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    DateWed, 18 Jul 2007 22:06:52 +0200
    FromJean Delvare <>
    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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