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    SubjectRe: [PATCH tip/core/rcu 16/23] rcu: Prevent initialization-time quiescent-state race
    On Wed, Sep 05, 2012 at 11:55:34AM -0700, Josh Triplett wrote:
    > On Wed, Sep 05, 2012 at 11:19:20AM -0700, Paul E. McKenney wrote:
    > > On Mon, Sep 03, 2012 at 02:37:42AM -0700, Josh Triplett wrote:
    > > > On Thu, Aug 30, 2012 at 11:18:31AM -0700, Paul E. McKenney wrote:
    > > > > From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
    > > > >
    > > > > Now the the grace-period initialization procedure is preemptible, it is
    > > > > subject to the following race on systems whose rcu_node tree contains
    > > > > more than one node:
    > > > >
    > > > > 1. CPU 31 starts initializing the grace period, including the
    > > > > first leaf rcu_node structures, and is then preempted.
    > > > >
    > > > > 2. CPU 0 refers to the first leaf rcu_node structure, and notes
    > > > > that a new grace period has started. It passes through a
    > > > > quiescent state shortly thereafter, and informs the RCU core
    > > > > of this rite of passage.
    > > > >
    > > > > 3. CPU 0 enters an RCU read-side critical section, acquiring
    > > > > a pointer to an RCU-protected data item.
    > > > >
    > > > > 4. CPU 31 removes the data item referenced by CPU 0 from the
    > > > > data structure, and registers an RCU callback in order to
    > > > > free it.
    > > > >
    > > > > 5. CPU 31 resumes initializing the grace period, including its
    > > > > own rcu_node structure. In invokes rcu_start_gp_per_cpu(),
    > > > > which advances all callbacks, including the one registered
    > > > > in #4 above, to be handled by the current grace period.
    > > > >
    > > > > 6. The remaining CPUs pass through quiescent states and inform
    > > > > the RCU core, but CPU 0 remains in its RCU read-side critical
    > > > > section, still referencing the now-removed data item.
    > > > >
    > > > > 7. The grace period completes and all the callbacks are invoked,
    > > > > including the one that frees the data item that CPU 0 is still
    > > > > referencing. Oops!!!
    > > > >
    > > > > This commit therefore moves the callback handling to precede initialization
    > > > > of any of the rcu_node structures, thus avoiding this race.
    > > >
    > > > I don't think it makes sense to introduce and subsequently fix a race in
    > > > the same patch series. :)
    > > >
    > > > Could you squash this patch into the one moving grace-period
    > > > initialization into a kthread?
    > >
    > > I tried that, and got a surprisingly large set of conflicts. Ah, OK,
    > > the problem is that breaking up rcu_gp_kthread() into subfunctions
    > > did enough code motion to defeat straightforward rebasing. Is there
    > > some way to tell "git rebase" about such code motion, or would this
    > > need to be carried out carefully by hand?
    >
    > To the extent rebase knows how to handle that, I think it does so
    > automatically as part of merge attempts. Fortunately, in this case, the
    > change consists of moving two lines of code and their attached comment,
    > which seems easy enough to change in the original code; you'll then get
    > a conflict on the commit that moves the newly fixed code (easily
    > resolved by moving the change to the new code), and conflicts on any
    > changes next to the change in the new code (hopefully handled by
    > three-way merge, and if not then easily fixed by keeping the new lines).

    Good point, perhaps if I do the code movement manually and use multiple
    rebases it will go more easily.

    Thanx, Paul



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