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

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Message-Id: <20110217.151147.35033921.davem@davemloft.net>
Date:	Thu, 17 Feb 2011 15:11:47 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	cmetcalf@...era.com
Cc:	xiyou.wangcong@...il.com, cypher.w@...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

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.
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