Re: [PATCH v3 10/15] futex: Add sys_futex_requeue()

Jens Axboe
2023-09-22T13:56:38Z

On 9/22/23 5:03 AM, Peter Zijlstra wrote:
> On Fri, Sep 22, 2023 at 11:35:03AM +0200, Ingo Molnar wrote:
>>
>> * peterz@infradead.org <peterz@infradead.org> wrote:
>>
>>> --- linux-2.6.orig/kernel/futex/syscalls.c
>>> +++ linux-2.6/kernel/futex/syscalls.c
>>> @@ -396,6 +396,44 @@ SYSCALL_DEFINE6(futex_wait,
>>> return ret;
>>> }
>>>
>>> +/*
>>> + * sys_futex_requeue - Requeue a waiter from one futex to another
>>> + * @waiters: array describing the source and destination futex
>>> + * @flags: unused
>>> + * @nr_wake: number of futexes to wake
>>> + * @nr_requeue: number of futexes to requeue
>>> + *
>>> + * Identical to the traditional FUTEX_CMP_REQUEUE op, except it is part of the
>>> + * futex2 family of calls.
>>> + */
>>> +
>>> +SYSCALL_DEFINE4(futex_requeue,
>>> + struct futex_waitv __user *, waiters,
>>> + unsigned int, flags,
>>> + int, nr_wake,
>>> + int, nr_requeue)
>>> +{
>>> + struct futex_vector futexes[2];
>>> + u32 cmpval;
>>> + int ret;
>>> +
>>> + if (flags)
>>> + return -EINVAL;
>>
>> Small detail, but isn't -ENOSYS the canonical error code for functionality
>> not yet implemented - which the unused 'flags' ABI is arguably?
>>
>> -EINVAL is for recognized but incorrect parameters, such as:
>
> IIUC 'unknown flag' falls into the -EINVAL return category. Here we
> happen to have no known flags, but that should not matter.

Yep, -ENOSYS is for not having the syscall at all, -EINVAL for unknown
flags set.

--
Jens Axboe