Re: Lockless file reading

[Nagendra Singh Tomar <nagendra_tomar@adaptec.com>]

On Thu, 28 Aug 2003, Timo Sirainen wrote:

> 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.

What is this mixing we are talking of ??

> 
> > 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.

But I don't see any problem with a single writer and >=1 reader. There is 
no question of portability.

tomar
> 
>