linux-kernel - Re: [PATCH RFC] slub: do not drop slab_mutex for sysfs_slab

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Date:	Thu, 6 Feb 2014 13:13:20 -0600 (CST)
From:	Christoph Lameter <cl@...ux.com>
To:	Vladimir Davydov <vdavydov@...allels.com>
cc:	penberg@...nel.org, akpm@...ux-foundation.org, rientjes@...gle.com,
	mhocko@...e.cz, linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	devel@...nvz.org
Subject: Re: [PATCH RFC] slub: do not drop slab_mutex for
 sysfs_slab_{add,remove}

On Thu, 6 Feb 2014, Vladimir Davydov wrote:

> Hmm... IIUC the only function of concern is kobject_uevent() -
> everything else called from sysfs_slab_{add,remove} is a mix of kmalloc,
> kfree, mutex_lock/unlock - in short, nothing dangerous. There we do
> call_usermodehelper(), but we do it with UMH_WAIT_EXEC, which means
> "wait for exec only, but not for the process to complete". An exec
> shouldn't issue any slab-related stuff AFAIU. At least, I tried to run
> the patched kernel with lockdep enabled and got no warnings at all when
> getting uevents about adding/removing caches. That's why I started to
> doubt whether we really need this lock...
>
> Please correct me if I'm wrong.

I have had this deadlock a couple of years ago. Sysfs seems to change over
time. Not sure if that is still the case.

> > I would be very thankful, if you can get that actually working reliably
> > without deadlock issues.
>
> If there is no choice rather than moving sysfs_slab_{add,remove} out of
> the slab_mutex critical section, I'll have to do it that way. But first
> I'd like to make sure it cannot be done with less footprint.

I am all for holding the lock as long as possible.

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