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Date:   Wed, 21 Jul 2021 13:30:29 +0200
From:   Greg KH <>
To:     Luis Chamberlain <>
Cc:     Tejun Heo <>,,,,,,,,,,,,,,,,,,,,,,,,,
Subject: Re: [PATCH v4] sysfs: fix kobject refcount to address races with
 kobject removal

On Thu, Jul 01, 2021 at 03:48:16PM -0700, Luis Chamberlain wrote:
> On Fri, Jun 25, 2021 at 02:56:03PM -0700, Luis Chamberlain wrote:
> > On Thu, Jun 24, 2021 at 01:09:03PM +0200, Greg KH wrote:
> > > thanks for making this change and sticking with it!
> > > 
> > > Oh, and with this change, does your modprobe/rmmod crazy test now work?
> > 
> > It does but I wrote a test_syfs driver and I believe I see an issue with
> > this. I'll debug a bit more and see what it was, and I'll then also use
> > the driver to demo the issue more clearly, and then verification can be
> > an easy selftest test.
> OK my conclusion based on a new selftest driver I wrote is we can drop
> this patch safely. The selftest will cover this corner case well now.
> In short: the kernfs active reference will ensure the store operation
> still exists. The kernfs mutex is not enough, but if the driver removes
> the operation prior to getting the active reference, the write will just
> fail. The deferencing inside of the sysfs operation is abstract to
> kernfs, and while kernfs can't do anything to prevent a driver from
> doing something stupid, it at least can ensure an open file ensure the
> op is not removed until the operation completes.

Ok, so all is good?  Then why is your zram test code blowing up so
badly?  Where is the reference counting going wrong?


greg k-h

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