linux-kernel - Re: [PATCH -v2 4/6] locking, arch: Update spin_unlock

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Date:	Fri, 27 May 2016 15:34:13 -0400
From:	Chris Metcalf <cmetcalf@...lanox.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	<linux-kernel@...r.kernel.org>, <torvalds@...ux-foundation.org>,
	<manfred@...orfullife.com>, <dave@...olabs.net>,
	<paulmck@...ux.vnet.ibm.com>, <will.deacon@....com>,
	<boqun.feng@...il.com>, <Waiman.Long@....com>, <tj@...nel.org>,
	<pablo@...filter.org>, <kaber@...sh.net>, <davem@...emloft.net>,
	<oleg@...hat.com>, <netfilter-devel@...r.kernel.org>,
	<sasha.levin@...cle.com>, <hofrat@...dl.org>, <rth@...ddle.net>,
	<vgupta@...opsys.com>, <linux@...linux.org.uk>,
	<realmz6@...il.com>, <rkuo@...eaurora.org>, <tony.luck@...el.com>,
	<james.hogan@...tec.com>, <ralf@...ux-mips.org>,
	<dhowells@...hat.com>, <jejb@...isc-linux.org>,
	<mpe@...erman.id.au>, <schwidefsky@...ibm.com>,
	<ysato@...rs.sourceforge.jp>, <chris@...kel.net>
Subject: Re: [PATCH -v2 4/6] locking, arch: Update spin_unlock_wait()

On 5/27/2016 5:05 AM, Peter Zijlstra wrote:
> On Thu, May 26, 2016 at 05:10:36PM -0400, Chris Metcalf wrote:
>> On 5/26/2016 10:19 AM, Peter Zijlstra wrote:
>>> --- a/arch/tile/lib/spinlock_32.c
>>> +++ b/arch/tile/lib/spinlock_32.c
>>> @@ -72,10 +72,14 @@ void arch_spin_unlock_wait(arch_spinlock
>>>   	if (next == curr)
>>>   		return;
>>> +	smp_rmb();
>>> +
>>>   	/* Wait until the current locker has released the lock. */
>>>   	do {
>>>   		delay_backoff(iterations++);
>>>   	} while (READ_ONCE(lock->current_ticket) == curr);
>>> +
>>> +	smp_acquire__after_ctrl_dep();
>>>   }
>>>   EXPORT_SYMBOL(arch_spin_unlock_wait);
>>> --- a/arch/tile/lib/spinlock_64.c
>>> +++ b/arch/tile/lib/spinlock_64.c
>>> @@ -72,10 +72,14 @@ void arch_spin_unlock_wait(arch_spinlock
>>>   	if (arch_spin_next(val) == curr)
>>>   		return;
>>> +	smp_rmb();
>>> +
>>>   	/* Wait until the current locker has released the lock. */
>>>   	do {
>>>   		delay_backoff(iterations++);
>>>   	} while (arch_spin_current(READ_ONCE(lock->lock)) == curr);
>>> +
>>> +	smp_acquire__after_ctrl_dep();
>>>   }
>>>   EXPORT_SYMBOL(arch_spin_unlock_wait);
>> The smp_rmb() are unnecessary for tile.  We READ_ONCE next/curr from the
>> lock and compare them, so we know the load(s) are complete.  There's no
>> microarchitectural speculation going on so that's that.  Then we READ_ONCE
>> the next load on the lock from within the wait loop, so our load/load
>> ordering is guaranteed.
> Does TILE never speculate reads? Because in that case the control
> dependency already provides a full load->load,store barrier and you'd
> want smp_acquire__after_ctrl_dep() to be a barrier() instead of
> smp_rmb().

Yes, that's a good point.  I didn't look at the definition of smp_acquire__after_ctrl_dep(),
but it certainly sounds like that's exactly a compiler barrier for tile.  There is no load
speculation performed.  The only out-of-order stuff that happens is in the memory
subsystem: stores will become visible in arbitrary order, and loads will arrive in
arbitrary order, but as soon as the result of a load is used in any other kind of
instruction, the instruction issue will halt until the pending load(s) for the instruction
operands are available.

-- 
Chris Metcalf, Mellanox Technologies
http://www.mellanox.com