On Thu, 2011-02-24 at 03:58 +0000, Al Viro wrote: > > Anyway, I wonder why you care about __d_lookup_rcu() and ->d_inode stability; > d_mountpoint() _is_ stable at that point (we hold vfsmount_lock) and you > don't seem to look at ->d_inode at all in RCU case. Note that ->d_automount() > is never called in RCU case at all; nor is ->lookup() and friends, so you > really only have ->d_manage() to cope with, what with autofs4 having no > ->d_revalidate() anymore. It does now but it doesn't do a whole lot, just checks for a negative dentry, returning false, and drops out of RCU mode when the dentry isn't selected or being checked for expiry yet. Indeed, I shouldn't need to care about it and checking for it doesn't seem to do a whole lot. I've seen walks fail with a dentry that has gone away (ie. I'm walking in a tree that is being expired, they seem to be trees with no actual mount at their base), which should be because I'm not catching the block request at a higher level dentry. Unfortunately, including a check for the base dentry in __follow_mount_rcu(), regardless of it being a mountpoint, doesn't always catch the need to block either. That the tree is being selected for expire is probably because the expire can't detect a walk within the mount tree when the walker is between the __d_lookup_rcu() and __follow_mount_rcu() calls. Jumping out of rcu-walk mode in an added revalidate reduces the possibility of this happening (assuming walks are sneaking in between dropping and acquiring the vfsmount_lock) but doesn't eliminate it. At this point I loop back to the trying to work out why a reference gets picked up and the cycle repeats with various small changes to the overall approach. Actually, it's not just the root of a mount that is getting an extra reference either. When I notice a root dentry has picked up a reference (from a printk) and SIGKILL everything and start umounting manually I get to a point in the tree where I need to umount autofs mounts twice, which usually means nothing more than a get/put mismatch somewhere, but in that case I normally don't see a root dentry pick up a reference. That's confusing me too. > > BTW, Nick has moved to kernel.dk; whether he's still reachable there is > a question, though - he seems to have disappeared since mid-January. Yeah, I've forwarded a couple of messages where I forgot to change the email. A few emails back Nick said he was following the thread anyway. Ian -- To unsubscribe from this list: send the line "unsubscribe linux-nfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
