linux-kernel - Re: [PATCH v2] fs: fall back to file_ref

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Message-ID: <r6r5osn7darsuaqv7cjkonaq63zioo6kup7ds3wyeyxjyohiso@hb3ftogkuw6m>
Date: Tue, 22 Apr 2025 15:38:41 +0200
From: Jan Kara <jack@...e.cz>
To: Mateusz Guzik <mjguzik@...il.com>
Cc: brauner@...nel.org, viro@...iv.linux.org.uk, jack@...e.cz, 
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org, 
	kernel test robot <oliver.sang@...el.com>
Subject: Re: [PATCH v2] fs: fall back to file_ref_put() for non-last reference

On Fri 18-04-25 14:57:56, Mateusz Guzik wrote:
> This reduces the slowdown in face of multiple callers issuing close on
> what turns out to not be the last reference.
> 
> Signed-off-by: Mateusz Guzik <mjguzik@...il.com>
> Reported-by: kernel test robot <oliver.sang@...el.com>
> Closes: https://lore.kernel.org/oe-lkp/202504171513.6d6f8a16-lkp@intel.com

Yeah, this looks good. Feel free to add:

Reviewed-by: Jan Kara <jack@...e.cz>

								Honza

> ---
> 
> v2:
> - fix a copy pasto with bad conditional
> 
>  fs/file.c                |  2 +-
>  include/linux/file_ref.h | 19 ++++++-------------
>  2 files changed, 7 insertions(+), 14 deletions(-)
> 
> diff --git a/fs/file.c b/fs/file.c
> index dc3f7e120e3e..3a3146664cf3 100644
> --- a/fs/file.c
> +++ b/fs/file.c
> @@ -26,7 +26,7 @@
>  
>  #include "internal.h"
>  
> -bool __file_ref_put_badval(file_ref_t *ref, unsigned long cnt)
> +static noinline bool __file_ref_put_badval(file_ref_t *ref, unsigned long cnt)
>  {
>  	/*
>  	 * If the reference count was already in the dead zone, then this
> diff --git a/include/linux/file_ref.h b/include/linux/file_ref.h
> index 7db62fbc0500..31551e4cb8f3 100644
> --- a/include/linux/file_ref.h
> +++ b/include/linux/file_ref.h
> @@ -61,7 +61,6 @@ static inline void file_ref_init(file_ref_t *ref, unsigned long cnt)
>  	atomic_long_set(&ref->refcnt, cnt - 1);
>  }
>  
> -bool __file_ref_put_badval(file_ref_t *ref, unsigned long cnt);
>  bool __file_ref_put(file_ref_t *ref, unsigned long cnt);
>  
>  /**
> @@ -178,20 +177,14 @@ static __always_inline __must_check bool file_ref_put(file_ref_t *ref)
>   */
>  static __always_inline __must_check bool file_ref_put_close(file_ref_t *ref)
>  {
> -	long old, new;
> +	long old;
>  
>  	old = atomic_long_read(&ref->refcnt);
> -	do {
> -		if (unlikely(old < 0))
> -			return __file_ref_put_badval(ref, old);
> -
> -		if (old == FILE_REF_ONEREF)
> -			new = FILE_REF_DEAD;
> -		else
> -			new = old - 1;
> -	} while (!atomic_long_try_cmpxchg(&ref->refcnt, &old, new));
> -
> -	return new == FILE_REF_DEAD;
> +	if (likely(old == FILE_REF_ONEREF)) {
> +		if (likely(atomic_long_try_cmpxchg(&ref->refcnt, &old, FILE_REF_DEAD)))
> +			return true;
> +	}
> +	return file_ref_put(ref);
>  }
>  
>  /**
> -- 
> 2.48.1
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR