netdev - Re: [PATCH 1/2 v5] usbnet: allow status interrupt URB to always be active

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Date:	Fri, 12 Apr 2013 18:08:44 +0800
From:	Ming Lei <tom.leiming@...il.com>
To:	Oliver Neukum <oliver@...kum.org>
Cc:	Dan Williams <dcbw@...hat.com>,
	Elina Pasheva <epasheva@...rrawireless.com>,
	Network Development <netdev@...r.kernel.org>,
	linux-usb <linux-usb@...r.kernel.org>,
	Rory Filer <rfiler@...rrawireless.com>,
	Phil Sutter <phil@....cc>
Subject: Re: [PATCH 1/2 v5] usbnet: allow status interrupt URB to always be active

On Fri, Apr 12, 2013 at 6:02 PM, Oliver Neukum <oliver@...kum.org> wrote:
> On Friday 12 April 2013 17:42:46 Ming Lei wrote:
>> On Fri, Apr 12, 2013 at 2:37 PM, Oliver Neukum <oliver@...kum.org> wrote:
>> >> The work will complete when memory is reclaimed, and the rx/tx path is
>> >> still working, so memory reclaim can continue and the deadlock may not
>> >> be caused, may it?
>> >
>> > Only if the memory allocation goes to the same interface. If the blocking interface
>> > is storage, something bad happens (data loss not deadlock)
>>
>> OK, got it, it should be both since reset can't move on, so
>> memory reclaim can't complete to satisfy the allocation.
>>
>> But I am wondering if it is a case which is worth the consideration.
>
> The remedy is close to trivial, so yes.
>
>> Almost all USB probe() allocate memory with GFP_KERNEL, then
>> the similar scenario(data loss only this time) might happen too. Do we
>> need to fix all USB drivers?
>
> No. We assume that probing happens before the interfaces are used.
> Furthermore, if this turns out to be real, we can reuse your fix for the
> general (w.o. work queue) reset case.

See driver load/bind path(driver_attach()/driver_bound()), both may
happen anytime on unbind interface.

Yes, we can easily address it with the PF_MEMALLOC_NOIO trick,
but can we afford NOIO allocation in probe() path for all drivers?

>
>> Wrt. the usbnet_status_start() API, looks no good reason to call
>> it in another queue context, it should be enough to call it in probe() or
>> bind() if init_status() can be put before info->bind() in usbnet_probe().
>
> We are talking about a generic helper. And the fix is really simple.
> We just pass mem_flags and all is well. If the API can be made better
> at next to no cost, we do it.
>
>> Then looks the mem_flags isn't needed in both cases, and we should
>> always take GFP_NOIO inside the API?
>
> We cannot. GFP_ATOMIC may be needed. And no, we atre not going to
> unconditionally use GFP_ATOMIC :-;

It depend on the probe() case, :-)

Thanks,
-- 
Ming Lei
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