Re: [dm-devel] [PATCH 5.15] dm init: add dm-mod.waitfor to wait for asynchronously probed block devices

[Greg KH <gregkh@linuxfoundation.org>]

On Mon, Jul 17, 2023 at 09:57:28AM +0800, Mark-PK Tsai wrote:
> > On Sun, Jul 16, 2023, 11:16 AM Greg KH <gregkh@linuxfoundation.org> wrote:
> > 
> > > On Thu, Jul 13, 2023 at 01:58:37PM +0800, Mark-PK Tsai wrote:
> > > > From: Peter Korsgaard <peter@korsgaard.com>
> > > > 
> > > > Just calling wait_for_device_probe() is not enough to ensure that
> > > > asynchronously probed block devices are available (E.G. mmc, usb), so
> > > > add a "dm-mod.waitfor=<device1>[,..,<deviceN>]" parameter to get
> > > > dm-init to explicitly wait for specific block devices before
> > > > initializing the tables with logic similar to the rootwait logic that
> > > > was introduced with commit  cc1ed7542c8c ("init: wait for
> > > > asynchronously scanned block devices").
> > > > 
> > > > E.G. with dm-verity on mmc using:
> > > > dm-mod.waitfor="PARTLABEL=hash-a,PARTLABEL=root-a"
> > > > 
> > > > [    0.671671] device-mapper: init: waiting for all devices to be 
> > > available before creating mapped devices
> > > > [    0.671679] device-mapper: init: waiting for device PARTLABEL=hash-a 
> > > ...
> > > > [    0.710695] mmc0: new HS200 MMC card at address 0001
> > > > [    0.711158] mmcblk0: mmc0:0001 004GA0 3.69 GiB
> > > > [    0.715954] mmcblk0boot0: mmc0:0001 004GA0 partition 1 2.00 MiB
> > > > [    0.722085] mmcblk0boot1: mmc0:0001 004GA0 partition 2 2.00 MiB
> > > > [    0.728093] mmcblk0rpmb: mmc0:0001 004GA0 partition 3 512 KiB, 
> > > chardev (249:0)
> > > > [    0.738274]  mmcblk0: p1 p2 p3 p4 p5 p6 p7
> > > > [    0.751282] device-mapper: init: waiting for device PARTLABEL=root-a 
> > > ...
> > > > [    0.751306] device-mapper: init: all devices available
> > > > [    0.751683] device-mapper: verity: sha256 using implementation 
> > > "sha256-generic"
> > > > [    0.759344] device-mapper: ioctl: dm-0 (vroot) is ready
> > > > [    0.766540] VFS: Mounted root (squashfs filesystem) readonly on 
> > > device 254:0.
> > > > 
> > > > Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
> > > > Signed-off-by: Mike Snitzer <snitzer@kernel.org>
> > > > Cc: stable@vger.kernel.org
> > > > Signed-off-by: Mark-PK Tsai <mark-pk.tsai@mediatek.com>
> > > > ---
> > > >  .../admin-guide/device-mapper/dm-init.rst     |  8 +++++++
> > > >  drivers/md/dm-init.c                          | 22 ++++++++++++++++++-
> > > >  2 files changed, 29 insertions(+), 1 deletion(-)
> > >
> > > What is the git commit id of this change in Linus's tree?
> > >
> > > thanks,
> > >
> > > greg k-h
> > >
> > >
> > 
> > Hey Greg,
> > 
> > This change shouldn't be backported to stable@. It is a feature, if
> > Mark-PK feels they need it older kernels they need to carry the change
> > in their own tree. Or at a minimum they need to explain why this
> > change is warranted in stable@.
> 
> Thanks for your comment.
> The reason why we think this should be backported to stable kernel is
> that it actually fix the potential race condition when make block
> device probe async in stable kernel.
> And we'd like to fix this upstream rather than just take it in
> our custom tree.

Potential race condition, is this actually able to be hit in real life?

thanks,

greg k-h

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