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shared: fix race in nm_ref_string_unref()

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We cannot drop the reference count to zero while having
no lock. Otherwise, another thread might race doing

  s = nm_ref_string_new("...");
  nm_ref_string_unref(s);

and already successfully delete the instance.

Hitting this race should be rather difficult, especially because
we tend to use NMRefString only from one thread. But still, access
to global variables must be race free.

Fixes: 908fadec96 ('shared: add NMRefString')
(cherry picked from commit 3490a09a7d)
(cherry picked from commit 1a68a54471)
This commit is contained in:
Thomas Haller 2020-12-08 19:37:49 +01:00
parent a744397ad5
commit 71f56a5d76
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GPG key ID: 29C2366E4DFC5728
1 changed files with 9 additions and 8 deletions
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17
shared/nm-glib-aux/nm-ref-string.c
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@ -169,24 +169,25 @@ void
_nm_ref_string_unref_non_null (NMRefString *rstr)
{
RefString *const rstr0 = (RefString *) rstr;
int r;
_ASSERT (rstr0);
if (G_LIKELY (!g_atomic_int_dec_and_test (&rstr0->ref_count)))
/* fast-path: first try to decrement the ref-count without bringing it
* to zero. */
r = rstr0->ref_count;
if (G_LIKELY (r > 1 && g_atomic_int_compare_and_exchange (&rstr0->ref_count, r, r - 1)))
return;
/* We apparently are about to return the last reference. Take a lock. */
G_LOCK (gl_lock);
/* in the fast-path above, we already decremented the ref-count to zero.
* We need recheck that the ref-count is still zero. */
nm_assert (g_hash_table_lookup (gl_hash, rstr0) == rstr0);
if (g_atomic_int_get (&rstr0->ref_count) == 0) {
if (G_LIKELY(g_atomic_int_dec_and_test (&rstr0->ref_count))) {
if (!g_hash_table_remove (gl_hash, rstr0))
nm_assert_not_reached ();
} else {
#if NM_MORE_ASSERTS > 5
nm_assert (g_hash_table_lookup (gl_hash, rstr0) == rstr0);
#endif
}
G_UNLOCK (gl_lock);