netdev - Re: [PATCH 01/12] usbnet: introduce usbnet 3 command helpers

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Date:	Fri, 12 Oct 2012 23:17:57 +0800
From:	Ming Lei <ming.lei@...onical.com>
To:	Oliver Neukum <oneukum@...e.de>
Cc:	Alan Stern <stern@...land.harvard.edu>,
	"David S. Miller" <davem@...emloft.net>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	netdev@...r.kernel.org, linux-usb@...r.kernel.org, jkosina@...e.com
Subject: Re: [PATCH 01/12] usbnet: introduce usbnet 3 command helpers

On Fri, Oct 12, 2012 at 9:51 PM, Oliver Neukum <oneukum@...e.de> wrote:
> On Thursday 11 October 2012 10:36:22 Alan Stern wrote:
>
>> It's worse than you may realize.  When a SCSI disk is suspended, all of
>> its ancestor devices may be suspended too.  Pages can't be read in from
>> the drive until all those ancestors are resumed.  This means that all
>> runtime resume code paths for all drivers that could be bound to an
>> ancestor of a block device must avoid GFP_KERNEL.  In practice it's
>> probably easiest for the runtime PM core to use tsk_set_allowd_gfp()
>> before calling any runtime_resume method.
>>
>> Or at least, this will be true when sd supports nontrivial autosuspend.
>
> Up to now, I've found three driver for which tsk_set_allowd_gfp() wouldn't
> do the job. They boil down into two types of errors. That is surprisingly good.

Looks all are very good examples, :-)

>
> First we have workqueues. bas-gigaset is a good example.
> The driver kills a scheduled work in pre_reset(). If this is done synchronously
> the driver may need to wait for a memory allocation inside the work.
> In principle we could provide a workqueue limited to GFP_NOIO. Is that worth
> it, or do we just check?

The easiest way is to always call tsk_set_allowd_gfp(~GFP_IOFS) in the
start of work function under the situation, and restore the flag in the end
of the work function.

>
> Second there is a problem just like priority inversion with realtime tasks.
> usb-skeleton and ati_remote2
> They take mutexes which are also taken in other code paths. So the error
> handler may need to wait for a mutex to be dropped which can only happen
> if a memory allocation succeeds, which is waiting for the error handler.

Suppose mutex_lock(A) is called in pre_reset(), one solution is that
always calling tsk_set_allowd_gfp(~GFP_IOFS) before each mutex_lock(A).
We can do it only for devices with storage interface in current
configuration.


Thanks，
--
Ming Lei
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