Re: Lockless file reading

[Timo Sirainen <tss@iki.fi>]

On Wed, 2003-08-27 at 21:42, Nagendra Singh Tomar wrote:
> Hi,
>     I beleive ur original post was to address the case of a reader reading 
> a file getting *incorrect* data due to the file being written 
> simultaneously by another writer process.

Well, "old" data, which mixed with new data would become incorrect as a
whole.

> Why do u require file locking if there is a *single* writer ?? I don't 
> understand why a 123 written over XXX can result in 1X3. The kernel should 
> take care of this. When the writer process is writing 123 it will first be 
> written to the page cache. The page cache lock will be taken inside the 
> kernel before writing to it, so we know that writing 123 over XXX will be 
> atomic.  Now even when this page is flushed to disk, the page lock would 
> be taken. So I cannot see a possibility of 123 written over XXX being read 
> as 1X3.

That was my original plan, to just rely on such kernel behaviour. I just
don't know if it's such a good idea to rely on that, especially if I
want to keep my program portable. I'll probably fallback to that anyway
if my checksumming ideas won't work.