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" <paul...@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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/