Commit bf22a697 authored by Thomas Gleixner's avatar Thomas Gleixner Committed by Peter Zijlstra
Browse files

futex: Provide FUTEX_LOCK_PI2 to support clock selection 



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: default avatarKurt Kanzenbach <kurt@linutronix.de>
Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20210422194705.440773992@linutronix.de
parent e112c413

include/uapi/linux/futex.h

+2 −0
Original line number Diff line number Diff line
@@ -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
@@ -32,6 +33,7 @@ 
#define FUTEX_CMP_REQUEUE_PRIVATE (FUTEX_CMP_REQUEUE | FUTEX_PRIVATE_FLAG)

#define FUTEX_WAKE_OP_PRIVATE	(FUTEX_WAKE_OP | FUTEX_PRIVATE_FLAG)

#define FUTEX_LOCK_PI_PRIVATE	(FUTEX_LOCK_PI | FUTEX_PRIVATE_FLAG)

#define FUTEX_LOCK_PI2_PRIVATE	(FUTEX_LOCK_PI2 | FUTEX_PRIVATE_FLAG)

#define FUTEX_UNLOCK_PI_PRIVATE	(FUTEX_UNLOCK_PI | FUTEX_PRIVATE_FLAG)

#define FUTEX_TRYLOCK_PI_PRIVATE (FUTEX_TRYLOCK_PI | FUTEX_PRIVATE_FLAG)

#define FUTEX_WAIT_BITSET_PRIVATE	(FUTEX_WAIT_BITSET | FUTEX_PRIVATE_FLAG)

kernel/futex.c

+6 −1
Original line number Diff line number Diff line
@@ -3707,12 +3707,14 @@ long do_futex(u32 __user *uaddr, int op, u32 val, ktime_t *timeout, 


	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;

	}



	switch (cmd) {

	case FUTEX_LOCK_PI:

	case FUTEX_LOCK_PI2:

	case FUTEX_UNLOCK_PI:

	case FUTEX_TRYLOCK_PI:

	case FUTEX_WAIT_REQUEUE_PI:
@@ -3740,6 +3742,8 @@ long do_futex(u32 __user *uaddr, int op, u32 val, ktime_t *timeout, 
		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);
@@ -3760,6 +3764,7 @@ static __always_inline bool futex_cmd_has_timeout(u32 cmd) 
	switch (cmd) {

	case FUTEX_WAIT:

	case FUTEX_LOCK_PI:

	case FUTEX_LOCK_PI2:

	case FUTEX_WAIT_BITSET:

	case FUTEX_WAIT_REQUEUE_PI:

		return true;