Re: [PATCH v12 15/21] futex: Allow to make the private hash immutable

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Apr 16, 2025 at 06:29:15PM +0200, Sebastian Andrzej Siewior wrote:
> My initial testing showed that
> 	perf bench futex hash
> 
> reported less operations/sec with private hash. After using the same
> amount of buckets in the private hash as used by the global hash then
> the operations/sec were about the same.
> 
> This changed once the private hash became resizable. This feature added
> a RCU section and reference counting via atomic inc+dec operation into
> the hot path.
> The reference counting can be avoided if the private hash is made
> immutable.
> Extend PR_FUTEX_HASH_SET_SLOTS by a fourth argument which denotes if the
> private should be made immutable. Once set (to true) the a further
> resize is not allowed (same if set to global hash).
> Add PR_FUTEX_HASH_GET_IMMUTABLE which returns true if the hash can not
> be changed.
> Update "perf bench" suite.

Does the below make sense? This changes arg4 into a flags field and uses
bit0 for immutable.

(the point where I got upset is where arg4==2 was accepted :-)

--- a/include/uapi/linux/prctl.h
+++ b/include/uapi/linux/prctl.h
@@ -367,6 +367,7 @@ 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
 
--- a/kernel/futex/core.c
+++ b/kernel/futex/core.c
@@ -1440,7 +1440,10 @@ static bool futex_hash_less(struct futex
 	return false; /* equal */
 }
 
-static int futex_hash_allocate(unsigned int hash_slots, unsigned int immutable, bool custom)
+#define FH_CUSTOM	0x01
+#define FH_IMMUTABLE	0x02
+
+static int futex_hash_allocate(unsigned int hash_slots, unsigned int flags)
 {
 	struct mm_struct *mm = current->mm;
 	struct futex_private_hash *fph;
@@ -1448,8 +1451,6 @@ static int futex_hash_allocate(unsigned
 
 	if (hash_slots && (hash_slots == 1 || !is_power_of_2(hash_slots)))
 		return -EINVAL;
-	if (immutable > 2)
-		return -EINVAL;
 
 	/*
 	 * Once we've disabled the global hash there is no way back.
@@ -1469,8 +1470,8 @@ static int futex_hash_allocate(unsigned
 
 	rcuref_init(&fph->users, 1);
 	fph->hash_mask = hash_slots ? hash_slots - 1 : 0;
-	fph->custom = custom;
-	fph->immutable = !!immutable;
+	fph->custom = !!(flags & FH_CUSTOM);
+	fph->immutable = !!(flags & FH_IMMUTABLE);
 	fph->mm = mm;
 
 	for (i = 0; i < hash_slots; i++)
@@ -1563,7 +1564,7 @@ int futex_hash_allocate_default(void)
 	if (current_buckets >= buckets)
 		return 0;
 
-	return futex_hash_allocate(buckets, 0, false);
+	return futex_hash_allocate(buckets, 0);
 }
 
 static int futex_hash_get_slots(void)
@@ -1592,7 +1593,7 @@ static int futex_hash_get_immutable(void
 
 #else
 
-static int futex_hash_allocate(unsigned int hash_slots, unsigned int immutable, bool custom)
+static int futex_hash_allocate(unsigned int hash_slots, unsigned int flags)
 {
 	return -EINVAL;
 }
@@ -1610,11 +1611,16 @@ static int futex_hash_get_immutable(void
 
 int futex_hash_prctl(unsigned long arg2, unsigned long arg3, unsigned long arg4)
 {
+	unsigned int flags = FH_CUSTOM;
 	int ret;
 
 	switch (arg2) {
 	case PR_FUTEX_HASH_SET_SLOTS:
-		ret = futex_hash_allocate(arg3, arg4, true);
+		if (arg4 & ~FH_FLAG_IMMUTABLE)
+			return -EINVAL;
+		if (arg4 & FH_FLAG_IMMUTABLE)
+			flags |= FH_IMMUTABLE;
+		ret = futex_hash_allocate(arg3, flags);
 		break;
 
 	case PR_FUTEX_HASH_GET_SLOTS: