Re: [PATCH]: Fix SMP-reordering race in mark_buffer_dirty

[Andrew Morton]

On Wed, 2 Apr 2008 12:44:05 -0700 (PDT)
Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:

> 
> 
> On Wed, 2 Apr 2008, Mikulas Patocka wrote:
> > +	/*
> > +	 * Make sure that the test for buffer_dirty(bh) is not reordered with
> > +	 * previous modifications to the buffer data.
> > +	 * -- mikulas
> > +	 */
> > +	smp_mb();
> >  	WARN_ON_ONCE(!buffer_uptodate(bh));
> >  	if (!buffer_dirty(bh) && !test_set_buffer_dirty(bh))
> 
> At that point, the better patch is to just *remove* the buffer_dirty() 
> test, and rely on the stronger ordering requirements of 
> test_set_buffer_dirty().
> 
> The whole - and only - point of the buffer_dirty() check was to avoid the 
> more expensive test_set_buffer_dirty() call, but it's only more expensive 
> because of the barrier semantics. So if you add a barrier, the point goes 
> away and you should instead remove the optimization.

But then the test-and-set of an already-set flag would newly cause the
cacheline to be dirtied, requiring additional bus usage to write it back?

The CPU's test-and-set-bit operation could of course optimise that away in
this case.  But does it?


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