netdev - Re: IGMP and rwlock: Dead ocurred again on TILEPro

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Date:	Fri, 18 Feb 2011 11:19:03 +0800
From:	Cypher Wu <cypher.w@...il.com>
To:	Chris Metcalf <cmetcalf@...era.com>
Cc:	David Miller <davem@...emloft.net>, xiyou.wangcong@...il.com,
	linux-kernel@...r.kernel.org, eric.dumazet@...il.com,
	netdev@...r.kernel.org
Subject: Re: IGMP and rwlock: Dead ocurred again on TILEPro

On Fri, Feb 18, 2011 at 7:18 AM, Chris Metcalf <cmetcalf@...era.com> wrote:
> On 2/17/2011 6:11 PM, David Miller wrote:
>> From: Chris Metcalf <cmetcalf@...era.com>
>> Date: Thu, 17 Feb 2011 18:04:13 -0500
>>
>>> On 2/17/2011 5:53 PM, David Miller wrote:
>>>> From: Chris Metcalf <cmetcalf@...era.com>
>>>> Date: Thu, 17 Feb 2011 17:49:46 -0500
>>>>
>>>>> The fix is to disable interrupts for the arch_read_lock family of methods.
>>>> How does that help handle the race when it happens between different
>>>> cpus, instead of between IRQ and non-IRQ context on the same CPU?
>>> There's no race in that case, since the lock code properly backs off and
>>> retries until the other cpu frees it.  The distinction here is that the
>>> non-IRQ context is "wedged" by the IRQ context.
>>>
>>>> Why don't you just use the generic spinlock based rwlock code on Tile,
>>>> since that is all that your atomic instructions can handle
>>>> sufficiently?
>>> The tile-specific code encodes reader/writer information in the same 32-bit
>>> word that the test-and-set instruction manipulates, so it's more efficient
>>> both in space and time.  This may not really matter for rwlocks, since no
>>> one cares much about them any more, but that was the motivation.
>> Ok, but IRQ disabling is going to be very expensive.
>
> The interrupt architecture on Tile allows a write to a special-purpose
> register to put you into a "critical section" where no interrupts or faults
> are delivered.  So we just need to bracket the read_lock operations with
> two SPR writes; each takes six machine cycles, so we're only adding 12
> cycles to the total cost of taking or releasing a read lock on an rwlock.
>
> --
> Chris Metcalf, Tilera Corp.
> http://www.tilera.com
>
>


Bye the way, other RISC platforms, say ARM and MIPS, use store
conditional rather that TNS a temp value for lock-val, does Fx have
similar instructions?


-- 
Cyberman Wu
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