Re: [linux-pm] Is it supposed to be ok to call del_gendisk while userspace is frozen?

[Jens Axboe <jens.axboe@oracle.com>]

On Tue, Feb 16 2010, Alan Stern wrote:
> On Mon, 15 Feb 2010, Rafael J. Wysocki wrote:
> 
> > On Monday 15 February 2010, Maxim Levitsky wrote:
> > > On Sat, 2010-02-13 at 15:29 +0200, Maxim Levitsky wrote: 
> > > > I noticed that currently calling del_gendisk leads to sure deadlock if
> > > > attemped from .suspend or .resume functions.
> > 
> > Well, it shouldn't be called from there, then.
> 
> Even if drivers avoid calling it from within suspend methods, they have
> to be able to call it from within resume methods.  After all, the
> resume method may find that the disk's device has vanished.

del_gendisk() needs process context at least, since it'll sleep (not
just for sync/invalidate, but other parts of the destruction as well).

> > > > Something like that:
> > > > 
> > > > [<ffffffff8106620a>] ? prepare_to_wait+0x2a/0x90
> > > > [<ffffffff810790bd>] ? trace_hardirqs_on+0xd/0x10
> > > > [<ffffffff8140db12>] ? _raw_spin_unlock_irqrestore+0x42/0x80
> > > > [<ffffffff8112a390>] ? bdi_sched_wait+0x0/0x20
> > > > [<ffffffff8112a39e>] bdi_sched_wait+0xe/0x20
> > > > [<ffffffff8140af6f>] __wait_on_bit+0x5f/0x90
> > > > [<ffffffff8112a390>] ? bdi_sched_wait+0x0/0x20
> > > > [<ffffffff8140b018>] out_of_line_wait_on_bit+0x78/0x90
> > > > [<ffffffff81065fd0>] ? wake_bit_function+0x0/0x40
> > > > [<ffffffff8112a2d3>] ? bdi_queue_work+0xa3/0xe0
> > > > [<ffffffff8112a37f>] bdi_sync_writeback+0x6f/0x80
> > > > [<ffffffff8112a3d2>] sync_inodes_sb+0x22/0x120
> > > > [<ffffffff8112f1d2>] __sync_filesystem+0x82/0x90
> > > > [<ffffffff8112f3db>] sync_filesystem+0x4b/0x70
> > > > [<ffffffff811391de>] fsync_bdev+0x2e/0x60
> > > > [<ffffffff812226be>] invalidate_partition+0x2e/0x50
> > > > [<ffffffff8116b92f>] del_gendisk+0x3f/0x140
> > > > [<ffffffffa00c0233>] mmc_blk_remove+0x33/0x60 [mmc_block]
> > > > [<ffffffff81338977>] mmc_bus_remove+0x17/0x20
> > > > [<ffffffff812ce746>] __device_release_driver+0x66/0xc0
> > > > [<ffffffff812ce89d>] device_release_driver+0x2d/0x40
> > > > [<ffffffff812cd9b5>] bus_remove_device+0xb5/0x120
> > > > [<ffffffff812cb46f>] device_del+0x12f/0x1a0
> > > > [<ffffffff81338a5b>] mmc_remove_card+0x5b/0x90
> > > > [<ffffffff8133ac27>] mmc_sd_remove+0x27/0x50
> > > > [<ffffffff81337d8c>] mmc_resume_host+0x10c/0x140
> > > > [<ffffffffa00850e9>] sdhci_resume_host+0x69/0xa0 [sdhci]
> > > > [<ffffffffa0bdc39e>] sdhci_pci_resume+0x8e/0xb0 [sdhci_pci]
> > > > 
> > > > bdi_queue_work seems to be the problem.
> > > > 
> > > > Some device drivers need to remove their cards logically in .suspend,
> > > > because the card is removable, and can be changed while system is
> > > > suspended.
> > 
> > I don't know how to resolve this right now.
> 
> This is a matter for Jens.  Is the bdi writeback task freezable?  If it
> is, should it be made unfreezable?

I'm not a big expect on what tasks should be freezable or not. As it
stands, the writeback tasks will attempt to freeze and thaw with the
system. I guess that screws the sync from resume call, since it's not
running and the sync will wait for it to retrieve and finish that work
item.

To the suspend experts - can we safely mark the writeback tasks as
non-freezable?

-- 
Jens Axboe