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

[Paul E. McKenney]

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@xxxxxxx 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
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