are potentially
> vulnerable to compiler misbehavior due to lack of ACCESS_ONCE(),
> READ_ONCE(), or other sources of the required volatility:
> 

> o	tile: For 32-bit, looks like a bug.  Compares ->current_ticket and
> 	->next_ticket with no obvious protection.  The compiler is free to
> 	load them in either order, so it is possible that the two fields
> 	could compare equal despite never having actually been equal at
> 	any given time.  Needs something like arm, arm64, mips, or x86
> 	to do single fetch, then compare fields in quantity fetched.
> 
> 	Except that this appears to be using int on a 32-bit system,
> 	thus might not have a 64-bit load.  If that is the case, the
> 	trick would be to load them in order.  Except that this can be
> 	defeated by overflow.  Are there really 32-bit tile systems with
> 	enough CPUs to overflow an unsigned short?
> 
> 	For 64-bit, a READ_ONCE() appears to be in order -- no obvious
> 	volatility present.
> 

Chris?
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