• Cody P Schafer's avatar
    rbtree: clarify documentation of rbtree_postorder_for_each_entry_safe() · 8de1ee7e
    Cody P Schafer authored
    I noticed that commit a20135ff ("writeback: don't drain
    bdi_writeback_congested on bdi destruction") added a usage of
    rbtree_postorder_for_each_entry_safe() in mm/backing-dev.c which appears
    to try to rb_erase() elements from an rbtree while iterating over it using
    rbtree_postorder_for_each_entry_safe().
    
    Doing this will cause random nodes to be missed by the iteration because
    rb_erase() may rebalance the tree, changing the ordering that we're trying
    to iterate over.
    
    The previous documentation for rbtree_postorder_for_each_entry_safe()
    wasn't clear that this wasn't allowed, it was taken from the docs for
    list_for_each_entry_safe(), where erasing isn't a problem due to
    list_del() not reordering.
    
    Explicitly warn developers about this potential pit-fall.
    
    Note that I haven't fixed the actual issue that (it appears) the commit
    referenced above introduced (not familiar enough with that code).
    
    In general (and in this case), the patterns to follow are:
     - switch to rb_first() + rb_erase(), don't use
       rbtree_postorder_for_each_entry_safe().
     - keep the postorder iteration and don't rb_erase() at all. Instead
       just clear the fields of rb_node & cgwb_congested_tree as required by
       other users of those structures.
    
    [akpm@linux-foundation.org: tweak comments]
    Signed-off-by: default avatarCody P Schafer <dev@codyps.com>
    Cc: John de la Garza <john@jjdev.com>
    Cc: Michel Lespinasse <walken@google.com>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Rusty Russell <rusty@rustcorp.com.au>
    Cc: Tejun Heo <tj@kernel.org>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    8de1ee7e
rbtree.h 4.38 KB