[patch 6/6] futex: Provide FUTEX_LOCK_PI2 to support clock selection

[Thomas Gleixner]

The FUTEX_LOCK_PI futex operand uses a CLOCK_REALTIME based absolute
timeout since it was implemented, but it does not require that the
FUTEX_CLOCK_REALTIME flag is set, because that was introduced later.

In theory as none of the user space implementations can set the
FUTEX_CLOCK_REALTIME flag on this operand, it would be possible to
creatively abuse it and make the meaning invers, i.e. select CLOCK_REALTIME
when not set and CLOCK_MONOTONIC when set. But that's a nasty hackery.

Another option would be to have a new FUTEX_CLOCK_MONOTONIC flag only for
FUTEX_LOCK_PI, but that's also awkward because it does not allow libraries
to handle the timeout clock selection consistently.

So provide a new FUTEX_LOCK_PI2 operand which implements the timeout
semantics which the other operands use and leave FUTEX_LOCK_PI alone.

Reported-by: Kurt Kanzenbach <kurt@xxxxxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
---
 include/uapi/linux/futex.h |    1 +
 kernel/futex.c             |    6 +++++-
 2 files changed, 6 insertions(+), 1 deletion(-)

--- a/include/uapi/linux/futex.h
+++ b/include/uapi/linux/futex.h
@@ -21,6 +21,7 @@
 #define FUTEX_WAKE_BITSET	10
 #define FUTEX_WAIT_REQUEUE_PI	11
 #define FUTEX_CMP_REQUEUE_PI	12
+#define FUTEX_LOCK_PI2		13
 
 #define FUTEX_PRIVATE_FLAG	128
 #define FUTEX_CLOCK_REALTIME	256
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -3711,7 +3711,8 @@ long do_futex(u32 __user *uaddr, int op,
 
 	if (op & FUTEX_CLOCK_REALTIME) {
 		flags |= FLAGS_CLOCKRT;
-		if (cmd != FUTEX_WAIT_BITSET && cmd != FUTEX_WAIT_REQUEUE_PI)
+		if (cmd != FUTEX_WAIT_BITSET && cmd != FUTEX_WAIT_REQUEUE_PI &&
+		    cmd != FUTEX_LOCK_PI2)
 			return -ENOSYS;
 	}
 
@@ -3744,6 +3745,8 @@ long do_futex(u32 __user *uaddr, int op,
 		return futex_wake_op(uaddr, flags, uaddr2, val, val2, val3);
 	case FUTEX_LOCK_PI:
 		flags |= FLAGS_CLOCKRT;
+		fallthrough;
+	case FUTEX_LOCK_PI2:
 		return futex_lock_pi(uaddr, flags, timeout, 0);
 	case FUTEX_UNLOCK_PI:
 		return futex_unlock_pi(uaddr, flags);
@@ -3764,6 +3767,7 @@ static inline bool futex_cmd_has_timeout
 	switch (cmd) {
 	case FUTEX_WAIT:
 	case FUTEX_LOCK_PI:
+	case FUTEX_LOCK_PI2:
 	case FUTEX_WAIT_BITSET:
 	case FUTEX_WAIT_REQUEUE_PI:
 		return true;