Re: __rcu_process_callbacks() in Linux 2.6

[Paul E. McKenney]

On Mon, Nov 26, 2007 at 06:39:58PM -0800, Paul E. McKenney wrote:
> On Mon, Nov 26, 2007 at 02:48:08PM -0800, James Huang wrote:
> > 
> > > -----Original Message-----
> > > From: James Huang [mailto:jamesclhuang@xxxxxxxxx]
> > > Sent: Monday, November 26, 2007 2:21 PM
> > > To: James Huang
> > > Subject: Fw: __rcu_process_callbacks() in Linux 2.6
> > > 
> > > ----- Forwarded Message ----
> > > From: Manfred Spraul <manfred@xxxxxxxxxxxxxxxx>
> > > To: James Huang <jamesclhuang@xxxxxxxxx>
> > > Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>; linux-
> > > kernel@xxxxxxxxxxxxxxx
> > > Sent: Monday, November 26, 2007 10:28:37 AM
> > > Subject: __rcu_process_callbacks() in Linux 2.6
> > > 
> > > Hi James,
> > > 
> > > If I understand the issue correctly, then the race is:
> > > 
> > > step 1: cpu 1: starts a new rcu batch (i.e. rcp->cur++, smb_mb)
> > > 
> > > step 2: cpu 2: completes the quiet state
> > > step 3: cpu 2: reads pointer 0x123 (ptr to a rcu protected struct)
> > > 
> > > step 4: cpu 3: call_rcu(0x123): rcu protected struct added to
> > rdp->nxtlist
> > > step 5: cpu 3: moves a new batch into rdp->curlist, rdp->batch = rcp-
> > > >cur+1.
> > > xxxxxxxxxxxxxxx Problem: where is the smp_rmb() that guarantees that
> > > xxxxxxxxxxxxxxx  update to rcp->cur from step 1 is seen by cpu 3?
> > > step 6: cpu 3: completes quiet state
> > > step 7: cpu 3: struct 0x123 destroyed
> > > 
> > > step 8: cpu 2: accesses pointer 0x123, but the struct is already
> > destroyed
> > > 
> > > James: Is that the race?
> > 
> > 
> > [James Huang] 
> > 
> > Yes, this is the race condition that I am concerned about.
> > 
> > 
> > > 
> > > I agree with Paul, there are smb_rmb's on cpu 3 between Step 1 and
> > Step 5:
> > > Either the test_and_set_bit in tasklet_action for rcu_process_callback
> > > if step 4 happens before the tasklet or somewhere in the irq handler
> > > path if step 4 happens in an irq handler that interrupted
> > > rcu_process_callback.
> > > 
> > > Thus theoretically no additional smb_rmb() should be necessary.
> > > What is missing is proper documentation.
> > > 
> > 
> > 
> > [James Huang] 
> > 
> > Is it true that a smb_rmb() before a read operation (say from variable
> > X) will guarantee that the read will always retrieve the most "current"
> > value of X?   I can not find such a guarantee in atomic_ops.txt or
> > memory-barriers.txt under Linux's documentation directory.  What is
> > described in both documents is relative ordering, e.g.
> > 
> >             CPU1                       CPU2
> >            ------                     ------
> >           write X = x1
> >           smp_wmb()  
> >           write Y = y1 
> > 
> >                                       read Y
> >                                       smp_rmb()
> >                                       read X
> > 
> > Then CPU2 will read X with a value of x1 if it reads Y with a value of
> > y1.
> > 
> > Please point me to the right section in the document if smp_rmb() does
> > provide such a guarantee.
> 
> You are correct, smp_rmb() is about ordering rather than about any sort
> of immediacy.  For one thing, it can be quite difficult to say exactly what
> the most "current" version of X might be at a given point in time from
> the viewpoint of a given CPU -- the different CPUs might well disagree as
> to what the "current" version is for awhile (though they are guaranteed
> to come to agreement).
> 
> > Thanks,
> > -- James Huang
> > 
> > > I'm analyzing the code right now:
> > > Is it really true that typically a cpu only completes data in every
> > other
> > > rcu
> > > cycle? I.e. that most structures are stored in the rcu callback list
> > until
> > > two
> > > quiet states happened?
> 
> That is correct.  This does mean that we should be able to leverage
> locking primitives and memory barriers executed from the scheduling
> clock interrupt.

And I managed to get some time on a 64-CPU POWER5+ system.  Been running
rcutorture for about 2.5 hours without a failure (128 reader processes)
running through not quite 1.5M RCU updates.  Of course, this is not
proof that the Classic RCU implementation works, but is should provide
some reassurance.

I will keep it running until I get kicked off (probably rather soon).

						Thanx, Paul

> > > I've tried to track the values of rcp->cur and rdp->batch.
> > > If next_pending is set, then cpu_quiet() immetiately starts
> > > the next rcu cycle and a cpu cannot both complete the currently
> > > pending rcu callbacks and add new callbacks to the next cycle,
> > > thus a cpu only takes part in every other rcu cycle.
> > > 
> > > The oocalc file is at
> > > http://www.colorfullife.com/~manfred/rcu.ods
> > > http://www.colorfullife.com/~manfred/rcu.pdf
> > > 
> > > Is that analysis correct? Perhaps the whole code should be rewritten?
> 
> I believe that the sequencing in spreadsheet is correct (and thank
> you very much for going through it!!!), but it seems to be silent on
> memory-barrier issues.
> 
> I also believe that Gautham's new CPU-hotplug setup will make
> it possible to simplify the code quite a bit.  And given that the
> grace-period-detection code is not on any sort of hot code path, it should
> be possible to use a less-aggressive design, perhaps one using straight
> locking to guard the shared structures.  Also, we are working in the
> -rt implementation on a scheme that allows CPUs to stay asleep through
> a grace period without the heavy overhead that is otherwise required to
> interact with them.  The trick is to maintain a per-CPU counter that is
> incremented on each entry and exit to low-power state.  But I would like
> to get this right in -rt before trying it in Classic RCU.  ;-)
> 
> 						Thanx, Paul
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