linux-kernel - Re: [PATCH v2 01/14] futex: Clarify FUTEX2 flags

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Date:   Mon, 7 Aug 2023 20:51:42 -0300
From:   André Almeida <andrealmeid@...lia.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, mingo@...hat.com,
        dvhart@...radead.org, dave@...olabs.net,
        Andrew Morton <akpm@...ux-foundation.org>, urezki@...il.com,
        hch@...radead.org, lstoakes@...il.com,
        Arnd Bergmann <arnd@...db.de>, tglx@...utronix.de,
        linux-api@...r.kernel.org, linux-mm@...ck.org,
        linux-arch@...r.kernel.org, malteskarupke@....de, axboe@...nel.dk
Subject: Re: [PATCH v2 01/14] futex: Clarify FUTEX2 flags

Hi Peter, thank you for your patches.

Em 07/08/2023 09:18, Peter Zijlstra escreveu:
> sys_futex_waitv() is part of the futex2 series (the first and only so
> far) of syscalls and has a flags field per futex (as opposed to flags
> being encoded in the futex op).
> 
> This new flags field has a new namespace, which unfortunately isn't
> super explicit. Notably it currently takes FUTEX_32 and
> FUTEX_PRIVATE_FLAG.
> 
> Introduce the FUTEX2 namespace to clarify this
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> Reviewed-by: Thomas Gleixner <tglx@...utronix.de>

Reviewed-by: André Almeida <andrealmeid@...lia.com>

> ---
>   include/uapi/linux/futex.h |   16 +++++++++++++---
>   kernel/futex/syscalls.c    |    7 +++----
>   2 files changed, 16 insertions(+), 7 deletions(-)
> 
> --- a/include/uapi/linux/futex.h
> +++ b/include/uapi/linux/futex.h
> @@ -44,10 +44,20 @@
>   					 FUTEX_PRIVATE_FLAG)
>   
>   /*
> - * Flags to specify the bit length of the futex word for futex2 syscalls.
> - * Currently, only 32 is supported.
> + * Flags for futex2 syscalls.
>    */
> -#define FUTEX_32		2
> +			/*	0x00 */
> +			/*	0x01 */
> +#define FUTEX2_SIZE_U32		0x02
> +			/*	0x04 */
> +			/*	0x08 */
> +			/*	0x10 */
> +			/*	0x20 */
> +			/*	0x40 */
> +#define FUTEX2_PRIVATE		FUTEX_PRIVATE_FLAG
> +
> +/* do not use */
> +#define FUTEX_32		FUTEX2_SIZE_U32 /* historical accident :-( */
>   
>   /*
>    * Max numbers of elements in a futex_waitv array
> --- a/kernel/futex/syscalls.c
> +++ b/kernel/futex/syscalls.c
> @@ -183,8 +183,7 @@ SYSCALL_DEFINE6(futex, u32 __user *, uad
>   	return do_futex(uaddr, op, val, tp, uaddr2, (unsigned long)utime, val3);
>   }
>   
> -/* Mask of available flags for each futex in futex_waitv list */
> -#define FUTEXV_WAITER_MASK (FUTEX_32 | FUTEX_PRIVATE_FLAG)
> +#define FUTEX2_VALID_MASK (FUTEX2_SIZE_U32 | FUTEX2_PRIVATE)
>   
>   /**
>    * futex_parse_waitv - Parse a waitv array from userspace
> @@ -205,10 +204,10 @@ static int futex_parse_waitv(struct fute
>   		if (copy_from_user(&aux, &uwaitv[i], sizeof(aux)))
>   			return -EFAULT;
>   
> -		if ((aux.flags & ~FUTEXV_WAITER_MASK) || aux.__reserved)
> +		if ((aux.flags & ~FUTEX2_VALID_MASK) || aux.__reserved)
>   			return -EINVAL;
>   
> -		if (!(aux.flags & FUTEX_32))
> +		if (!(aux.flags & FUTEX2_SIZE_U32))
>   			return -EINVAL;
>   
>   		futexv[i].w.flags = aux.flags;
> 
>