Re: [PATCH] firmware loader: don't cancel _nowait requests when helper is not yet available

[Stephen Boyd <sboyd@codeaurora.org>]

On 3/16/2012 10:51 PM, Linus Torvalds wrote:
> On Fri, Mar 16, 2012 at 7:47 PM, Stephen Boyd <sboyd@codeaurora.org> wrote:
>> -       if (WARN_ON(usermodehelper_is_disabled())) {
>> +       if (is_sleep_task()) {
>>                dev_err(device, "firmware: %s will not be loaded\n", name);
> Stop these idiotic games already!
>
> It's very simple: you cannot load firmware while the system is not
> readt. Your moronic "let's change the test to something else" is
> entirely and utterly misguided and totally misses the point.
>
> It's not about sleeping, and it's not about anything even *remotely*
> about that. Stop the idiocy already.
>
> How hard is it to understand? How many times do people have to tell you?
>
> That warning is very much valid during bootup, and that warning has
> been *seen* during bootup. For example, try to compile in most
> wireless drivers as non-modular, and that warning *has* to trigger.

Ok. I like where nowait() is going in the other part of the thread but
I'm still confused about when request_firmware() is correct to use. It
seems that the function is inherently racy with freezing. Does every
user of request_firmware() need to synchronize with freezing?

For example, if one CPU is in the middle of a driver probe that makes a
request_firmware() call and another CPU is starting to suspend we will
have a race between usermodehelpers being disabled and the
request_firmware() call acquiring the usermodehelper rwsem. If the
suspending CPU wins the race it will disable usermodehelpers and the
request_firmware() call will return -EBUSY and warn.

CPU0                           CPU1
driver_probe()                 suspend_prepare()
  ...                            usermodehelper_disable()
  _request_firmware()              down_write(&umhelper_sem)
                                   usermodehelper_disabled = 1
                                   up_write(&umhelper_sem)
    down_read(&umhelper_sem)       ....
    WARN_ON(...)                   freeze_processes()
    <freeze>                       


Hopefully I'm missing something here? This is all theoretical right now
as I haven't actually seen any of this in practice.

-- 
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The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum.