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Message-ID: <ec7cf6ad-3081-47a8-be83-af6eb7befa35@kernel.dk>
Date:   Fri, 22 Sep 2023 07:56:28 -0600
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Subject: Re: [PATCH v3 10/15] futex: Add sys_futex_requeue()
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To:     Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@kernel.org>
Cc:     tglx@linutronix.de, linux-kernel@vger.kernel.org, mingo@redhat.com,
        dvhart@infradead.org, dave@stgolabs.net, andrealmeid@igalia.com,
        Andrew Morton <akpm@linux-foundation.org>, urezki@gmail.com,
        hch@infradead.org, lstoakes@gmail.com,
        Arnd Bergmann <arnd@arndb.de>, linux-api@vger.kernel.org,
        linux-mm@kvack.org, linux-arch@vger.kernel.org,
        malteskarupke@web.de, Geert Uytterhoeven <geert@linux-m68k.org>
References: <20230921104505.717750284@noisy.programming.kicks-ass.net>
 <20230921105248.511860556@noisy.programming.kicks-ass.net>
 <ZQ1fx29+b8PmLVk6@gmail.com>
 <20230922110335.GC7080@noisy.programming.kicks-ass.net>
From:   Jens Axboe <axboe@kernel.dk>
In-Reply-To: <20230922110335.GC7080@noisy.programming.kicks-ass.net>
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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