[RFC 0/4] futex2: Add new futex interface
Thu Jun 25 06:48:29 GMT 2020
On Fri, Jun 12, 2020 at 8:51 PM André Almeida <firstname.lastname@example.org> wrote:
> - The proposed interface uses ktime_t type for absolute timeout, and I
> assumed that it should use values in a nsec resolution. If this is true,
> we have some problems with i386 ABI, please check out the
> COMPAT_32BIT_TIME implementation in patch 1 for more details. I
> haven't added a time64 implementation yet, until this is clarified.
ktime_t is not part of the uapi headers, and has always been considered
an implementation detail of the kernel so far. I would argue it should
stay that way. The most sensible alternatives would be to either use
a "__u64 *timeout" argument for a relative timeout, or a
"struct __kernel_timespec *timeout" for an absolute timeout.
old_time32_t also makes no sense for multiple reasons:
- It's another kernel internal type and not part of the uapi headers
- your time32 call has different calling conventions from your time64
version, not just a different type.
- there should be no need to add syscalls that are known to be buggy
when there is a replacement type that does not have that bug.
> - Is expected to have a x32 ABI implementation as well? In the case of
> wait and wake, we could use the same as x86_64 ABI. However, for the
> waitv (aka wait on multiple futexes) we would need a proper x32 entry
> since we are dealing with 32bit pointers.
For new syscalls, I'd actually recommend not having a separate
entry point, but just checking 'if (in_compat_syscall())' inside of the
implementation to pick one behavior vs the other when accessing
the user pointers. This keeps the implementation simpler and
avoids assigning a new x32 syscall number that would be different
from all the other architectures.
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