Re: Update of file offset on write() etc. is non-atomic with I/O

From: "George Spelvin" <linux@horizon.com>
To: torvalds@linux-foundation.org
Cc: linux@horizon.com, linux-fsdevel@vger.kernel.org,
	linux-kernel@vger.kernel.org, viro@ZenIV.linux.org.uk
Subject: Re: Update of file offset on write() etc. is non-atomic with I/O
Date: 3 Mar 2014 16:03:59 -0500	[thread overview]
Message-ID: <20140303210359.26624.qmail@science.horizon.com> (raw)

>  struct fd {
>  	struct file *file;
> -	int need_put;
> +	unsigned need_put:1, need_pos_unlock:1;
>  };

Since we're rounding up to 2*sizeof(struct file *) anyway, is this a case
where wasting space on a couple of char (or bool) flags would generate
better code than a bitfield?

In particular, the code to set need_pos_unlock (which will be executed
each read/write for most files) is messy in the bitfield case.
A byte store is much cleaner.

(If you want to use bits, why not use the two lsbits of the file pointer
for the purpose?  That would save a lot of space.)