Re: [PATCH 5.19 0191/1157] hwmon: (sht15) Fix wrong assumptions in device remove callback

[Uwe Kleine-König]

Hello Greg,

On Tue, Aug 16, 2022 at 12:40:50PM +0200, Greg Kroah-Hartman wrote:
> On Mon, Aug 15, 2022 at 11:49:11PM +0200, Uwe Kleine-König wrote:
> > On Mon, Aug 15, 2022 at 07:52:27PM +0200, Greg Kroah-Hartman wrote:
> > > From: Uwe Kleine-König <u.kleine-koenig@xxxxxxxxxxxxxx>
> > > 
> > > [ Upstream commit 7d4edccc9bbfe1dcdff641343f7b0c6763fbe774 ]
> > > 
> > > Taking a lock at the beginning of .remove() doesn't prevent new readers.
> > > With the existing approach it can happen, that a read occurs just when
> > > the lock was taken blocking the reader until the lock is released at the
> > > end of the remove callback which then accessed *data that is already
> > > freed then.
> > > 
> > > To actually fix this problem the hwmon core needs some adaption. Until
> > > this is implemented take the optimistic approach of assuming that all
> > > readers are gone after hwmon_device_unregister() and
> > > sysfs_remove_group() as most other drivers do. (And once the core
> > > implements that, taking the lock would deadlock.)
> > > 
> > > So drop the lock, move the reset to after device unregistration to keep
> > > the device in a workable state until it's deregistered. Also add a error
> > > message in case the reset fails and return 0 anyhow. (Returning an error
> > > code, doesn't stop the platform device unregistration and only results
> > > in a little helpful error message before the devm cleanup handlers are
> > > called.)
> > > 
> > > Signed-off-by: Uwe Kleine-König <u.kleine-koenig@xxxxxxxxxxxxxx>
> > > Link: https://lore.kernel.org/r/20220725194344.150098-1-u.kleine-koenig@xxxxxxxxxxxxxx
> > > Signed-off-by: Guenter Roeck <linux@xxxxxxxxxxxx>
> > > Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
> > 
> > Does this mean my concerns I expressed in the mail with Message-Id:
> > 20220814155638.idxnihylofsxqlql@xxxxxxxxxxxxxx were not taken into
> > consideration?
> 
> I can't find that message-id on lore.kernel.org, do you have a link?

Oh, I missed your request earlier. No I don't have a link, the mail was
sent to Sasha Levin, Jean Delvare, Guenter Roeck and stable-commits as I
just replied to the "note to let you know that I've just added the patch
titled [...] to the 5.19-stable tree".

I wrote:
> I'd say adding this patch to stable isn't right. The problem existed
> since v3.0 (commit cc15c7ebb424e45ba2c5ceecbe52d025219ee970) and was
> never reported to be hit in practise. And given that the problem doesn't
> go away completely but (AFAICT) just opens the possibility for a hwmon
> core fix, I'd say it's not worth a backport.

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | https://www.pengutronix.de/ |

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