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, 9 insertions, 8 deletions

diff --git a/accel/tcg/tb-maint.c b/accel/tcg/tb-maint.c
index 7246c1c..a173db1 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"
@@ -314,7 +315,7 @@ struct page_entry {
  * See also: page_collection_lock().
  */
 struct page_collection {
-    GTree *tree;
+    QTree *tree;
     struct page_entry *max;
 };
 
@@ -467,7 +468,7 @@ 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;
     }
@@ -478,7 +479,7 @@ 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
@@ -525,13 +526,13 @@ 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;
@@ -542,7 +543,7 @@ 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);
@@ -551,7 +552,7 @@ 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;
             }
         }
@@ -562,7 +563,7 @@ 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);
 }