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Message-ID: <20080407060602.GE9309@duo.random> Date: Mon, 7 Apr 2008 08:06:02 +0200 From: Andrea Arcangeli <andrea@...ranet.com> To: Christoph Lameter <clameter@....com> Cc: Robin Holt <holt@....com>, "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>, kvm-devel@...ts.sourceforge.net, Peter Zijlstra <a.p.zijlstra@...llo.nl>, general@...ts.openfabrics.org, steiner@....com, linux-kernel@...r.kernel.org, linux-mm@...ck.org 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). -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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