linux-kernel - Re: [PATCH] futex: Improve get_inode_sequence

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Message-ID: <390874e9-eddd-494d-9bc0-62b34a9bb29b@igalia.com>
Date: Thu, 3 Oct 2024 10:28:57 -0300
From: André Almeida <andrealmeid@...lia.com>
To: Uros Bizjak <ubizjak@...il.com>
Cc: Thomas Gleixner <tglx@...utronix.de>, linux-kernel@...r.kernel.org,
 Ingo Molnar <mingo@...nel.org>, Peter Zijlstra <peterz@...radead.org>,
 Darren Hart <dvhart@...radead.org>, Davidlohr Bueso <dave@...olabs.net>
Subject: Re: [PATCH] futex: Improve get_inode_sequence_number()

Hi Uros,

Em 03/10/2024 09:18, Uros Bizjak escreveu:
> Rewrite FOR loop to a DO-WHILE loop where returns are moved out of
> the loop. Use atomic64_inc_return() instead of atomic64_add_return().
> 
> Use !atomic64_try_cmpxchg_relaxed(*ptr, &old, new) instead of
> atomic64_cmpxchg_relaxed (*ptr, old, new) != old.  x86 CMPXCHG
> instruction returns success in ZF flag, so this change saves
> a compare after CMPXCHG..
> 
> Note that due to early return, "old" equals to 0 before
> atomic64_cmpxchg_relaxed(), so initialization of variable to 0
> is not needed.
> 

Despite the implicitly `old = 0`, I think it makes people life easier to 
know explicitly that `old = 0` in the cmpxchg() call.

Also, please state in the commit message the motivation of doing this 
change. Is to make the code simpler or to try to save some instructions? 
The compiler might be already saving such instructions for us :)

> Signed-off-by: Uros Bizjak <ubizjak@...il.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Ingo Molnar <mingo@...nel.org>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Darren Hart <dvhart@...radead.org>
> Cc: Davidlohr Bueso <dave@...olabs.net>
> Cc: "André Almeida" <andrealmeid@...lia.com>
> ---
>   kernel/futex/core.c | 17 +++++++----------
>   1 file changed, 7 insertions(+), 10 deletions(-)
> 
> diff --git a/kernel/futex/core.c b/kernel/futex/core.c
> index 136768ae2637..665501c885d0 100644
> --- a/kernel/futex/core.c
> +++ b/kernel/futex/core.c
> @@ -173,23 +173,20 @@ futex_setup_timer(ktime_t *time, struct hrtimer_sleeper *timeout,
>   static u64 get_inode_sequence_number(struct inode *inode)
>   {
>   	static atomic64_t i_seq;
> -	u64 old;
> +	u64 old, new;
>   
>   	/* Does the inode already have a sequence number? */
>   	old = atomic64_read(&inode->i_sequence);
>   	if (likely(old))
>   		return old;
>   
> -	for (;;) {
> -		u64 new = atomic64_add_return(1, &i_seq);
> -		if (WARN_ON_ONCE(!new))
> -			continue;
> +	do {
> +		new = atomic64_inc_return(&i_seq);
> +	} while	(WARN_ON_ONCE(!new));
>   
> -		old = atomic64_cmpxchg_relaxed(&inode->i_sequence, 0, new);
> -		if (old)
> -			return old;
> -		return new;
> -	}
> +	if (!atomic64_try_cmpxchg_relaxed(&inode->i_sequence, &old, new))
> +		return old;
> +	return new;
>   }
>   
>   /**