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authorCody P Schafer <dev@codyps.com>2015-11-07 03:31:28 +0300
committerLinus Torvalds <torvalds@linux-foundation.org>2015-11-07 04:50:42 +0300
commit8de1ee7ebfb4979c6444e81273e12e7a972c367d (patch)
treea1d45a54168a77a814537a50cc29cc4fdcf35615 /lib
parent90224350eaaf8b8043b19c393048f732bc2e4120 (diff)
downloadlinux-8de1ee7ebfb4979c6444e81273e12e7a972c367d.tar.xz
rbtree: clarify documentation of rbtree_postorder_for_each_entry_safe()
I noticed that commit a20135ffbc44 ("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: Cody 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: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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