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Message-Id: <200711162220.56494.rjw@sisk.pl>
Date: Fri, 16 Nov 2007 22:20:55 +0100
From: "Rafael J. Wysocki" <rjw@...k.pl>
To: "Franck Bui-Huu" <vagabon.xyz@...il.com>
Cc: linux-pm@...ts.linux-foundation.org,
lkml <linux-kernel@...r.kernel.org>
Subject: Re: apm emulation driver broken ?
On Friday, 16 of November 2007, Franck Bui-Huu wrote:
> 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.
Ah, that. Yes, I see your point.
However, using PF_NOFREEZE to prevent this from happening doesn't seem to be
a good idea.
I'd probably use wait_event_freezable() (defined in include/linux/freezer.h)
for that.
> As I said, I'm not familiar with this code, so please correct me if
> I'm wrong.
No, you're not wrong and I have overlooked the problem.
> BTW, how does try_to_freeze_tasks() deal with user land thread waiting
> in the UNINTERRUPTIBLE state ?
It tries to send them fake signals and waits for them to freeze. If they don't
freeze within the timeout, it fails and clears their TIF_FREEZE bits.
Greetings,
Rafael
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