futex: Remove support for IMMUTABLE

The FH_FLAG_IMMUTABLE flag was meant to avoid the reference counting on
the private hash and so to avoid the performance regression on big
machines.
With the switch to per-CPU counter this is no longer needed. That flag
was never useable on any released kernel.

Remove any support for IMMUTABLE while preserve the flags argument and
enforce it to be zero.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20250710110011.384614-5-bigeasy@linutronix.de

Author: Sebastian Andrzej Siewior

include/uapi/linux/prctl.h

@@ -367,8 +367,6 @@ struct prctl_mm_map {
 /* FUTEX hash management */
 #define PR_FUTEX_HASH			78
 # define PR_FUTEX_HASH_SET_SLOTS	1
-# define FH_FLAG_IMMUTABLE		(1ULL << 0)
 # define PR_FUTEX_HASH_GET_SLOTS	2
-# define PR_FUTEX_HASH_GET_IMMUTABLE	3
 
 #endif /* _LINUX_PRCTL_H */

kernel/futex/core.c

@@ -69,7 +69,6 @@ struct futex_private_hash {
 	struct rcu_head	rcu;
 	void		*mm;
 	bool		custom;
-	bool		immutable;
 	struct futex_hash_bucket queues[];
 };
 
@@ -145,15 +144,11 @@ static inline bool futex_key_is_private(union futex_key *key)
 
 static bool futex_private_hash_get(struct futex_private_hash *fph)
 {
-	if (fph->immutable)
-		return true;
 	return futex_ref_get(fph);
 }
 
 void futex_private_hash_put(struct futex_private_hash *fph)
 {
-	if (fph->immutable)
-		return;
 	if (futex_ref_put(fph))
 		wake_up_var(fph->mm);
 }
@@ -1530,7 +1525,6 @@ static void futex_hash_bucket_init(struct futex_hash_bucket *fhb,
 }
 
 #define FH_CUSTOM	0x01
-#define FH_IMMUTABLE	0x02
 
 #ifdef CONFIG_FUTEX_PRIVATE_HASH
 
@@ -1800,7 +1794,7 @@ static int futex_hash_allocate(unsigned int hash_slots, unsigned int flags)
 	 */
 	scoped_guard(rcu) {
 		fph = rcu_dereference(mm->futex_phash);
-		if (fph && (!fph->hash_mask || fph->immutable)) {
+		if (fph && !fph->hash_mask) {
 			if (custom)
 				return -EBUSY;
 			return 0;
@@ -1814,7 +1808,6 @@ static int futex_hash_allocate(unsigned int hash_slots, unsigned int flags)
 
 	fph->hash_mask = hash_slots ? hash_slots - 1 : 0;
 	fph->custom = custom;
-	fph->immutable = !!(flags & FH_IMMUTABLE);
 	fph->mm = mm;
 
 	for (i = 0; i < hash_slots; i++)
@@ -1838,7 +1831,7 @@ static int futex_hash_allocate(unsigned int hash_slots, unsigned int flags)
 		mm->futex_phash_new = NULL;
 
 		if (fph) {
-			if (cur && (!cur->hash_mask || cur->immutable)) {
+			if (cur && !cur->hash_mask) {
 				/*
 				 * If two threads simultaneously request the global
 				 * hash then the first one performs the switch,
@@ -1931,19 +1924,6 @@ static int futex_hash_get_slots(void)
 	return 0;
 }
 
-static int futex_hash_get_immutable(void)
-{
-	struct futex_private_hash *fph;
-
-	guard(rcu)();
-	fph = rcu_dereference(current->mm->futex_phash);
-	if (fph && fph->immutable)
-		return 1;
-	if (fph && !fph->hash_mask)
-		return 1;
-	return 0;
-}
-
 #else
 
 static int futex_hash_allocate(unsigned int hash_slots, unsigned int flags)
@@ -1956,10 +1936,6 @@ static int futex_hash_get_slots(void)
 	return 0;
 }
 
-static int futex_hash_get_immutable(void)
-{
-	return 0;
-}
 #endif
 
 int futex_hash_prctl(unsigned long arg2, unsigned long arg3, unsigned long arg4)
@@ -1969,21 +1945,15 @@ int futex_hash_prctl(unsigned long arg2, unsigned long arg3, unsigned long arg4)
 
 	switch (arg2) {
 	case PR_FUTEX_HASH_SET_SLOTS:
-		if (arg4 & ~FH_FLAG_IMMUTABLE)
+		if (arg4)
 			return -EINVAL;
-		if (arg4 & FH_FLAG_IMMUTABLE)
-			flags |= FH_IMMUTABLE;
 		ret = futex_hash_allocate(arg3, flags);
 		break;
 
 	case PR_FUTEX_HASH_GET_SLOTS:
 		ret = futex_hash_get_slots();
 		break;
 
-	case PR_FUTEX_HASH_GET_IMMUTABLE:
-		ret = futex_hash_get_immutable();
-		break;
-
 	default:
 		ret = -EINVAL;
 		break;