elf: Fix slow DSO sorting behavior in dynamic loader (BZ #17645)

This second patch contains the actual implementation of a new sorting algorithm
for shared objects in the dynamic loader, which solves the slow behavior that
the current "old" algorithm falls into when the DSO set contains circular
dependencies.

The new algorithm implemented here is simply depth-first search (DFS) to obtain
the Reverse-Post Order (RPO) sequence, a topological sort. A new l_visited:1
bitfield is added to struct link_map to more elegantly facilitate such a search.

The DFS algorithm is applied to the input maps[nmap-1] backwards towards
maps[0]. This has the effect of a more "shallow" recursion depth in general
since the input is in BFS. Also, when combined with the natural order of
processing l_initfini[] at each node, this creates a resulting output sorting
closer to the intuitive "left-to-right" order in most cases.

Another notable implementation adjustment related to this _dl_sort_maps change
is the removing of two char arrays 'used' and 'done' in _dl_close_worker to
represent two per-map attributes. This has been changed to simply use two new
bit-fields l_map_used:1, l_map_done:1 added to struct link_map. This also allows
discarding the clunky 'used' array sorting that _dl_sort_maps had to sometimes
do along the way.

Tunable support for switching between different sorting algorithms at runtime is
also added. A new tunable 'glibc.rtld.dynamic_sort' with current valid values 1
(old algorithm) and 2 (new DFS algorithm) has been added. At time of commit
of this patch, the default setting is 1 (old algorithm).

Signed-off-by: Chung-Lin Tang  <cltang@codesourcery.com>
Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

1 files changed, 11 insertions, 0 deletions

diff --git a/manual/tunables.texi b/manual/tunables.texi
index 658547c613..10f4d75993 100644
--- a/manual/tunables.texi
+++ b/manual/tunables.texi
@@ -309,6 +309,17 @@ changed once allocated at process startup.  The default allocation of
 optional static TLS is 512 bytes and is allocated in every thread.
 @end deftp
 
+@deftp Tunable glibc.rtld.dynamic_sort
+Sets the algorithm to use for DSO sorting, valid values are @samp{1} and
+@samp{2}.  For value of @samp{1}, an older O(n^3) algorithm is used, which is
+long time tested, but may have performance issues when dependencies between
+shared objects contain cycles due to circular dependencies.  When set to the
+value of @samp{2}, a different algorithm is used, which implements a
+topological sort through depth-first search, and does not exhibit the
+performance issues of @samp{1}.
+
+The default value of this tunable is @samp{1}.
+@end deftp
 
 @node Elision Tunables
 @section Elision Tunables