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Date:   Fri, 22 Sep 2023 13:03:35 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Ingo Molnar <mingo@kernel.org>
Cc:     tglx@linutronix.de, axboe@kernel.dk, 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>
Subject: Re: [PATCH v3 10/15] futex: Add sys_futex_requeue()
Message-ID: <20230922110335.GC7080@noisy.programming.kicks-ass.net>
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 <ZQ1fx29+b8PmLVk6@gmail.com>
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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.