On Wed, Jun 21, 2006 at 03:36:10PM -0700, Christoph Lameter wrote:
> On Sat, 17 Jun 2006, Paul E. McKenney wrote:
>
> > Careful performance testing is needed as well, the validity check does
> > appear to me to be pretty lightweight, but this stuff is a bit sensitive
> > to performance.
>
> Jens has done some testing with another approach like this and found a
> good increase in speed.
And I would be much more concerned if the change was in the single-threaded
fastpath in fget_light(), to be sure. But getting a view of multi-threaded
performance of this particular patch for a variety of workloads is still
warranted.
> > Some one less than a philistine than I might argue that this should
> > go back to the original name of f_list. ;-)
>
> Did that in V2 of the patch.
Cool!
> > o Not sure whether hfs_file_release()s check for zero f_count
> > still works, but am quite concerned. HFS experts?
> >
> > o Ditto for hfsplus_file_release(). And for affs_file_release().
>
> That is not a problem since the object has not been returned to the slab
> yet so it cannot have been reused.
Good enough!
Thanx, Paul
> A problem could occur if one could
> now establish a new reference to the object while it has a zero count.
> But if that would be possible then we would be already in trouble.
>
> The RCU check is only used to guarantee that the object is not
> vanishing from under us while we increase the refcount if it was not
> zero. Since it is zero in these cases that will never occur.
>
> In order to generate a wrong alias situation the following needs to
> happen:
>
> process 1 process 2:
>
> do table lookup
> free the filp structure (refcount becomes zero)
> modify table
> create a new filp structure (same address)
> increment counter to 1
> inc_if_nonzero
> (counter becomes two)
>
> <now we have a wrong reference>
>
> put_filp (reduce refcount)
>
> Table lookup fails->fget fails.
>
>
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