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

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Date:	Sun, 23 Feb 2014 08:38:07 +0100
From:	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To:	Kevin Easton <kevin@...rana.org>
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 Sun, Feb 23, 2014 at 2:18 AM, Kevin Easton <kevin@...rana.org> wrote:
> 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.

Oops -- yes, of course.

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

Yes. (I haven't tested that though.)

Cheers,

Michael

-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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