On Fri, 18 Jul 2014 15:21:49 -0400 "J. Bruce Fields" <bfields@xxxxxxxxxxxx> wrote: > On Fri, Jul 18, 2014 at 03:04:04PM -0400, Jeff Layton wrote: > > On Fri, 18 Jul 2014 13:49:57 -0400 > > "J. Bruce Fields" <bfields@xxxxxxxxxxxx> wrote: > > > > > On Fri, Jul 18, 2014 at 01:31:40PM -0400, Jeff Layton wrote: > > > > On Fri, 18 Jul 2014 12:28:25 -0400 > > > > "J. Bruce Fields" <bfields@xxxxxxxxxxxx> wrote: > > > > > > > > > On Fri, Jul 18, 2014 at 11:13:27AM -0400, Jeff Layton wrote: > > > > > > Move more of the delegation fields to be protected by the fi_lock. It's > > > > > > more granular than the state_lock and in later patches we'll want to > > > > > > be able to rely on it in addition to the state_lock. > > > > > > > > > > > > Also, the current code in nfs4_setlease calls vfs_setlease and uses the > > > > > > client_mutex to ensure that it doesn't disappear before we can hash the > > > > > > delegation. With the client_mutex gone, we'll have a potential race > > > > > > condition. > > > > > > > > > > > > It's possible that the delegation could be recalled after we acquire the > > > > > > lease but before we ever get around to hashing it. If that happens, then > > > > > > we'd have a nfs4_file that *thinks* it has a delegation, when it > > > > > > actually has none. > > > > > > > > > > I understand now, thanks: so the lease break code walks the list of > > > > > delegations associated with the file, finds none, and issues no recall, > > > > > but the open code continues merrily on and returns a delegation, with > > > > > the result that we return the client a delegation that will never be > > > > > recalled. > > > > > > > > > > That could be worded more carefully, and would be worth a separate patch > > > > > (since the bug predates the new locking). > > > > > > > > > > > > > Yes, that's basically correct. I'd have to think about how to fix that > > > > with the current code. It's probably doable if you think it's > > > > worthwhile, but I'll need to rebase this set on top of it. > > > > > > Well, I was wondering if this patch could just be split in two, no need > > > to backport further than that. > > > > > > > Erm, now that I've looked, I don't think it'll be that easy. The key > > here is to ensure that fi_had_conflict is set while holding the > > fi_lock. The trick here is that we need to take it in nfs4_setlease as > > well, and check the flag before hashing the delegation without dropping > > the fi_lock. > > OK, I'll live. For the sake of anyone that actually runs across that > bug I'll update the summary and changelog to emphasize the bugfix over > the locking change. > Ok, thanks. > > > > > > Attempt to acquire a delegation. If that succeeds, take the spinlocks > > > > > > and then check to see if the file has had a conflict show up since then. > > > > > > If it has, then we assume that the lease is no longer valid and that > > > > > > we shouldn't hand out a delegation. > > > > > > > > > > > > There's also one more potential (but very unlikely) problem. If the > > > > > > lease is broken before the delegation is hashed, then it could leak. > > > > > > In the event that the fi_delegations list is empty, reset the > > > > > > fl_break_time to jiffies so that it's cleaned up ASAP by > > > > > > the normal lease handling code. > > > > > > > > > > Is there actually any guarantee time_out_leases() will get called on > > > > > this inode again? > > > > > > > > > > --b. > > > > > > > > > > > > > Yes. Lease breaks are handled in two phases. We walk the i_flock list > > > > and issue a ->lm_break on each lease, and then later we walk the list > > > > again after putting the task to sleep, and try to time out the leases. > > > > So by doing this, we should ensure that the task will wake up after > > > > sleeping and delete it. > > > > > > In the case of an interrupt or a nonblocking break (which is what nfsd > > > will do), then time_out_leases isn't called again from what I could > > > tell. > > > > > > --b. > > > > > > > In both cases, time_out_leases is still called at the beginning of > > __break_lease. So the next time that function is called it'll > > get cleaned up, or when the filp is closed (in locks_remove_file). > > Right, but there's no guarantee another break_lease comes. E.g. the > process waiting on the lease break could get killed. > > --b. In that case, there's no harm in leaving the lease on the list until the filp closes. FWIW, I looked at trying to just remove the lease from the list, but that's not safe from the lm_break callback. So, I think this is the best we can reasonably do here. -- Jeff Layton <jlayton@xxxxxxxxxxxxxxx> -- 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
