linux-kernel - [HACK][PATCH 10/10] futex: Munge size and numa into the legacy interface

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Date:   Fri, 14 Jul 2023 15:39:09 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     tglx@...utronix.de, axboe@...nel.dk
Cc:     linux-kernel@...r.kernel.org, peterz@...radead.org,
        mingo@...hat.com, dvhart@...radead.org, dave@...olabs.net,
        andrealmeid@...lia.com, Andrew Morton <akpm@...ux-foundation.org>,
        urezki@...il.com, hch@...radead.org, lstoakes@...il.com,
        Arnd Bergmann <arnd@...db.de>, linux-api@...r.kernel.org,
        linux-mm@...ck.org, linux-arch@...r.kernel.org,
        malteskarupke@....de
Subject: [HACK][PATCH 10/10] futex: Munge size and numa into the legacy interface

Avert your eyes...

Arguably just the NUMA thing wouldn't be too bad.

Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
 include/uapi/linux/futex.h |   15 ++++++++++++---
 kernel/futex/futex.h       |    9 ++++++++-
 kernel/futex/syscalls.c    |   18 ++++++++++++++++++
 3 files changed, 38 insertions(+), 4 deletions(-)

--- a/include/uapi/linux/futex.h
+++ b/include/uapi/linux/futex.h
@@ -23,9 +23,18 @@
 #define FUTEX_CMP_REQUEUE_PI	12
 #define FUTEX_LOCK_PI2		13
 
-#define FUTEX_PRIVATE_FLAG	128
-#define FUTEX_CLOCK_REALTIME	256
-#define FUTEX_CMD_MASK		~(FUTEX_PRIVATE_FLAG | FUTEX_CLOCK_REALTIME)
+#define FUTEX_PRIVATE_FLAG	(1 << 7)
+#define FUTEX_CLOCK_REALTIME	(1 << 8)
+#define FUTEX_NUMA		(1 << 9)
+#define FUTEX_SIZE_32		(0 << 10) /* backwards compat */
+#define FUTEX_SIZE_64		(1 << 10)
+#define FUTEX_SIZE_8		(2 << 10)
+#define FUTEX_SIZE_16		(3 << 10)
+
+#define FUTEX_CMD_MASK		~(FUTEX_PRIVATE_FLAG	|	\
+				  FUTEX_CLOCK_REALTIME	|	\
+				  FUTEX_NUMA		|	\
+				  FUTEX_SIZE_16)
 
 #define FUTEX_WAIT_PRIVATE	(FUTEX_WAIT | FUTEX_PRIVATE_FLAG)
 #define FUTEX_WAKE_PRIVATE	(FUTEX_WAKE | FUTEX_PRIVATE_FLAG)
--- a/kernel/futex/futex.h
+++ b/kernel/futex/futex.h
@@ -39,7 +39,7 @@
 /* FUTEX_ to FLAGS_ */
 static inline unsigned int futex_to_flags(unsigned int op)
 {
-	unsigned int flags = FLAGS_SIZE_32;
+	unsigned int sz, flags = 0;
 
 	if (!(op & FUTEX_PRIVATE_FLAG))
 		flags |= FLAGS_SHARED;
@@ -47,6 +47,13 @@ static inline unsigned int futex_to_flag
 	if (op & FUTEX_CLOCK_REALTIME)
 		flags |= FLAGS_CLOCKRT;
 
+	if (op & FUTEX_NUMA)
+		flags |= FLAGS_NUMA;
+
+	/* { 2,3,0,1 } -> { 0,1,2,3 } */
+	sz = ((op + FUTEX_SIZE_8) & FUTEX_SIZE_16) >> 10;
+	flags |= sz;
+
 	return flags;
 }
 
--- a/kernel/futex/syscalls.c
+++ b/kernel/futex/syscalls.c
@@ -95,6 +95,24 @@ long do_futex(u32 __user *uaddr, int op,
 			return -ENOSYS;
 	}
 
+	/* can't support u64 with a u32 based interface */
+	if ((flags & FLAGS_SIZE_MASK) == FLAGS_SIZE_64)
+		return -ENOSYS;
+
+	switch (cmd) {
+	case FUTEX_WAIT:
+	case FUTEX_WAIT_BITSET:
+	case FUTEX_WAKE:
+	case FUTEX_WAKE_BITSET:
+		/* u8, u16, u32 */
+		break;
+
+	default:
+		/* only u32 for now */
+		if ((flags & FLAGS_SIZE_MASK) != FLAGS_SIZE_32)
+			return -ENOSYS;
+	}
+
 	switch (cmd) {
 	case FUTEX_WAIT:
 		val3 = FUTEX_BITSET_MATCH_ANY;