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

[André Almeida]

Hi Thomas,

Às 16:44 de 22/04/21, Thomas Gleixner escreveu:
> 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

To keep consistency with other operations, maybe add a 
FUTEX_LOCK_PI2_PRIVATE?

> --- 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;
>   	}

As FUTEX_LOCK_PI, FUTEX_LOCK_PI2 also requires FUTEX_CMPXCHG right? 
Then, add it here:

	switch (cmd) {
	case FUTEX_LOCK_PI:
+	case FUTEX_LOCK_PI2:
	case FUTEX_UNLOCK_PI:
	case FUTEX_TRYLOCK_PI:
	case FUTEX_WAIT_REQUEUE_PI:
	case FUTEX_CMP_REQUEUE_PI:
 		if (!futex_cmpxchg_enabled)
 			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;

Thanks,
	André