linux-kernel - Re: apm emulation driver broken ?

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Message-ID: <cda58cb80711160929i4dbac6b7xb9d1bcc9223ddd7b@mail.gmail.com>
Date:	Fri, 16 Nov 2007 18:29:56 +0100
From:	"Franck Bui-Huu" <vagabon.xyz@...il.com>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
Cc:	linux-pm@...ts.linux-foundation.org,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: apm emulation driver broken ?

On Nov 16, 2007 5:20 PM, Rafael J. Wysocki <rjw@...k.pl> wrote:
>
> The freezer doesn't regard the current task as freezable.
>

Hmm, I don't get your point.

If I understood this driver correctly, several processes can be
waiting for a suspend event by reading /dev/apm_bios, apmd (the _user_
space daemon) can be one of them.

Then another process asks to suspend the system by calling 'apm -s',
which results in a apm_ioctl() call. This process will basically
execute:

	err = queue_suspend_event(APM_USER_SUSPEND, as);
	flags = current->flags;
	wait_event_interruptible(apm_suspend_waitqueue,
			as->suspend_state == SUSPEND_DONE);

It's basically waiting for the waiters to ack the event. But it won't
be the process that is going to suspend the system, right ?

So now all waiting processes are waken up and need to acknolwedge the
event for the system to actually suspend. So they need to call
apm_ioctl(). They'll basically do:

	flags = current->flags;
	wait_event(apm_suspend_waitqueue,
			as->suspend_state == SUSPEND_DONE);

Except for the last acknowledging process which will do instead:

	apm_suspend();

It's a call to pm_suspend().

So you can see that the process which initiates the suspend, the one
that calls 'apm -s', is not the current process but is going to be
waken up by the fake signal sent by freeze_task().

One of the consequence I can see is at this time 'as->result' won't be
setup, so the return value of apm_ioctl() may be wrong.

As I said, I'm not familiar with this code, so please correct me if
I'm wrong.

BTW, how does try_to_freeze_tasks() deal with user land thread waiting
in the UNINTERRUPTIBLE state ?

Thanks.

               Franck
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