netdev - Re: Kernel crash after using new Intel NIC (igb)

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Message-ID: <4DDEAA3C.7020502@fb.com>
Date:	Thu, 26 May 2011 12:30:04 -0700
From:	Arun Sharma <asharma@...com>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	Maximilian Engelhardt <maxi@...monizer.de>,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	StuStaNet Vorstand <vorstand@...sta.mhn.de>
Subject: Re: Kernel crash after using new Intel NIC (igb)

On 5/24/11 11:35 PM, Eric Dumazet wrote:

>> Another possibility is to do the list_empty() check twice. Once without
>> taking the lock and again with the spinlock held.
>>
>
> Why ?
>

Part of the problem is that I don't have a precise understanding of the 
race condition that's causing the list to become corrupted.

All I know is that doing it under the lock fixes it. If it's slowing 
things down, we do a check outside the lock (since it's cheap). But if 
we get the wrong answer, we verify it again under the lock.

> list_del_init(&p->unused); (done under lock of course) is safe, you can
> call it twice, no problem.

Doing it twice is not a problem. But doing it when we shouldn't be doing 
it could be the problem.

The list modification under unused_peers.lock looks generally safe. But 
the control flow (based on refcnt) done outside the lock might have races.

Eg: inet_putpeer() might find the refcnt go to zero, but before it adds 
it to the unused list, another thread may be doing inet_getpeer() and 
set refcnt to 1. In the end, we end up with a node that's potentially in 
use, but ends up on the unused list.

  -Arun
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