linux-kernel - Re: [Suspend-devel] [BUG] 3.7-rc regression bisected: s2disk fails to resume image: Processes could not be frozen, cannot continue resuming

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Message-ID: <20130924002111.GB31569@amd.pavel.ucw.cz>
Date:	Tue, 24 Sep 2013 02:21:11 +0200
From:	Pavel Machek <pavel@....cz>
To:	Al Viro <viro@...IV.linux.org.uk>,
	"Rafael J. Wysocki" <rjw@...k.pl>
Cc:	Andrew Savchenko <bircoph@...il.com>,
	suspend-devel@...ts.sourceforge.net, linux-kernel@...r.kernel.org,
	linux-pm@...r.kernel.org
Subject: Re: [Suspend-devel] [BUG] 3.7-rc regression bisected: s2disk fails
 to resume image: Processes could not be frozen, cannot continue resuming

Hi!

> > And from suspend_ioctls.h:
> > #define SNAPSHOT_IOC_MAGIC      '3'
> > #define SNAPSHOT_FREEZE                 _IO(SNAPSHOT_IOC_MAGIC, 1)
> > 
> > My mistake, should be '3' instead of 3.
> 
> OK...  The thing to test, then, is what does __usermodehelper_disable()
> return to freeze_processes().  If that's where this -EAGAIN comes from,
> we at least have a plausible theory re what's going on.
> 
> freeze_processes() uses __usermodehelper_disable() to stop any new userland
> processes spawned by UMH (modprobe, etc.) and waits for ones it might be
> waiting for to complete.  Then it does try_to_freeze_tasks(), which
> freezes remaining userland, carefully skipping the current thread.
> However, it misses the possibility that current thread might have been
> spawned by something that had been launched by UMH, with UMH waiting
> for it.  Which is the case of everything spawned by linuxrc.
> 
> I'd try something like diff below, but I'm *NOT* familiar with swsusp at
> all; it's not for mainline until ACKed by swsusp folks.
> 
> diff --git a/kernel/kmod.c b/kernel/kmod.c
> index fb32636..d968882 100644
> --- a/kernel/kmod.c
> +++ b/kernel/kmod.c
> @@ -571,7 +571,8 @@ int call_usermodehelper_exec(struct subprocess_info *sub_info, int wait)
>  	DECLARE_COMPLETION_ONSTACK(done);
>  	int retval = 0;
>  
> -	helper_lock();
> +	if (!(current->flags & PF_FREEZER_SKIP))
> +		helper_lock();
>  	if (!khelper_wq || usermodehelper_disabled) {
>  		retval = -EBUSY;
>  		goto out;
> @@ -611,7 +612,8 @@ wait_done:
>  out:
>  	call_usermodehelper_freeinfo(sub_info);
>  unlock:
> -	helper_unlock();
> +	if (!(current->flags & PF_FREEZER_SKIP))
> +		helper_unlock();
>  	return retval;
>  }
>  EXPORT_SYMBOL(call_usermodehelper_exec);

PF_FREEZER_SKIP flag is manipulated at about 1000 places, so I'm not
sure this will nest correctly. They seem to be in form of 

|= FREEZER_SKIP
schedule()
&= ~FREEZER_SKIP

so this should be safe, but...

								Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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