[PATCH v2 2/2] tcg: use QTree instead of GTree

[Emilio Cota]

qemu-user can hang in a multi-threaded fork. One common
reason is that when creating a TB, between fork and exec
we manipulate a GTree whose memory allocator (GSlice) is
not fork-safe.

Although POSIX does not mandate it, the system's allocator
(e.g. tcmalloc, libc malloc) is probably fork-safe.

Fix some of these hangs by using QTree, which uses the system's
allocator regardless of the Glib version that we used at
configuration time.

Tested with the test program in the original bug report, i.e.:
```

void garble() {
  int pid = fork();
  if (pid == 0) {
    exit(0);
  } else {
    int wstatus;
    waitpid(pid, &wstatus, 0);
  }
}

void supragarble(unsigned depth) {
  if (depth == 0)
    return ;

  std::thread a(supragarble, depth-1);
  std::thread b(supragarble, depth-1);
  garble();
  a.join();
  b.join();
}

int main() {
  supragarble(10);
}
```

Fixes: #285

Signed-off-by: Emilio Cota <cota@braap.org>
---
 accel/tcg/tb-maint.c | 17 +++++++++--------
 tcg/region.c         | 19 ++++++++++---------
 2 files changed, 19 insertions(+), 17 deletions(-)

diff --git a/accel/tcg/tb-maint.c b/accel/tcg/tb-maint.c
index b3d6529ae2..e6370ddc71 100644
--- a/accel/tcg/tb-maint.c
+++ b/accel/tcg/tb-maint.c
@@ -19,6 +19,7 @@
 
 #include "qemu/osdep.h"
 #include "qemu/interval-tree.h"
+#include "qemu/qtree.h"
 #include "exec/cputlb.h"
 #include "exec/log.h"
 #include "exec/exec-all.h"
@@ -313,7 +314,7 @@ struct page_entry {
  * See also: page_collection_lock().
  */
 struct page_collection {
-    GTree *tree;
+    QTree *tree;
     struct page_entry *max;
 };
 
@@ -466,7 +467,7 @@ static bool page_trylock_add(struct page_collection *set, 
tb_page_addr_t addr)
     struct page_entry *pe;
     PageDesc *pd;
 
-    pe = g_tree_lookup(set->tree, &index);
+    pe = q_tree_lookup(set->tree, &index);
     if (pe) {
         return false;
     }
@@ -477,7 +478,7 @@ static bool page_trylock_add(struct page_collection *set, 
tb_page_addr_t addr)
     }
 
     pe = page_entry_new(pd, index);
-    g_tree_insert(set->tree, &pe->index, pe);
+    q_tree_insert(set->tree, &pe->index, pe);
 
     /*
      * If this is either (1) the first insertion or (2) a page whose index
@@ -524,13 +525,13 @@ static struct page_collection 
*page_collection_lock(tb_page_addr_t start,
     end   >>= TARGET_PAGE_BITS;
     g_assert(start <= end);
 
-    set->tree = g_tree_new_full(tb_page_addr_cmp, NULL, NULL,
+    set->tree = q_tree_new_full(tb_page_addr_cmp, NULL, NULL,
                                 page_entry_destroy);
     set->max = NULL;
     assert_no_pages_locked();
 
  retry:
-    g_tree_foreach(set->tree, page_entry_lock, NULL);
+    q_tree_foreach(set->tree, page_entry_lock, NULL);
 
     for (index = start; index <= end; index++) {
         TranslationBlock *tb;
@@ -541,7 +542,7 @@ static struct page_collection 
*page_collection_lock(tb_page_addr_t start,
             continue;
         }
         if (page_trylock_add(set, index << TARGET_PAGE_BITS)) {
-            g_tree_foreach(set->tree, page_entry_unlock, NULL);
+            q_tree_foreach(set->tree, page_entry_unlock, NULL);
             goto retry;
         }
         assert_page_locked(pd);
@@ -550,7 +551,7 @@ static struct page_collection 
*page_collection_lock(tb_page_addr_t start,
                 (tb_page_addr1(tb) != -1 &&
                  page_trylock_add(set, tb_page_addr1(tb)))) {
                 /* drop all locks, and reacquire in order */
-                g_tree_foreach(set->tree, page_entry_unlock, NULL);
+                q_tree_foreach(set->tree, page_entry_unlock, NULL);
                 goto retry;
             }
         }
@@ -561,7 +562,7 @@ static struct page_collection 
*page_collection_lock(tb_page_addr_t start,
 static void page_collection_unlock(struct page_collection *set)
 {
     /* entries are unlocked and freed via page_entry_destroy */
-    g_tree_destroy(set->tree);
+    q_tree_destroy(set->tree);
     g_free(set);
 }
 
