linux-kernel - Re: sysfs root link count broken in 2.6.22-git5

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Date:	Thu, 19 Jul 2007 02:44:54 +0200
From:	"Kay Sievers" <kay.sievers@...y.org>
To:	"Jean Delvare" <khali@...ux-fr.org>
Cc:	"Greg KH" <gregkh@...e.de>, "Tejun Heo" <htejun@...il.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: sysfs root link count broken in 2.6.22-git5

On 7/18/07, Jean Delvare <khali@...ux-fr.org> wrote:
> On Tue, 17 Jul 2007 20:38:28 -0700, Greg KH wrote:
> > On Tue, Jul 17, 2007 at 11:05:30PM +0200, Jean Delvare wrote:
> > > This breaks libsensors. libsensors uses libsysfs, and libsysfs is not
> > > very smart in that it will initialize successfully even if sysfs is not
> > > mounted.
> >
> > libsysfs isn't smart at all, and isn't even supported anymore.  I'd
> > really suggest droping it entirely, it isn't worth it.
>
> Agreed, except that I do not have the time for this right now. I want
> to get lm-sensors-3.0.0 ready for a release candidate first. What
> really matters for this is to get the API ready. Implementation details
> will come later.
>
> > > So I added tests after the initialization, to make sure that
> > > sysfs is really there. These tests are:
> > > * The mount point exists.
> > > * The mount point is really mounted.
> >
> > Do you know of a 2.6 based distro that does not mount sysfs at /sys?  We
> > took that check out a long time ago in udev and no one has complained :)
>
> I don't know of any 2.6-based distro not mounting sysfs at /sys, but I
> know of 2.4-based distros not mounting sysfs at all ;) libsensors
> supports both 2.4 and 2.6 kernels, so being able to tell whether sysfs
> is mounted or not, matters.
>
> > > The code looks like:
> > >
> > >        if (sysfs_get_mnt_path(sensors_sysfs_mount, NAME_MAX)
> > >          || stat(sensors_sysfs_mount, &statbuf) < 0
> > >          || statbuf.st_nlink <= 2)      /* Empty directory */
> > >                 return 0;           /* Failure */
> > >
> > > This works OK with 2.6.22.1, but the last test fails with the current
> > > git kernel even when sysfs is mounted.
> >
> > Yeah, but is checking the number of hard links in the directory a safe
> > way to always verify that it isn't empty?
>
> I think so, yes. To the best of my knowledge, it has worked on all
> Unix-like systems for decades. There are other ways, but this is by far
> the less expensive.

Well, just check if /sys/devices/ exists, that should be cheap enough. :)

Kay
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