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gthread-posix: Use named states to manage the mutex states

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We used to use raw numbers to control the mutex state, we can use a
nicer enum to avoid dealing with pure numbers.
This commit is contained in:
Marco Trevisan (Treviño) 2022-06-23 06:26:34 +02:00
parent 15d2053122
commit 8d98617297
1 changed files with 29 additions and 17 deletions
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46
glib/gthread-posix.c
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@ -1449,9 +1449,16 @@ g_system_thread_set_name (const gchar *name)
* *
* 1: acquired by one thread only, no contention * 1: acquired by one thread only, no contention
* *
* > 1: contended * 2: contended
* */
*
typedef enum {
G_MUTEX_STATE_EMPTY = 0,
G_MUTEX_STATE_OWNED,
G_MUTEX_STATE_CONTENDED,
} GMutexState;
/*
* As such, attempting to acquire the lock should involve an increment. * As such, attempting to acquire the lock should involve an increment.
* If we find that the previous value was 0 then we can return * If we find that the previous value was 0 then we can return
* immediately. * immediately.
@ -1470,13 +1477,13 @@ g_system_thread_set_name (const gchar *name)
void void
g_mutex_init (GMutex *mutex) g_mutex_init (GMutex *mutex)
{ {
mutex->i[0] = 0; mutex->i[0] = G_MUTEX_STATE_EMPTY;
} }
void void
g_mutex_clear (GMutex *mutex) g_mutex_clear (GMutex *mutex)
{ {
if G_UNLIKELY (mutex->i[0] != 0) if G_UNLIKELY (mutex->i[0] != G_MUTEX_STATE_EMPTY)
{ {
fprintf (stderr, "g_mutex_clear() called on uninitialised or locked mutex\n"); fprintf (stderr, "g_mutex_clear() called on uninitialised or locked mutex\n");
g_abort (); g_abort ();
@ -1486,13 +1493,16 @@ g_mutex_clear (GMutex *mutex)
static void __attribute__((noinline)) static void __attribute__((noinline))
g_mutex_lock_slowpath (GMutex *mutex) g_mutex_lock_slowpath (GMutex *mutex)
{ {
/* Set to 2 to indicate contention. If it was zero before then we /* Set to contended. If it was empty before then we
* just acquired the lock. * just acquired the lock.
* *
* Otherwise, sleep for as long as the 2 remains... * Otherwise, sleep for as long as the contended state remains...
*/ */
while (exchange_acquire (&mutex->i[0], 2) != 0) while (exchange_acquire (&mutex->i[0], G_MUTEX_STATE_CONTENDED) != G_MUTEX_STATE_EMPTY)
syscall (__NR_futex, &mutex->i[0], (gsize) FUTEX_WAIT_PRIVATE, (gsize) 2, NULL); {
syscall (__NR_futex, &mutex->i[0], (gsize) FUTEX_WAIT_PRIVATE,
G_MUTEX_STATE_CONTENDED, NULL);
}
} }
static void __attribute__((noinline)) static void __attribute__((noinline))
@ -1502,7 +1512,7 @@ g_mutex_unlock_slowpath (GMutex *mutex,
/* We seem to get better code for the uncontended case by splitting /* We seem to get better code for the uncontended case by splitting
* this out... * this out...
*/ */
if G_UNLIKELY (prev == 0) if G_UNLIKELY (prev == G_MUTEX_STATE_EMPTY)
{ {
fprintf (stderr, "Attempt to unlock mutex that was not locked\n"); fprintf (stderr, "Attempt to unlock mutex that was not locked\n");
g_abort (); g_abort ();
@ -1514,8 +1524,10 @@ g_mutex_unlock_slowpath (GMutex *mutex,
void void
g_mutex_lock (GMutex *mutex) g_mutex_lock (GMutex *mutex)
{ {
/* 0 -> 1 and we're done. Anything else, and we need to wait... */ /* empty -> owned and we're done. Anything else, and we need to wait... */
if G_UNLIKELY (g_atomic_int_add (&mutex->i[0], 1) != 0) if G_UNLIKELY (!g_atomic_int_compare_and_exchange (&mutex->i[0],
G_MUTEX_STATE_EMPTY,
G_MUTEX_STATE_OWNED))
g_mutex_lock_slowpath (mutex); g_mutex_lock_slowpath (mutex);
} }
@ -1524,22 +1536,22 @@ g_mutex_unlock (GMutex *mutex)
{ {
guint prev; guint prev;
prev = exchange_release (&mutex->i[0], 0); prev = exchange_release (&mutex->i[0], G_MUTEX_STATE_EMPTY);
/* 1-> 0 and we're done. Anything else and we need to signal... */ /* 1-> 0 and we're done. Anything else and we need to signal... */
if G_UNLIKELY (prev != 1) if G_UNLIKELY (prev != G_MUTEX_STATE_OWNED)
g_mutex_unlock_slowpath (mutex, prev); g_mutex_unlock_slowpath (mutex, prev);
} }
gboolean gboolean
g_mutex_trylock (GMutex *mutex) g_mutex_trylock (GMutex *mutex)
{ {
guint zero = 0; GMutexState empty = G_MUTEX_STATE_EMPTY;
/* We don't want to touch the value at all unless we can move it from /* We don't want to touch the value at all unless we can move it from
* exactly 0 to 1. * exactly empty to owned.
*/ */
return compare_exchange_acquire (&mutex->i[0], &zero, 1); return compare_exchange_acquire (&mutex->i[0], &empty, G_MUTEX_STATE_OWNED);
} }
/* Condition variables are implemented in a rather simple way as well. /* Condition variables are implemented in a rather simple way as well.