linux-kernel - Re: [patch V5 10/12] futex: Convert to get/put_user

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Message-ID: <942a27d9-135a-4e59-8eff-a44c46f8bb76@csgroup.eu>
Date: Tue, 4 Nov 2025 07:31:05 +0100
From: Christophe Leroy <christophe.leroy@...roup.eu>
To: Thomas Gleixner <tglx@...utronix.de>, LKML <linux-kernel@...r.kernel.org>
Cc: Peter Zijlstra <peterz@...radead.org>, Darren Hart
 <dvhart@...radead.org>, Davidlohr Bueso <dave@...olabs.net>,
 André Almeida <andrealmeid@...lia.com>,
 kernel test robot <lkp@...el.com>, Russell King <linux@...linux.org.uk>,
 linux-arm-kernel@...ts.infradead.org,
 Linus Torvalds <torvalds@...ux-foundation.org>, x86@...nel.org,
 Madhavan Srinivasan <maddy@...ux.ibm.com>,
 Michael Ellerman <mpe@...erman.id.au>, Nicholas Piggin <npiggin@...il.com>,
 linuxppc-dev@...ts.ozlabs.org, Paul Walmsley <pjw@...nel.org>,
 Palmer Dabbelt <palmer@...belt.com>, linux-riscv@...ts.infradead.org,
 Heiko Carstens <hca@...ux.ibm.com>,
 Christian Borntraeger <borntraeger@...ux.ibm.com>,
 Sven Schnelle <svens@...ux.ibm.com>, linux-s390@...r.kernel.org,
 Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
 Andrew Cooper <andrew.cooper3@...rix.com>,
 David Laight <david.laight.linux@...il.com>,
 Julia Lawall <Julia.Lawall@...ia.fr>, Nicolas Palix <nicolas.palix@...g.fr>,
 Alexander Viro <viro@...iv.linux.org.uk>,
 Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
 linux-fsdevel@...r.kernel.org
Subject: Re: [patch V5 10/12] futex: Convert to get/put_user_inline()



Le 27/10/2025 à 09:44, Thomas Gleixner a écrit :
> From: Thomas Gleixner <tglx@...utronix.de>
> 
> Replace the open coded implementation with the new get/put_user_inline()
> helpers. This might be replaced by a regular get/put_user(), but that needs
> a proper performance evaluation.
> 
> No functional change intended
> 
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Darren Hart <dvhart@...radead.org>
> Cc: Davidlohr Bueso <dave@...olabs.net>
> Cc: "André Almeida" <andrealmeid@...lia.com>

Reviewed-by: Christophe Leroy <christophe.leroy@...roup.eu>

> ---
> V5: Rename again and remove the helpers
> V4: Rename once moar
> V3: Adapt to scope changes
> V2: Convert to scoped variant
> ---
>   kernel/futex/core.c  |    4 +--
>   kernel/futex/futex.h |   58 ++-------------------------------------------------
>   2 files changed, 5 insertions(+), 57 deletions(-)
> ---
> --- a/kernel/futex/core.c
> +++ b/kernel/futex/core.c
> @@ -581,7 +581,7 @@ int get_futex_key(u32 __user *uaddr, uns
>   	if (flags & FLAGS_NUMA) {
>   		u32 __user *naddr = (void *)uaddr + size / 2;
>   
> -		if (futex_get_value(&node, naddr))
> +		if (get_user_inline(node, naddr))
>   			return -EFAULT;
>   
>   		if ((node != FUTEX_NO_NODE) &&
> @@ -601,7 +601,7 @@ int get_futex_key(u32 __user *uaddr, uns
>   			node = numa_node_id();
>   			node_updated = true;
>   		}
> -		if (node_updated && futex_put_value(node, naddr))
> +		if (node_updated && put_user_inline(node, naddr))
>   			return -EFAULT;
>   	}
>   
> --- a/kernel/futex/futex.h
> +++ b/kernel/futex/futex.h
> @@ -281,63 +281,11 @@ static inline int futex_cmpxchg_value_lo
>   	return ret;
>   }
>   
> -/*
> - * This does a plain atomic user space read, and the user pointer has
> - * already been verified earlier by get_futex_key() to be both aligned
> - * and actually in user space, just like futex_atomic_cmpxchg_inatomic().
> - *
> - * We still want to avoid any speculation, and while __get_user() is
> - * the traditional model for this, it's actually slower than doing
> - * this manually these days.
> - *
> - * We could just have a per-architecture special function for it,
> - * the same way we do futex_atomic_cmpxchg_inatomic(), but rather
> - * than force everybody to do that, write it out long-hand using
> - * the low-level user-access infrastructure.
> - *
> - * This looks a bit overkill, but generally just results in a couple
> - * of instructions.
> - */
> -static __always_inline int futex_get_value(u32 *dest, u32 __user *from)
> -{
> -	u32 val;
> -
> -	if (can_do_masked_user_access())
> -		from = masked_user_access_begin(from);
> -	else if (!user_read_access_begin(from, sizeof(*from)))
> -		return -EFAULT;
> -	unsafe_get_user(val, from, Efault);
> -	user_read_access_end();
> -	*dest = val;
> -	return 0;
> -Efault:
> -	user_read_access_end();
> -	return -EFAULT;
> -}
> -
> -static __always_inline int futex_put_value(u32 val, u32 __user *to)
> -{
> -	if (can_do_masked_user_access())
> -		to = masked_user_access_begin(to);
> -	else if (!user_write_access_begin(to, sizeof(*to)))
> -		return -EFAULT;
> -	unsafe_put_user(val, to, Efault);
> -	user_write_access_end();
> -	return 0;
> -Efault:
> -	user_write_access_end();
> -	return -EFAULT;
> -}
> -
> +/* Read from user memory with pagefaults disabled */
>   static inline int futex_get_value_locked(u32 *dest, u32 __user *from)
>   {
> -	int ret;
> -
> -	pagefault_disable();
> -	ret = futex_get_value(dest, from);
> -	pagefault_enable();
> -
> -	return ret;
> +	guard(pagefault)();
> +	return get_user_inline(*dest, from);
>   }
>   
>   extern void __futex_unqueue(struct futex_q *q);
>