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    SubjectRe: sysfs root link count broken in 2.6.22-git5
    Hi Greg,

    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.

    > Isn't there some glibc
    > function that can detect the mount point of a filesystem or directory?
    > Something in glibc parses /proc/mounts for something, I can't remember
    > what it is right now though, sorry.

    Maybe getmntent(3)? Sure I could use this, but how expensive compared
    to a single stat(2).

    > Again, I recommend dropping libsysfs, it's gone from some distros
    > already :)

    Really? I'm curious how such distributions support libsensors and the
    other tools which still rely on libsysfs. If they have already
    converted libsensors for me, that would be good news :)

    > And yes, the bug should be fixed, I agree. Thanks for letting us know.

    Tejun already fixed it, that was quick :)

    --
    Jean Delvare
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