linux-kernel - Re: freeze vs freezer

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Date:	Thu, 3 Jan 2008 23:31:05 +0100
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	nigel@...el.suspend2.net
Cc:	Pavel Machek <pavel@....cz>, Kyle Moffett <mrmacman_g4@....com>,
	Matthew Garrett <mjg59@...f.ucam.org>,
	David Chinner <dgc@....com>,
	Jeremy Fitzhardinge <jeremy@...p.org>, xfs-masters@....sgi.com,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: freeze vs freezer

On Thursday, 3 of January 2008, Nigel Cunningham wrote:
> Hi.
> 
> Rafael J. Wysocki wrote:
> > On Wednesday, 2 of January 2008, Nigel Cunningham wrote:
> >> Pavel Machek wrote:
> >>>>>>>> So how do you handle threads that are blocked on I/O or a lock  
> >>>>>>>> during the system freeze process, then?
> >>>>>>> We wait until they can continue.
> >>>>>> So if I have a process blocked on an unavilable NFS mount, I can't
> >>>>>> suspend?
> >>>>> That's correct, you can't.
> >>>>>
> >>>>> [And I know what you're going to say. ;-)]
> >>>> Why exactly does suspend/hibernation depend on "TASK_INTERRUPTIBLE"  
> >>>> instead of a zero preempt_count()?  Really what we should do is just  
> >>>> iterate over all of the actual physical devices and tell each one  
> >>>> "Block new IO requests preemptably, finish pending DMA, put the  
> >>>> hardware in low-power mode, and prepare for suspend/hibernate".  As  
> >>>> long as each driver knows how to do those simple things we can have  
> >>>> an entirely consistent kernel image for both suspend and for  
> >>>> hibernation.
> >>> "each driver" means this is a lot of work. But yes, that is probably
> >>> way to go, and patch would be welcome.
> >> Yes, that does work. It's what I've done in my (preliminary) support for
> >> fuse.
> > 
> > Hmm, can you please elaborate a bit?
> 
> Sorry. I wasn't very unambiguous, was I? And I'm not sure now whether
> you're meaning "How does fuse support relate to freezing block devices?"
> or "What's this about fuse support?". Let me therefore seek to answer
> both questions:
> 
> Higher level, I know (filesystems rather than block devices), but I was
> meaning the general concept of blocking new requests and completing
> existing ones worked fine for the supposedly impossible fuse support.
> 
> Re fuse support, let me start by saying "I know this doesn't handle all
> situations, but I think it's a good enough proof-of-concept implementation".
> 
> I added some simple hooks to the code for submitting new work to fuse
> threads.
> 
> #define FUSE_MIGHT_FREEZE(superblock, desc) \
> do { \
>        int printed = 0; \
>        while(superblock->s_frozen != SB_UNFROZEN) { \
>                if (!printed) { \
>                        printk("%d frozen in " desc ".\n", current->pid); \
>                        printed = 1; \
>                } \
>                try_to_freeze(); \
>                yield(); \
>        } \
> } while (0)
> 
> On top of this, I made a (too simple at the moment) freeze_filesystems
> function which iterates through &super_blocks in reverse order, freezing
> fuse filesystems or ordinary ones. I say 'too simple' because it doesn't
> currently allow for the possibility of someone mounting (say) ext3 on
> fuse, but that would just be an extension of what's already done.
> 
> The end result is:
> 
> int freeze_processes(void)
> {
>         int error;
> 
>         printk(KERN_INFO "Stopping fuse filesystems.\n");
>         freeze_filesystems(FS_FREEZER_FUSE);
>         freezer_state = FREEZER_FILESYSTEMS_FROZEN;
>         printk(KERN_INFO "Freezing user space processes ... ");
>         error = try_to_freeze_tasks(FREEZER_USER_SPACE);
>         if (error)
>                 goto Exit;
>         printk(KERN_INFO "done.\n");
> 
>         sys_sync();
>         printk(KERN_INFO "Stopping normal filesystems.\n");
>         freeze_filesystems(FS_FREEZER_NORMAL);
>         freezer_state = FREEZER_USERSPACE_FROZEN;
>         printk(KERN_INFO "Freezing remaining freezable tasks ... ");
>         error = try_to_freeze_tasks(FREEZER_KERNEL_THREADS);
>         if (error)
>                 goto Exit;
>         printk(KERN_INFO "done.");
>         freezer_state = FREEZER_FULLY_ON;
>  Exit:
>         BUG_ON(in_atomic());
>         printk("\n");
>         return error;
> }
> 
> Sorry if that's more info than you wanted.

No, that's fine, thanks.

Greetings,
Rafael
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