Messages in this thread Patch in this message | | | Subject | Re: [patch 6/6] futex: Provide FUTEX_LOCK_PI2 to support clock selection | From | André Almeida <> | Date | Fri, 23 Apr 2021 19:20:44 -0300 |
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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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