Re: [PATCH tip/core/rcu 16/23] rcu: Prevent initialization-timequiescent-state race

[Paul E. McKenney]

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@xxxxxxxxxxxxxxxxxx>
> > > > 
> > > > 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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