netdev - Re: System blocks (hangs) on ifconfig up

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Date:	Mon, 13 Dec 2010 15:11:25 +0200
From:	Shmulik Hen <shmulik@...go.co.il>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	netdev@...r.kernel.org, Shmulik Hen <shmulik.hen@...il.com>,
	Ben Hutchings <bhutchings@...arflare.com>,
	shemminger@...tta.com
Subject: Re: System blocks (hangs) on ifconfig up

On 12/13/2010 02:37 PM, Eric Dumazet wrote:
>>
>> But why does this happen only the second time I run ifconfig up?
>> How come the entire system is totally frozen?
>> I can't even switch to other applications running. If I run 'top' in another
>> console, it stops refreshing for the entire period.
>>
>> I'll try to explain better;
>> The driver I'm referring to is part of an embedded system development kit.
>> It runs on the controlling side, which may be a PC or some Linux embedded
>> system. It exposes a virtual interface that allows to communicate via
>> ethernet connection to a remote board, and performs the firmware download
>> to that board.
>> Unfortunately, the firmware download stage is  done during dev->open() of
>> this virtual interface. The call to wait_event_interruptible_timeout()
>> is there to make sure the boot process of the remote board is complete via a
>> message. If all goes well the first time, there is no delay, but if the
>> operation
>> fails for any reason the first time, and a second attempt is made (another
>> ifconfig up), we see the freezing.
>>
>> Since this driver is (mostly) closed source, I had to try and reproduce
>> the situation
>> in an all open-source driver - this is the sample code I attached to my
>> original
>> message. The call to mdelay() there is meant to simulate the delay of
>> the original
>> driver - it schedules.
>>
> mdelay() does a busy wait. If you are not SMP, this means a 'freeze'
>
> If you want to schedule, you should use msleep()

Correct - my bad. When I use msleep() in the sample code there is no freeze.

But I still don't get it - when using mdelay(), why does the system 
freeze only
the second time but not in the first time?
And what about wait_event_interruptible_timeout()? surely it doesn't
do a busy loop - I can see  the source calling schedule().

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