diff --git a/tcg/region.c b/tcg/region.c
index 88d6bb273f..bef4c4756f 100644
--- a/tcg/region.c
+++ b/tcg/region.c
@@ -28,6 +28,7 @@
 #include "qemu/mprotect.h"
 #include "qemu/memalign.h"
 #include "qemu/cacheinfo.h"
+#include "qemu/qtree.h"
 #include "qapi/error.h"
 #include "exec/exec-all.h"
 #include "tcg/tcg.h"
@@ -36,7 +37,7 @@
 
 struct tcg_region_tree {
     QemuMutex lock;
-    GTree *tree;
+    QTree *tree;
     /* padding to avoid false sharing is computed at run-time */
 };
 
@@ -163,7 +164,7 @@ static void tcg_region_trees_init(void)
         struct tcg_region_tree *rt = region_trees + i * tree_size;
 
         qemu_mutex_init(&rt->lock);
-        rt->tree = g_tree_new_full(tb_tc_cmp, NULL, NULL, tb_destroy);
+        rt->tree = q_tree_new_full(tb_tc_cmp, NULL, NULL, tb_destroy);
     }
 }
 
@@ -202,7 +203,7 @@ void tcg_tb_insert(TranslationBlock *tb)
 
     g_assert(rt != NULL);
     qemu_mutex_lock(&rt->lock);
-    g_tree_insert(rt->tree, &tb->tc, tb);
+    q_tree_insert(rt->tree, &tb->tc, tb);
     qemu_mutex_unlock(&rt->lock);
 }
 
@@ -212,7 +213,7 @@ void tcg_tb_remove(TranslationBlock *tb)
 
     g_assert(rt != NULL);
     qemu_mutex_lock(&rt->lock);
-    g_tree_remove(rt->tree, &tb->tc);
+    q_tree_remove(rt->tree, &tb->tc);
     qemu_mutex_unlock(&rt->lock);
 }
 
@@ -232,7 +233,7 @@ TranslationBlock *tcg_tb_lookup(uintptr_t tc_ptr)
     }
 
     qemu_mutex_lock(&rt->lock);
-    tb = g_tree_lookup(rt->tree, &s);
+    tb = q_tree_lookup(rt->tree, &s);
     qemu_mutex_unlock(&rt->lock);
     return tb;
 }
@@ -267,7 +268,7 @@ void tcg_tb_foreach(GTraverseFunc func, gpointer user_data)
     for (i = 0; i < region.n; i++) {
         struct tcg_region_tree *rt = region_trees + i * tree_size;
 
-        g_tree_foreach(rt->tree, func, user_data);
+        q_tree_foreach(rt->tree, func, user_data);
     }
     tcg_region_tree_unlock_all();
 }
@@ -281,7 +282,7 @@ size_t tcg_nb_tbs(void)
     for (i = 0; i < region.n; i++) {
         struct tcg_region_tree *rt = region_trees + i * tree_size;
 
-        nb_tbs += g_tree_nnodes(rt->tree);
+        nb_tbs += q_tree_nnodes(rt->tree);
     }
     tcg_region_tree_unlock_all();
     return nb_tbs;
@@ -296,8 +297,8 @@ static void tcg_region_tree_reset_all(void)
         struct tcg_region_tree *rt = region_trees + i * tree_size;
 
         /* Increment the refcount first so that destroy acts as a reset */
-        g_tree_ref(rt->tree);
-        g_tree_destroy(rt->tree);
+        q_tree_ref(rt->tree);
+        q_tree_destroy(rt->tree);
     }
     tcg_region_tree_unlock_all();
 }
-- 
2.34.1