tcg: use QTree instead of GTree

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);
}
```

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/285
Reported-by: Valentin David <me@valentindavid.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Emilio Cota <cota@braap.org>
Message-Id: <20230205163758.416992-3-cota@braap.org>
[rth: Add QEMU_DISABLE_CFI for all callback using functions.]
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

1 files changed, 10 insertions, 9 deletions

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();
 }