Hello, Tejun Heo wrote: > Al Viro wrote: >> On Sat, Jan 05, 2008 at 11:30:25PM +0900, Tejun Heo wrote: >>>> Assuming that this is what we get, everything looks explainable - we >>>> have sysfs_rename_dir() calling sysfs_get_dentry() while the parent >>>> gets evicted. We don't have any exclusion, so while we are playing >>>> silly buggers with lookups in sysfs_get_dentry() we have parent become >>>> negative; the rest is obvious... >>> That part of code is walking down the sysfs tree from the s_root of >>> sysfs hierarchy and on each step parent is held using dget() while being >>> referenced, so I don't think they can turn negative there. >> Turn? Just what stops you from getting a negative (and unhashed) from >> lookup_one_noperm() and on the next iteration being buggered on mutex_lock()? > > Right, I haven't thought about that. When sysfs_get_dentry() is called, > @sd is always valid so unless there was existing negative dentry, lookup > is guaranteed to return positive dentry, but by populating dcache with > negative dentry before a node is created, things can go wrong. I don't > think that's what's going on here tho. If that was the case, the > while() loop looking up the next sd to lookup (@cur) should have blown > up as negative dentry will have NULL d_fsdata which doesn't match any sd. > > I guess what's needed here is d_revalidate() as other distributed > filesystems do. I'll test whether this can be actually triggered and > prepare a fix. Thanks a lot for pointing out the problem.
This can't happen because lookup of non-existent entry doesn't create a negative dentry. The new dentry is never hashed and killed after lookup failure, the above scenario can't happen. That said, the mechanism is a bit too fragile. sysfs currently ensures that dentry/inode point to the associated sysfs_dirent. This is mainly remanent of conversion from previous VFS based implementation. I think the right thing to do here is to make sysfs behave like other proper distributed filesystems using d_revalidate. Thanks. -- tejun -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
