On Tue, Jul 22, 2014 at 06:40:49AM +1000, NeilBrown wrote: > On Mon, 21 Jul 2014 07:44:12 -0400 Jeff Layton <jeff.layton@xxxxxxxxxxxxxxx> > wrote: > > > On Mon, 21 Jul 2014 17:02:54 +1000 > > NeilBrown <neilb@xxxxxxx> wrote: > > > > > hash = arch_fast_hash(&fh->fh_base, fh->fh_size, 0); > > > > > > > > __set_bit(hash&255, bd->set[bd->new]); > > > > __set_bit((hash>>8)&255, bd->set[bd->new]); > > > > __set_bit((hash>>16)&255, bd->set[bd->new]); > > > > + spin_lock(&blocked_delegations_lock); > > > > > > __set_bit isn't atomic. The spin_lock should be taken *before* these > > > __set_bit() calls. > > > > > > Otherwise, looks fine. > > > > > > Thanks, > > > NeilBrown > > > > > > > > > > Ok. I guess the worry is that we could end up setting bits in the > > middle of swapping the two fields? Makes sense -- fixed in my repo. > > It is more subtle than that. > __set_bit() will: > read a value from memory to a register > set a bit in the register > write the register back out to memory > > If two threads both run __set_bit on the same word of memory at the same > time, one of the updates can get lost. > set_bit() (no underscore) performs an atomic RMW to avoid this, but is more > expensive. > spin_lock() obviously ensures the required exclusion and as we are going to > take the lock anyway we may as well take it before setting bits so we can use > the non-atomic (cheaper) __set_bit function. > > > I'll send out the updated set later today (it also includes a few nits > > that HCH pointed out last week). > > > > As a side note...I wonder how much we'll get in the way of false > > positives with this scheme? > > If a future version of NFSv4 could allow delegations to be granted while a > file is open (oh, it seems you are the only client using this file at the > moment, you can treat this "open" as a delegation if you like) a few false > positives would be a complete non-issue. For what it's worth, I think 4.1 provides what you're asking for here; see http://tools.ietf.org/html/rfc5661#section-20.7 and the discussion of the various WANT_ flags in http://tools.ietf.org/html/rfc5661#section-18.16.3 As far as I know none of that is implemented yet. --b. -- 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
