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SubjectRe: [patch 6/6] futex: Provide FUTEX_LOCK_PI2 to support clock selection
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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@linutronix.de>
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> ---
> 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

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