netdev - Re: [PATCH 2/3] b43: Fix oops if firmware is not available

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Date:	Sun, 12 Jan 2014 04:24:07 +0000
From:	Ben Hutchings <ben@...adent.org.uk>
To:	Larry Finger <Larry.Finger@...inger.net>
Cc:	linville@...driver.com, linux-wireless@...r.kernel.org,
	netdev@...r.kernel.org, Stable <stable@...r.kernel.org>
Subject: Re: [PATCH 2/3] b43: Fix oops if firmware is not available

On Sat, 2014-01-11 at 21:55 -0600, Larry Finger wrote:
> On 01/11/2014 09:27 PM, Ben Hutchings wrote:
> > On Sat, 2014-01-11 at 13:48 -0600, Larry Finger wrote:
> >> On openSUSE systems, the script that installs the firmware for b43 also
> >> unloads and reloads the driver. When the firmware was not previously
> >> available, the driver has stalled at a wait_for_completion(). When the
> >> unload routine releases that hold, the driver encounters structures
> >> that have already been deleted and generates a fatal condition. When
> >> the user does a manual restart, the file system cleanup frequently
> >> results in the firmware files being deleted and the user is never able
> >> to install the firmware. The fix is to change the wait_for_completion()
> >> with a wait_for_completion_timeout() with a 60 second wait period.
> >>
> >> There is a potential race condition; however, the chances that less
> >> than a minute has elapsed between the initial driver load and a
> >> subsequent unload is very unlikely.
> >
> > A minute-long race is 'unlikely' to be hit?  Seriously?!
> 
> Ben,
> 
> If you force a reboot before the minute expires, nothing weird happens. The only 
> race condition happens when the user has to

...remove the module.  Exactly how the bug reporter triggered module
removal seems irrelevant.

> log in, open a terminal, run a 
> script that downloads 13.5 MiB of files from the Internet, and then executes the 
> firmware extraction program. On my 10 Mbps external line and a 2 GHz CPU, that 
> takes 32 s, plus any time to enter the password for a sudo operation. That was 
> the basis for my conclusion that a race is unlikely.
> 
> What is the minimum time that should be allowed for a request_firmware_nowait() 
> to respond? I know we had to go to asynchronous fw loading because the 
> synchronous  version would timeout at 30 s.

You could switch back to synchronous firmware loading soon, as it's not
going to support a usermode helper any more.

But until then, the proper fix for this is going to be to cancel the
waiter earlier in teardown.

Ben.

-- 
Ben Hutchings
Quantity is no substitute for quality, but it's the only one we've got.

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