linux-kernel - Re: [PATCH] fs: use debug-only asserts around fd allocation and install

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Message-ID: <nlsxinmlymfa6jkafzu4d2xdbrrwjpkvdnz5hxnfn7qahltqws@nagqp77yuk6f>
Date: Wed, 12 Mar 2025 17:35:03 +0100
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
Subject: Re: [PATCH] fs: use debug-only asserts around fd allocation and
 install

On Wed 12-03-25 17:19:41, Mateusz Guzik wrote:
> This also restores the check which got removed in 52732bb9abc9ee5b
> ("fs/file.c: remove sanity_check and add likely/unlikely in alloc_fd()")
> for performance reasons -- they no longer apply with a debug-only
> variant.
> 
> Signed-off-by: Mateusz Guzik <mjguzik@...il.com>

Looks good. Feel free to add:

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

								Honza

> ---
> 
> I have about 0 opinion whether this should be BUG or WARN, the code was
> already inconsistent on this front. If you want the latter, I'll have 0
> complaints if you just sed it and commit as yours.
> 
> This reminded me to sort out that litmus test for smp_rmb, hopefully
> soon(tm) as it is now nagging me.
> 
>  fs/file.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/file.c b/fs/file.c
> index 6c159ede55f1..09460ec74ef8 100644
> --- a/fs/file.c
> +++ b/fs/file.c
> @@ -582,6 +582,7 @@ static int alloc_fd(unsigned start, unsigned end, unsigned flags)
>  
>  	__set_open_fd(fd, fdt, flags & O_CLOEXEC);
>  	error = fd;
> +	VFS_BUG_ON(rcu_access_pointer(fdt->fd[fd]) != NULL);
>  
>  out:
>  	spin_unlock(&files->file_lock);
> @@ -647,7 +648,7 @@ void fd_install(unsigned int fd, struct file *file)
>  		rcu_read_unlock_sched();
>  		spin_lock(&files->file_lock);
>  		fdt = files_fdtable(files);
> -		WARN_ON(fdt->fd[fd] != NULL);
> +		VFS_BUG_ON(fdt->fd[fd] != NULL);
>  		rcu_assign_pointer(fdt->fd[fd], file);
>  		spin_unlock(&files->file_lock);
>  		return;
> @@ -655,7 +656,7 @@ void fd_install(unsigned int fd, struct file *file)
>  	/* coupled with smp_wmb() in expand_fdtable() */
>  	smp_rmb();
>  	fdt = rcu_dereference_sched(files->fdt);
> -	BUG_ON(fdt->fd[fd] != NULL);
> +	VFS_BUG_ON(fdt->fd[fd] != NULL);
>  	rcu_assign_pointer(fdt->fd[fd], file);
>  	rcu_read_unlock_sched();
>  }
> -- 
> 2.43.0
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR