• Glauber Costa's avatar
    list_lru: remove special case function list_lru_dispose_all. · 4e717f5c
    Glauber Costa authored
    The list_lru implementation has one function, list_lru_dispose_all, with
    only one user (the dentry code).  At first, such function appears to make
    sense because we are really not interested in the result of isolating each
    dentry separately - all of them are going away anyway.  However, it's
    implementation is buggy in the following way:
    
    When we call list_lru_dispose_all in fs/dcache.c, we scan all dentries
    marking them with DCACHE_SHRINK_LIST.  However, this is done without the
    nlru->lock taken.  The imediate result of that is that someone else may
    add or remove the dentry from the LRU at the same time.  When list_lru_del
    happens in that scenario we will see an element that is not yet marked
    with DCACHE_SHRINK_LIST (even though it will be in the future) and
    obviously remove it from an lru where the element no longer is.  Since
    list_lru_dispose_all will in effect count down nlru's nr_items and
    list_lru_del will do the same, this will lead to an imbalance.
    
    The solution for this would not be so simple: we can obviously just keep
    the lru_lock taken, but then we have no guarantees that we will be able to
    acquire the dentry lock (dentry->d_lock).  To properly solve this, we need
    a communication mechanism between the lru and dentry code, so they can
    coordinate this with each other.
    
    Such mechanism already exists in the form of the list_lru_walk_cb
    callback.  So it is possible to construct a dcache-side prune function
    that does the right thing only by calling list_lru_walk in a loop until no
    more dentries are available.
    
    With only one user, plus the fact that a sane solution for the problem
    would involve boucing between dcache and list_lru anyway, I see little
    justification to keep the special case list_lru_dispose_all in tree.
    Signed-off-by: default avatarGlauber Costa <glommer@openvz.org>
    Cc: Michal Hocko <mhocko@suse.cz>
    Acked-by: default avatarDave Chinner <dchinner@redhat.com>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarAl Viro <viro@zeniv.linux.org.uk>
    4e717f5c
dcache.c 85.1 KB