linux-kernel - Re: [PATCH v3 10/15] futex: Add sys_futex

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Date:   Fri, 22 Sep 2023 07:56:28 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>
Cc:     tglx@...utronix.de, linux-kernel@...r.kernel.org, mingo@...hat.com,
        dvhart@...radead.org, dave@...olabs.net, andrealmeid@...lia.com,
        Andrew Morton <akpm@...ux-foundation.org>, urezki@...il.com,
        hch@...radead.org, lstoakes@...il.com,
        Arnd Bergmann <arnd@...db.de>, linux-api@...r.kernel.org,
        linux-mm@...ck.org, linux-arch@...r.kernel.org,
        malteskarupke@....de, Geert Uytterhoeven <geert@...ux-m68k.org>
Subject: Re: [PATCH v3 10/15] futex: Add sys_futex_requeue()

On 9/22/23 5:03 AM, Peter Zijlstra wrote:
> On Fri, Sep 22, 2023 at 11:35:03AM +0200, Ingo Molnar wrote:
>>
>> * peterz@...radead.org <peterz@...radead.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