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

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Message-ID: <20140223011806.GA13677@chicago.guarana.org>
Date:	Sun, 23 Feb 2014 12:18:06 +1100
From:	Kevin Easton <kevin@...rana.org>
To:	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
Cc:	Al Viro <viro@...iv.linux.org.uk>,
	"Zuckerman, Boris" <borisz@...com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	lkml <linux-kernel@...r.kernel.org>,
	Miklos Szeredi <miklos@...redi.hu>,
	Theodore T'so <tytso@....edu>, Christoph Hellwig <hch@....de>,
	Chris Mason <clm@...com>, DaveC@...cago.guarana.org
Subject: Re: Update of file offset on write() etc. is non-atomic with I/O

On Fri, Feb 21, 2014 at 07:01:31AM +0100, Michael Kerrisk (man-pages) wrote:
> Here's the fulll list from POSIX.1-2008/SUSv4 Section XSI 2.9.7:
> 
> [[
> 2.9.7 Thread Interactions with Regular File Operations
> 
> All of the following functions shall be atomic with respect to each
> other in the effects specified in
> POSIX.1-2008 when they operate on regular files or symbolic links:
> 
> chmod( )
> chown( )
> close( )
> creat( )
> dup2( )
> fchmod( )
> fchmodat( )
> fchown( )
> fchownat( )
> fcntl( )
> fstat( )
> fstatat( )
> ftruncate( )
> lchown( )
> link( )
> linkat( )
> lseek( )
> lstat( )
> open( )
> openat( )
> pread( )
> read( )
> readlink( )
> readlinkat( )
> readv( )
> pwrite( )
> rename( )
> renameat( )
> stat( )
> symlink( )
> symlinkat( )
> truncate( )
> unlink( )
> unlinkat( )
> utime( )
> utimensat( )
> utimes( )
> write( )
> writev( )
> 
> If two threads each call one of these functions, each call shall
> either see all of the specified effects
> of the other call, or none of them.
> ]]
> 
> I'd bet that there's a bunch of violations to be found, but the
> read/write f_pos case is one of the most egregious.
> 
> For example, I got curious about stat() versus rename(). If one
> stat()s a directory() while a subdirectory is being renamed to a new
> name within that directory, does the link count of the parent
> directory ever change--that is, could stat() ever see a changed link
> count in the middle of the rename()? My experiments suggest that it
> can. I suppose it would have to be a very unusual application that
> would be troubled by that, but it does appear to be a violation of
> 2.9.7.

A directory isn't a regular file or symlink, though, so the warranty
would seem to be void in that case.

If you {f}stat() a regular file that is being concurrently renamed() then
the link count shouldn't vary, though.

    - Kevin
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