Re: Reviewing determinism of xfs_reclaim_inodes_ag()

["Luis R. Rodriguez" <mcgrof@kernel.org>]

On Wed, Apr 26, 2017 at 10:04:26AM +1000, Dave Chinner wrote:
> On Tue, Apr 25, 2017 at 10:25:03AM +0200, Luis R. Rodriguez wrote:
> > I checked with Jan Kara and he believes the current code is correct but that
> > its the comment that that may be misleading. As per Jan the race is between
> > getting an inode reclaimed and grabbing it. Ie, XFS frees the inodes by RCU.
> > However it doesn't actually *reuse* the inode until RCU period passes
> > (unlike inodes allocated from slab with SLAB_RCU can be). So it can happen
> 
> ..... I initially tried using SLAB_DESTROY_BY_RCU which meant the
> RCU grace period did not prevent reallocation of inodes that had
> been freed. Hence this check was (once) necessary to prevent the
> reclaim index going whacky on a reallocated inode.

Alright this helps, but why does *having* the RCU grace period prevent
such type of race ? I can see it helping but removing completely such
a race as a possibility ? Also, just so I understand I am following, this
then implicates our reclaim rate is directly linked to the RCU grace
period ?

  Luis