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From: Alan Stern <stern@rowland.harvard.edu>
To: Miklos Szeredi <miklos@szeredi.hu>
Cc: rjw@sisk.pl, <linux-kernel@vger.kernel.org>,
	<ncunningham@crca.org.au>, <linux-pm@lists.linux-foundation.org>
Subject: Re: [linux-pm] Freezer: Don't count threads waiting for frozen filesystems.
Date: Wed, 29 Oct 2008 11:28:34 -0400 (EDT)	[thread overview]
Message-ID: <Pine.LNX.4.44L0.0810291122220.2189-100000@iolanthe.rowland.org> (raw)
In-Reply-To: <E1KvCN9-00081i-KF@pomaz-ex.szeredi.hu>

On Wed, 29 Oct 2008, Miklos Szeredi wrote:

> Actually I was thinking of an rw-semaphore, not a mutex.  But yeah
> that still has scalability problems.  But it could be done with custom
> locking primitives, optimized for this case:
> 
>    suspend_disable();
>    /* driver stuff */
>    suspend_enable();

Yes, it could be done.  And the overhead could be minimized by using
per-CPU variables.  It would still be an awful lot of work, and easy to
get wrong.

> > The problem with unrestricted freezing shows up when you freeze tasks
> > that hold a mutex or other sort of lock.  If this mutex is needed later
> > on for suspending a device then the suspend will hang, because a frozen
> > task can't release any mutexes.
> 
> I did a random sampling of ->suspend() callbacks, and they don't seem
> to be taking mutexes.  Does that happen at all?

It does, particularly among drivers that do runtime PM, which is 
becoming more and more important.

Besides, suspend has to synchronize with I/O somehow.  Right now that 
is handled by making suspend wait until no tasks are doing I/O (because 
they are all frozen).  If you allow tasks to be frozen at more or less 
arbitrary times, while holding arbitrary locks, then you may end up 
freezing a task that's in the middle of I/O.  That should certainly 
block the suspend (not to mention messing up the I/O operation).

> Did anybody ever try modifying the freezer for suspend (not
> hibernate), so that it allows tasks not in running state to freeze?
> If not, I think that's an experiment worth doing.

What happens if the reason the task isn't running is because it's 
waiting for I/O to complete?  I just don't think this can be made to 
work.

Alan Stern


  reply	other threads:[~2008-10-29 15:28 UTC|newest]

Thread overview: 59+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <1224886068.6478.21.camel@nigel-laptop>
2008-10-26 20:01 ` Rafael J. Wysocki
2008-10-27 11:12   ` Miklos Szeredi
2008-10-27 11:20     ` Nigel Cunningham
2008-10-27 11:37       ` Rafael J. Wysocki
2008-10-27 11:40         ` Nigel Cunningham
2008-10-27 12:38           ` Miklos Szeredi
2008-10-27 20:59             ` Nigel Cunningham
2008-10-27 21:09               ` Miklos Szeredi
2008-10-27 22:13                 ` Nigel Cunningham
2008-10-28 20:25                   ` Miklos Szeredi
2008-10-28 21:29                     ` Nigel Cunningham
2008-10-28 21:51                       ` Miklos Szeredi
2008-10-28 22:00                         ` Rafael J. Wysocki
2008-10-28 22:02                           ` Miklos Szeredi
2008-10-28 22:21                             ` Rafael J. Wysocki
2008-10-28 23:21                               ` Miklos Szeredi
2008-10-28 23:59                                 ` Rafael J. Wysocki
2008-10-29  8:10                                   ` Miklos Szeredi
2008-10-29 13:51                                     ` Alan Stern
2008-10-29 14:50                                       ` Miklos Szeredi
2008-10-29 15:28                                         ` Alan Stern [this message]
2008-10-29 15:50                                           ` Miklos Szeredi
2008-10-29 16:17                                             ` Alan Stern
2008-10-29 16:10                                           ` Miklos Szeredi
2008-10-29 20:37                                             ` Alan Stern
2008-10-29 21:11                                               ` Rafael J. Wysocki
2008-10-29 21:45                                                 ` Nigel Cunningham
2008-10-29 22:07                                                   ` Rafael J. Wysocki
2008-10-29 23:53                                                     ` Miklos Szeredi
2008-11-09 13:44                                                       ` Pavel Machek
2008-10-29 23:48                                                   ` Miklos Szeredi
2008-10-30 13:04                                                     ` Nigel Cunningham
2008-10-30 13:56                                                       ` Alan Stern
2008-10-30 21:44                                                         ` Nigel Cunningham
2008-10-31  8:49                                                           ` Miklos Szeredi
2008-10-31  9:10                                                             ` Nigel Cunningham
2008-10-31  9:16                                                               ` Miklos Szeredi
2008-10-31 11:28                                                                 ` Nigel Cunningham
2008-10-31 12:44                                                                   ` Miklos Szeredi
2008-10-31 21:11                                                                     ` Nigel Cunningham
2008-10-29 23:43                                               ` Miklos Szeredi
2008-10-30 13:54                                                 ` Alan Stern
2008-10-30 14:39                                                   ` Miklos Szeredi
2008-10-30 17:07                                                     ` Alan Stern
2008-10-30 17:43                                                       ` Miklos Szeredi
2008-10-30 20:17                                                     ` Rafael J. Wysocki
2008-11-15 16:58                                                     ` Pavel Machek
2008-10-29 23:56                                               ` Matthew Garrett
2008-10-28 22:03                         ` Nigel Cunningham
2008-10-28 23:04                     ` Nigel Cunningham
2008-10-28 23:12                       ` Miklos Szeredi
2008-10-28 23:17                         ` Nigel Cunningham
2008-10-28 23:24                           ` Miklos Szeredi
2008-10-28 23:41                             ` Nigel Cunningham
2008-10-28 23:45                               ` Miklos Szeredi
2008-10-28 23:50                                 ` Miklos Szeredi
2008-10-28 23:58                                   ` Nigel Cunningham
2008-10-28 23:54                                 ` Nigel Cunningham
2008-10-27 11:37       ` Miklos Szeredi

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