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	Sun, 06 Oct 2024 21:04:26 +0000
Date: Sun, 6 Oct 2024 22:04:26 +0100
From: Al Viro <viro@zeniv.linux.org.uk>
To: Amir Goldstein <amir73il@gmail.com>
Cc: Miklos Szeredi <miklos@szeredi.hu>,
	Christian Brauner <brauner@kernel.org>,
	linux-fsdevel@vger.kernel.org, linux-unionfs@vger.kernel.org
Subject: Re: [PATCH v2 2/4] ovl: stash upper real file in backing_file struct
Message-ID: <20241006210426.GG4017910@ZenIV>
References: <20241006082359.263755-1-amir73il@gmail.com>
 <20241006082359.263755-3-amir73il@gmail.com>
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Sender: Al Viro <viro@ftp.linux.org.uk>

On Sun, Oct 06, 2024 at 10:23:57AM +0200, Amir Goldstein wrote:
> +	/*
> +	 * Usually, if we operated on a stashed upperfile once, all following
> +	 * operations will operate on the stashed upperfile, but there is one
> +	 * exception - ovl_fsync(datasync = false) can populate the stashed
> +	 * upperfile to perform fsync on upper metadata inode.  In this case,
> +	 * following read/write operations will not use the stashed upperfile.
> +	 */
> +	if (upperfile && likely(ovl_is_real_file(upperfile, realpath))) {
> +		realfile = upperfile;
> +		goto checkflags;
>  	}
>  
> +	/*
> +	 * If realfile is lower and has been copied up since we'd opened it,
> +	 * open the real upper file and stash it in backing_file_private().
> +	 */
> +	if (unlikely(!ovl_is_real_file(realfile, realpath))) {
> +		struct file *old;
> +
> +		/* Either stashed realfile or upperfile must match realinode */
> +		if (WARN_ON_ONCE(upperfile))
> +			return -EIO;
> +
> +		upperfile = ovl_open_realfile(file, realpath);
> +		if (IS_ERR(upperfile))
> +			return PTR_ERR(upperfile);
> +
> +		old = cmpxchg_release(backing_file_private_ptr(realfile), NULL,
> +				      upperfile);
> +		if (old) {
> +			fput(upperfile);
> +			upperfile = old;
> +		}
> +
> +		/* Stashed upperfile that won the race must match realinode */
> +		if (WARN_ON_ONCE(!ovl_is_real_file(upperfile, realpath)))
> +			return -EIO;
> +
> +		realfile = upperfile;
> +	}
> +
> +checkflags:

Hmm...  That still feels awkward.  Question: can we reach that code with
	* non-NULL upperfile
	* false ovl_is_real_file(upperfile, realpath)
	* true ovl_is_real_file(realfile, realpath)
Is that really possible?