linux-kernel - Re: [linux-pm] [PATCH 2/9] PM: suspend_block: Add driver to access suspend blockers from user-space

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Message-ID: <z2wd6200be21004222054qaf4a0b0doa13aea4c7ef049d7@mail.gmail.com>
Date:	Thu, 22 Apr 2010 20:54:59 -0700
From:	Arve Hjønnevåg <arve@...roid.com>
To:	Matt Helsley <matthltc@...ibm.com>
Cc:	linux-pm@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
	Len Brown <len.brown@...el.com>, linux-doc@...r.kernel.org,
	Jesse Barnes <jbarnes@...tuousgeek.org>,
	Magnus Damm <damm@...l.co.jp>
Subject: Re: [linux-pm] [PATCH 2/9] PM: suspend_block: Add driver to access 
	suspend blockers from user-space

2010/4/22 Matt Helsley <matthltc@...ibm.com>:
...
> On Thu, Apr 22, 2010 at 06:08:51PM -0700, Arve Hjønnevåg wrote:
>> +To create a suspend_blocker from user-space, open the suspend_blocker device:
>> +    fd = open("/dev/suspend_blocker", O_RDWR | O_CLOEXEC);
>> +then call:
>> +    ioctl(fd, SUSPEND_BLOCKER_IOCTL_INIT(strlen(name)), name);
>
> Why not initialize the user suspend blocker struct by default and then
> allow each BLOCK to specify the name? Also, my guess is it's not
> really a name so much as a description of why we're blocking suspend,
> right?
>

There are stats tracked as long as the suspend blocker exists.
Specifying a new name every time you block suspend would make this
less useful.

> Should the kernel reject empty strings or strings composed only of
> "non-printing" characters?
>

Is there an existing function that check if a sting is "unsafe". If
so, I can add a call to this.

>> +
>> +To activate a suspend_blocker call:
>> +    ioctl(fd, SUSPEND_BLOCKER_IOCTL_BLOCK);
>> +
>> +To unblock call:
>> +    ioctl(fd, SUSPEND_BLOCKER_IOCTL_UNBLOCK);
>
> lsof will show which tasks hold the device open but not which ones
> are blocking suspend. If merely keeping the device open corresponded to
> blocking suspend then this would be obvious and no ioctls would be
> necessary -- just write() the name/description.

We track more information about suspend blockers than this.

>
> Do you block/unblock often enough that frequent open/close of the device are
> a problem?

Yes.

...
>> +
>> +static DEFINE_MUTEX(ioctl_lock);
>
> nit: Usually locks protect data -- not functions.
>
> Couldn't this be part of the user_suspend_blocker struct? That would allow
> the description/name to change as described above.

It mainly protects the allocation of that struct, so no. Allocating a
separate struct in open would work, but does not seem worth it at the
moment.

>
>> +
>> +struct user_suspend_blocker {
>> +     struct suspend_blocker  blocker;
>> +     char                    name[0];
>> +};
>
> <snip>
>
> Cheers,
>        -Matt Helsley
>



-- 
Arve Hjønnevåg
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