Messages in this thread Patch in this message | | | Date | Thu, 22 Apr 2021 21:44:23 +0200 | From | Thomas Gleixner <> | Subject | [patch 6/6] futex: Provide FUTEX_LOCK_PI2 to support clock selection |
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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 --- 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;
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