linux-kernel - Re: [PATCH 0/5] V2 usb console improvements series

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Date:	Wed, 17 Mar 2010 09:46:47 -0500
From:	Jason Wessel <jason.wessel@...driver.com>
To:	Alan Stern <stern@...land.harvard.edu>
CC:	gregkh@...e.de, linux-usb@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/5] V2 usb console improvements series

On 03/17/2010 09:30 AM, Alan Stern wrote:
> On Tue, 16 Mar 2010, Jason Wessel wrote:
>
>   
>> Part of having the console stable requires that usb sysrq handler get
>> executed in a tasklet.  The hcd device lock is always held while the
>> driver call backs are executed.  The means you cannot get the sysrq
>> printk's out to the usb console device after the in_flight urb queue
>> is filled.
>>     
>
> Either I don't understand this comment properly or else it is simply
> wrong.  HCDs do not continue to hold their locks when handing completed
> URBs back to drivers.  The giveback call occurs with interrupts
> disabled, but no locks are held.
>
>   

The call back from the urb processing holds a lock.  The sysrq is
invoked from that context, which will do a whole lot of printk()
(sysrq-t for example).   The problem is that writes will queue for a bit
and then cannot get drained because the hcd lock is not available. 
Using a tasklet seemed like the obvious choice to move the sysrq
processing to a context where the hcd lock is not likely to be in use.

> Does this mean one of the patches in your series can be simplified?
>   

I will further simplify if you have some recommendations.

Thanks,
Jason.
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