Re: [patch 24/52] fs: dcache reduce d_parent locking

Paul E. McKenney
2010-06-24T15:32:28Z

On Fri, Jun 25, 2010 at 01:07:06AM +1000, Nick Piggin wrote:
> On Thu, Jun 24, 2010 at 10:44:22AM +0200, Peter Zijlstra wrote:
> > On Thu, 2010-06-24 at 13:02 +1000, npiggin@suse.de wrote:
> > > Use RCU property of dcache to simplify locking in some places where we
> > > take d_parent and d_lock.
> > >
> > > Comment: don't need rcu_deref because we take the spinlock and recheck it.
> >
> > But does the LOCK barrier imply a DATA DEPENDENCY barrier? (It does on
> > x86, and the compiler barrier implied by spin_lock() suffices to replace
> > ACCESS_ONCE()).
>
> Well the dependency we care about is from loading the parent pointer
> to acquiring its spinlock. But we can't possibly have stale data given
> to the spin lock operation itself because it is a RMW.

As long as you check for the structure being valid after acquiring the
lock, I agree. Otherwise, I would be concerned about the following
sequence of events:

1. CPU 0 picks up a pointer to a given data element.

2. CPU 1 removes this element from the list, drops any locks that
it might have, and starts waiting for a grace period to
elapse.

3. CPU 0 acquires the lock, does some operation that would
be appropriate had the element not been removed, then
releases the lock.

4. After the grace period, CPU 1 frees the element, negating
CPU 0's hard work.

The usual approach is to have a "deleted" flag or some such in the
element that CPU 0 would set when removing the element and that CPU 1
would check after acquiring the lock. Which you might well already
be doing! ;-)

Thanx, Paul