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tcg: use QTree instead of GTree

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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>
This commit is contained in:
Emilio Cota 2023-02-05 11:37:58 -05:00 • committed by Richard Henderson
parent e3feb2cc22
commit 1ff4a81bd3
3 changed files with 23 additions and 21 deletions
Download patch file Download diff file Expand all files Collapse all files

17
accel/tcg/tb-maint.c
View file

@ -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);
}