On Wed 26-04-17 11:12:06, Luis R. Rodriguez wrote: > 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 ? Well, if the inode is freed only after RCU period expires and we are doing xfs_reclaim_inode_grab() under rcu_read_lock - which we are - then this surely prevents us from seeing inode reallocated. What are you missing? > Also, just so I understand I am following, this then implicates our > reclaim rate is directly linked to the RCU grace period ? Yes, as for any RCU-freed object... Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR -- To unsubscribe from this list: send the line "unsubscribe linux-xfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
