Messages in this thread |  | | | Date | Mon, 7 Apr 2008 08:06:02 +0200 | | From | Andrea Arcangeli <> | | Subject | Re: [patch 02/10] emm: notifier logic |
On Sun, Apr 06, 2008 at 10:48:56PM -0700, Christoph Lameter wrote:
> On Sat, 5 Apr 2008, Andrea Arcangeli wrote:> > > > + rcu_assign_pointer(mm->emm_notifier, e);> > > + mm_unlock(mm);> > > > My mm_lock solution makes all rcu serialization an unnecessary
> > overhead so you should remove it like I already did in #v11. If it
> > wasn't the case, then mm_lock wouldn't be a definitive fix for the
> > race.> > There still could be junk in the cache of one cpu. If you just read the
> new pointer but use the earlier content pointed to then you have a
> problem.
There can't be junk, spinlocks provides semantics of proper memory
barriers, just like rcu, so it's entirely superflous.
There could be junk only if any of the mmu_notifier_* methods would be
invoked _outside_ the i_mmap_lock and _outside_ the anon_vma and
outside the mmap_sem, that is never the case of course.
> So a memory fence / barrier is needed to guarantee that the contents
> pointed to are fetched after the pointer.
It's not needed... if you were right we could never possibly run a
list_for_each inside any spinlock protected critical section and we'd
always need to use the _rcu version instead. The _rcu version is
needed only when the list walk happens outside the spinlock critical
section of course (rcu = no spinlock cacheline exlusive write
operation in the read side, here the read side takes the spinlock big time).
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