This is the mail archive of the
libc-alpha@sourceware.org
mailing list for the glibc project.
[PATCH] malloc: Preseve arena free list for attached threads [BZ #20370]
- From: fweimer at redhat dot com (Florian Weimer)
- To: libc-alpha at sourceware dot org
- Date: Thu, 14 Jul 2016 17:28:48 +0200
- Subject: [PATCH] malloc: Preseve arena free list for attached threads [BZ #20370]
- Authentication-results: sourceware.org; auth=none
It is necessary to preserve the invariant that if an arena is
on the free list, it has thread attach count zero. Otherwise,
when arena_thread_freeres sees the zero attach count, it will
add it, and without the invariant, an arena could get pushed
to the list twice, resulting in a cycle.
One possible execution trace looks like this:
Thread 1 examines free list and observes it as empty.
Thread 2 exits and adds its arena to the free list,
with attached_threads == 0).
Thread 1 selects this arena in reused_arena (not from the free list).
Thread 1 increments attached_threads and attaches itself.
(The arena remains on the free list.)
Thread 1 exits, decrements attached_threads,
and adds the arena to the free list.
The final step creates a cycle in the usual way (by overwriting the
next_free member with the former list head, while there is another
list item pointing to the arena structure).
tst-malloc-thread-exit exhibits this issue, but it was only visible
with a debugger because the incorrect fix in bug 19243 removed
the assert from get_free_list.
2016-07-14 Florian Weimer <fweimer@redhat.com>
* malloc/arena.c (get_free_list): Update comment. Assert that
arenas on the free list have no attached threads.
(remove_from_free_list): New function.
(reused_arena): Call it.
diff --git a/malloc/arena.c b/malloc/arena.c
index 229783f..4e16593 100644
--- a/malloc/arena.c
+++ b/malloc/arena.c
@@ -702,8 +702,7 @@ _int_new_arena (size_t size)
}
-/* Remove an arena from free_list. The arena may be in use because it
- was attached concurrently to a thread by reused_arena below. */
+/* Remove an arena from free_list. */
static mstate
get_free_list (void)
{
@@ -718,7 +717,8 @@ get_free_list (void)
free_list = result->next_free;
/* The arena will be attached to this thread. */
- ++result->attached_threads;
+ assert (result->attached_threads == 0);
+ result->attached_threads = 1;
detach_arena (replaced_arena);
}
@@ -735,6 +735,26 @@ get_free_list (void)
return result;
}
+/* Remove the arena from the free list (if it is present).
+ free_list_lock must have been acquired by the caller. */
+static void
+remove_from_free_list (mstate arena)
+{
+ mstate *previous = &free_list;
+ for (mstate p = free_list; p != NULL; p = p->next_free)
+ {
+ assert (p->attached_threads == 0);
+ if (p == arena)
+ {
+ /* Remove the requested arena from the list. */
+ *previous = p->next_free;
+ break;
+ }
+ else
+ previous = &p->next_free;
+ }
+}
+
/* Lock and return an arena that can be reused for memory allocation.
Avoid AVOID_ARENA as we have already failed to allocate memory in
it and it is currently locked. */
@@ -782,14 +802,25 @@ reused_arena (mstate avoid_arena)
(void) mutex_lock (&result->mutex);
out:
- /* Attach the arena to the current thread. Note that we may have
- selected an arena which was on free_list. */
+ /* Attach the arena to the current thread. */
{
/* Update the arena thread attachment counters. */
mstate replaced_arena = thread_arena;
(void) mutex_lock (&free_list_lock);
detach_arena (replaced_arena);
+
+ /* We may have picked up an arena on the free list. We need to
+ preserve the invariant that no arena on the free list has a
+ positive attached_threads counter (otherwise,
+ arena_thread_freeres cannot use the counter to determine if the
+ arena needs to be put on the free list). We unconditionally
+ remove the selected arena from the free list. The caller of
+ reused_arena checked the free list and observed it to be empty,
+ so the list is very short. */
+ remove_from_free_list (result);
+
++result->attached_threads;
+
(void) mutex_unlock (&free_list_lock);
}