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, 9 insertions, 0 deletions

diff --git a/NEWS b/NEWS
index 220d327071..b39b1e5e33 100644
--- a/NEWS
+++ b/NEWS
@@ -51,6 +51,15 @@ Major new features:
 
 * The ISO C2X macro _PRINTF_NAN_LEN_MAX has been added to <stdio.h>.
 
+* A new DSO sorting algorithm has been added in the dynamic linker that uses
+  topological sorting by depth-first search (DFS), solving performance issues
+  of the existing sorting algorithm when encountering particular circular
+  object dependency cases.
+
+* A new tunable, glibc.rtld.dynamic_sort, can be used to select between the two
+  DSO sorting algorithms.  The default setting of '1' uses the current existing
+  algorithm, while a value of '2' selects the new DFS-based algorithm.
+
 Deprecated and removed features, and other changes affecting compatibility:
 
 * The r_version update in the debugger interface makes the glibc binary