linux-kernel - Re: [PATCH v1 05/14] futex: Add sys_futex

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Message-Id: <2a1f8ae6-ed2b-4fe8-85af-df64e9c84794@app.fastmail.com>
Date:   Fri, 21 Jul 2023 17:41:20 +0200
From:   "Arnd Bergmann" <arnd@...db.de>
To:     "Peter Zijlstra" <peterz@...radead.org>,
        "Thomas Gleixner" <tglx@...utronix.de>,
        "Jens Axboe" <axboe@...nel.dk>
Cc:     linux-kernel@...r.kernel.org, "Ingo Molnar" <mingo@...hat.com>,
        "Darren Hart" <dvhart@...radead.org>, dave@...olabs.net,
        andrealmeid@...lia.com,
        "Andrew Morton" <akpm@...ux-foundation.org>, urezki@...il.com,
        "Christoph Hellwig" <hch@...radead.org>,
        "Lorenzo Stoakes" <lstoakes@...il.com>, linux-api@...r.kernel.org,
        linux-mm@...ck.org, Linux-Arch <linux-arch@...r.kernel.org>,
        malteskarupke@....de
Subject: Re: [PATCH v1 05/14] futex: Add sys_futex_wake()

On Fri, Jul 21, 2023, at 12:22, Peter Zijlstra wrote:
> --- a/arch/arm/tools/syscall.tbl
> +++ b/arch/arm/tools/syscall.tbl
> @@ -465,3 +465,4 @@
>  449	common	futex_waitv			sys_futex_waitv
>  450	common	set_mempolicy_home_node		sys_set_mempolicy_home_node
>  451	common	cachestat			sys_cachestat
> +452	common	futex_wake			sys_futex_wake

This clashes with __NR_fchmodat2 in linux-next, which also wants number 452.

> --- a/arch/arm64/include/asm/unistd32.h
> +++ b/arch/arm64/include/asm/unistd32.h
> @@ -909,6 +909,8 @@ __SYSCALL(__NR_futex_waitv, sys_futex_wa
>  __SYSCALL(__NR_set_mempolicy_home_node, sys_set_mempolicy_home_node)
>  #define __NR_cachestat 451
>  __SYSCALL(__NR_cachestat, sys_cachestat)
> +#define __NR_futex_wake 452
> +__SYSCALL(__NR_futex_wake, sys_futex_wake)
> 
>  /*
>   * Please add new compat syscalls above this comment and update

Unfortunately, changing this file still requires updating __NR_compat_syscalls
in arch/arm64/include/asm/unistd.h as well.

> --- a/kernel/sys_ni.c
> +++ b/kernel/sys_ni.c
> @@ -87,6 +87,7 @@ COND_SYSCALL_COMPAT(set_robust_list);
>  COND_SYSCALL(get_robust_list);
>  COND_SYSCALL_COMPAT(get_robust_list);
>  COND_SYSCALL(futex_waitv);
> +COND_SYSCALL(futex_wake);
>  COND_SYSCALL(kexec_load);
>  COND_SYSCALL_COMPAT(kexec_load);
>  COND_SYSCALL(init_module);

This is fine for the moment, but I wonder if we should start making
futex mandatory at some point. Right now, sparc32 with CONFIG_SMP
cannot support futex because of the lack of atomics in early
sparc processors, but sparc32 glibc actually requires futexes
and consequently only works on uniprocessor machines, on sparc64
compat mode, or on Leon3 with out of tree patches.

      Arnd