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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>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20210422194705.440773992@linutronix.de
This commit is contained in:
Thomas Gleixner 2021-04-22 21:44:23 +02:00 • committed by Peter Zijlstra
parent e112c41341
commit bf22a69768
2 changed files with 8 additions and 1 deletions
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2
include/uapi/linux/futex.h
View file

@ -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)