Hello, 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. >>> AFAICS, the locking here is quite broken and frankly, sysfs_get_dentry() >>> and the way it plays with fs/namei.c are ucking fugly. >> Can you elaborate a bit? The locking in sysfs is unconventional but >> that's mostly from necessity. It has dual interface - vfs and driver >> model && vfs data structures (dentry and inode) are too big to always >> keep around, so it basically becomes a small distributed file system >> where the backing data can change asynchronously. > > ... with all fun that creates. As it is, you have those async changers > of backing data using VFS locking _under_ sysfs locks via lookup_one_noperm() > and yet it needs sysfs_mutex inside sysfs_lookup(). So you can't have > sysfs_get_dentry() under it. So you don't have exclusion with arseloads > of sysfs tree changes in there. Joy... There are two locks. sysfs_rename_mutex and sysfs_mutex. sysfs_rename_mutex is above VFS locks while sysfs_mutex is below VFS locks. sysfs_rename_mutex() protects against move/rename which can change the ancestry of a held sysfs_dirent while sysfs_mutex protects the sd hierarchy itself. Locking can be wrong if sysfs_rename_mutex locking is missing from the places where ancestry of a held sd can change but I can't find one ATM. If I'm missing your point again, feel free to scream at me. :-) As it's unnecessarily unintuitive, there's a pending change to rename sysfs_rename_mutex and use it to protect the whole tree structure to make locking simpler while using sysfs_mutex to guard VFS access such that the locking hierarchy plainly becomes sysfs_rename_mutex - VFS locks - sysfs_mutex where all internal sysfs structure is protected by the outer mutex and the inner one just protects VFS accesses. > Frankly, with the current state of sysfs the last vestiges of arguments > used to push it into the tree back then are dead and buried. I'm not > blaming you, BTW - the shitpile *did* grow past the point where its > memory footprint became far too large and something needed to be done. > Unfortunately, it happened too late for that something being "get rid > of the entire mess" and now we are saddled with it for good. Yeah, it's too late to get rid of sysfs and regardless implementation ugliness, which BTW I think has improved a lot during last six or so months, it's now pretty useful and important to drivers, so I guess the only option is trying hard to make it better. Oh, BTW, the ugly lookup_one_noperm() can be removed if LOOKUP_NOPERM flag is added. The only reason sysfs_lookup() uses the specialized lookup is to avoid permission check. 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/
