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GPrivate: eliminate a malloc for pthread_key_t on most platforms

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We don't know how big a pthread_key_t is, so we malloc() a big enough
chunk of memory for it and store a pointer into the GPrivate struct.

It turns out, on Linux, pthread_key_t is just an int, so we could much
easier just store it directly into the struct.

https://bugzilla.gnome.org/show_bug.cgi?id=737445
Fixes: #931
This commit is contained in:
Allison Karlitskaya 2023-07-21 17:05:26 +01:00 committed by Philip Withnall
parent 256b195ff1
commit 766663dafe
1 changed files with 86 additions and 9 deletions
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77
glib/gthread-posix.c
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@ -1047,9 +1047,66 @@ g_private_impl_free (pthread_key_t *key)
free (key);
}
static gpointer
g_private_impl_new_direct (GDestroyNotify notify)
{
gpointer impl = (void *) (gssize) -1;
pthread_key_t key;
gint status;
status = pthread_key_create (&key, notify);
if G_UNLIKELY (status != 0)
g_thread_abort (status, "pthread_key_create");
memcpy (&impl, &key, sizeof (pthread_key_t));
/* pthread_key_create could theoretically put a NULL value into key.
* If that happens, waste the result and create a new one, since we
* use NULL to mean "not yet allocated".
*
* This will only happen once per program run.
*
* We completely avoid this problem for the case where pthread_key_t
* is smaller than void* (for example, on 64 bit Linux) by putting
* some high bits in the value of 'impl' to start with. Since we only
* overwrite part of the pointer, we will never end up with NULL.
*/
if (sizeof (pthread_key_t) == sizeof (gpointer))
{
if G_UNLIKELY (impl == NULL)
{
status = pthread_key_create (&key, notify);
if G_UNLIKELY (status != 0)
g_thread_abort (status, "pthread_key_create");
memcpy (&impl, &key, sizeof (pthread_key_t));
if G_UNLIKELY (impl == NULL)
g_thread_abort (status, "pthread_key_create (gave NULL result twice)");
}
}
return impl;
}
static void
g_private_impl_free_direct (gpointer impl)
{
pthread_key_t tmp;
gint status;
memcpy (&tmp, &impl, sizeof (pthread_key_t));
status = pthread_key_delete (tmp);
if G_UNLIKELY (status != 0)
g_thread_abort (status, "pthread_key_delete");
}
static inline pthread_key_t
g_private_get_impl (GPrivate *key)
{
if (sizeof (pthread_key_t) > sizeof (gpointer))
{
pthread_key_t *impl = g_atomic_pointer_get (&key->p);
if G_UNLIKELY (impl == NULL)
@ -1063,6 +1120,26 @@ g_private_get_impl (GPrivate *key)
}
return *impl;
}
else
{
gpointer impl = g_atomic_pointer_get (&key->p);
pthread_key_t tmp;
if G_UNLIKELY (impl == NULL)
{
impl = g_private_impl_new_direct (key->notify);
if (!g_atomic_pointer_compare_and_exchange (&key->p, NULL, impl))
{
g_private_impl_free_direct (impl);
impl = key->p;
}
}
memcpy (&tmp, &impl, sizeof (pthread_key_t));
return tmp;
}
}
/**