linux-kernel - Re: [PATCH 1/7] futex: abstract out futex_op_to

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Date:   Wed, 12 Jul 2023 10:16:17 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Jens Axboe <axboe@...nel.dk>
Cc:     io-uring@...r.kernel.org, linux-kernel@...r.kernel.org,
        tglx@...utronix.de, mingo@...hat.com
Subject: Re: [PATCH 1/7] futex: abstract out futex_op_to_flags() helper

On Tue, Jul 11, 2023 at 06:46:59PM -0600, Jens Axboe wrote:
> Rather than needing to duplicate this for the io_uring hook of futexes,
> abstract out a helper.
> 
> No functional changes intended in this patch.
> 
> Signed-off-by: Jens Axboe <axboe@...nel.dk>
> ---
>  kernel/futex/futex.h    | 15 +++++++++++++++
>  kernel/futex/syscalls.c | 11 ++---------
>  2 files changed, 17 insertions(+), 9 deletions(-)
> 
> diff --git a/kernel/futex/futex.h b/kernel/futex/futex.h
> index b5379c0e6d6d..d2949fca37d1 100644
> --- a/kernel/futex/futex.h
> +++ b/kernel/futex/futex.h
> @@ -291,4 +291,19 @@ extern int futex_unlock_pi(u32 __user *uaddr, unsigned int flags);
>  
>  extern int futex_lock_pi(u32 __user *uaddr, unsigned int flags, ktime_t *time, int trylock);
>  
> +static inline bool futex_op_to_flags(int op, int cmd, unsigned int *flags)
> +{
	*flags = 0;

> +	if (!(op & FUTEX_PRIVATE_FLAG))
> +		*flags |= FLAGS_SHARED;
> +
> +	if (op & FUTEX_CLOCK_REALTIME) {
> +		*flags |= FLAGS_CLOCKRT;
> +		if (cmd != FUTEX_WAIT_BITSET && cmd != FUTEX_WAIT_REQUEUE_PI &&
> +		    cmd != FUTEX_LOCK_PI2)
> +			return false;
> +	}
> +
> +	return true;
> +}


> diff --git a/kernel/futex/syscalls.c b/kernel/futex/syscalls.c
> index a8074079b09e..75ca8c41cc94 100644
> --- a/kernel/futex/syscalls.c
> +++ b/kernel/futex/syscalls.c
> @@ -88,15 +88,8 @@ long do_futex(u32 __user *uaddr, int op, u32 val, ktime_t *timeout,
>  	int cmd = op & FUTEX_CMD_MASK;
>  	unsigned int flags = 0;

and skip the initializer here.

>  
> -	if (!(op & FUTEX_PRIVATE_FLAG))
> -		flags |= FLAGS_SHARED;
> -
> -	if (op & FUTEX_CLOCK_REALTIME) {
> -		flags |= FLAGS_CLOCKRT;
> -		if (cmd != FUTEX_WAIT_BITSET && cmd != FUTEX_WAIT_REQUEUE_PI &&
> -		    cmd != FUTEX_LOCK_PI2)
> -			return -ENOSYS;
> -	}
> +	if (!futex_op_to_flags(op, cmd, &flags))
> +		return -ENOSYS;
>  

then the helper is more self sufficient and doesn't rely on the caller
to initialize the flags word.