• Ian Kent's avatar
    autofs4: deadlock during create · 1864f7bd
    Ian Kent authored
    Due to inconsistent locking in the VFS between calls to lookup and
    revalidate deadlock can occur in the automounter.
    
    The inconsistency is that the directory inode mutex is held for both lookup
    and revalidate calls when called via lookup_hash whereas it is held only
    for lookup during a path walk.  Consequently, if the mutex is held during a
    call to revalidate autofs4 can't release the mutex to callback the daemon
    as it can't know whether it owns the mutex.
    
    This situation happens when a process tries to create a directory within an
    automount and a second process also tries to create the same directory
    between the lookup and the mkdir.  Since the first process has dropped the
    mutex for the daemon callback, the second process takes it during
    revalidate leading to deadlock between the autofs daemon and the second
    process when the daemon tries to create the mount point directory.
    
    After spending quite a bit of time trying to resolve this on more than one
    occassion, using rather complex and ulgy approaches, it turns out that just
    delaying the hashing of the dentry until the create operation works fine.
    Signed-off-by: default avatarIan Kent <raven@themaw.net>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    1864f7bd
root.c 26.3 KB