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    SubjectRe: [PATCH] x86, mce: Fix rcu splat in drain_mce_log_buffer()
    On Mon, Feb 06, 2012 at 05:18:02PM +0100, Borislav Petkov wrote:
    > On Fri, Jan 13, 2012 at 01:44:35PM -0800, Paul E. McKenney wrote:
    > > Looks good to me, but I do need to defer to people who know this code
    > > better than do I. The key thing that (from what I can see) makes
    > > rcu_dereference() unnecessary is that the smp_rmb() used in conjunction
    > > with polling the .finished field takes care of ordering.
    >
    > Right, this was me trying hard not to screw up touching mcelog.next,
    > thus trying to use the rcu_dereference_index_check() primitive without
    > thinking it through too much. But you're right, I'm polling the
    > ->finished field 4 times (totally arbitrary, btw) which should suffice
    > while the mce_log() routine above writes those entries.
    >
    > Although, the question still remains, since mce_log() accesses
    > mcelog.next through the rcu_dereference_index_check() primitive,
    > shouldn't I do it the same way?

    I don't claim to be an mce_log() expert, but when I looked it over,
    I didn't see a need for rcu_dereference_index_check(). Unless I am
    confused (quite possible), the memory barriers are sufficient.

    The rcu_read_lock() and rcu_read_unlock() seem to be needed to avoid
    premature freeing, though.

    Thanx, Paul



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