On Tue, Apr 21, 2009 at 05:11:58PM -0400, Mathieu Desnoyers wrote:
> * Paul E. McKenney (paulmck@xxxxxxxxxxxxxxxxxx) wrote:
> > On Tue, Apr 21, 2009 at 11:10:35AM -0400, Mathieu Desnoyers wrote:
> > > * Paul E. McKenney (paulmck@xxxxxxxxxxxxxxxxxx) wrote:
> >
> > [ . . . ]
> >
> > > > +void synchronize_rcu_fgp(void)
> > > > +{
> > > > + mutex_lock(&rcu_fgp_mutex);
> > > > +
> > > > + /* CPUs must see earlier change before parity flip. */
> > > > + smp_call_function(rcu_fgp_do_mb, NULL, 1);
> > > > +
> > >
> > > Hrm, my original comment about missing smp_mb() here still applies, I
> > > don't think we have come to an agreement yet.
> >
> > My argument is that smp_call_function() must necessarily contain a
> > full memory barrier, otherwise it cannot function properly. ;-)
> >
>
> Looking at :
>
> kernel/smp.c
>
> smp_call_function_many() indeed has a smp_mb(). It is called by
> smp_call_function(). I wonder if it could eventually be turned into a
> smp_wmb() instead ? If this is even a remote possibility, then the fact
> that
>
> - The rcu_fgp code does not document that it expects smp_call_function()
> to have a smp_mb().
> - The fact that smp_call_function_many() comments do not state that this
> function provides the guarantee to run a smp_mb().
>
> are both asking for an eventual bug to creep into the kernel.
Many bugs -- I believe that a number of users of smp_call_function()
assume that it maintains ordering between the calling code and all
invocations of the function passed to smp_call_function().
> So your assumption seems OK, but I think it needs to be explicitly
> documented.
That might well be a good thing.
Thanx, Paul
> Mathieu
>
> > > > + /*
> > > > + * We must flip twice to correctly handle tasks that stall
> > > > + * in rcu_read_lock_fgp() between the time that they fetch
> > > > + * rcu_fgp_ctr and the time that the store to their CPU's
> > > > + * rcu_fgp_active_readers. No matter when they resume
> > > > + * execution, we will wait for them to get to the corresponding
> > > > + * rcu_read_unlock_fgp().
> > > > + */
> > > > + ACCESS_ONCE(rcu_fgp_ctr) ^= RCU_FGP_PARITY; /* flip parity 0 -> 1 */
> > > > + rcu_fgp_wait_for_quiescent_state(); /* wait for old readers */
> > > > + ACCESS_ONCE(rcu_fgp_ctr) ^= RCU_FGP_PARITY; /* flip parity 1 -> 0 */
> > > > + rcu_fgp_wait_for_quiescent_state(); /* wait for old readers */
> > > > +
> > > > + /* Prevent CPUs from reordering out of prior RCU critical sections. */
> > > > + smp_call_function(rcu_fgp_do_mb, NULL, 1);
> > > > +
> > >
> > > Same here.
> > >
> > > So we would need to either add a smp_mb() at both of these locations, or
> > > use on_each_cpu() rather than smp_call_function. Note that this is to
> > > ensure that the "updater" thread executes these memory barriers.
> >
> > Or rely on the barriers that must be part of smp_call_function. ;-)
> >
> > Thanx, Paul
> >
> > > Mathieu
> > >
> > >
> > > > + rcu_fgp_completed++;
> > > > + mutex_unlock(&rcu_fgp_mutex);
> > > > +}
> > > > +EXPORT_SYMBOL_GPL(synchronize_rcu_fgp);
> > > > +
> > > > +/**
> > > > + * rcu_fgp_batches_completed - return batches completed.
> > > > + * @sp: srcu_struct on which to report batch completion.
> > > > + *
> > > > + * Report the number of batches, correlated with, but not necessarily
> > > > + * precisely the same as, the number of grace periods that have elapsed.
> > > > + */
> > > > +long rcu_fgp_batches_completed(void)
> > > > +{
> > > > + return rcu_fgp_completed;
> > > > +}
> > > > +EXPORT_SYMBOL_GPL(rcu_fgp_batches_completed);
> > >
> > > --
> > > Mathieu Desnoyers
> > > OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68
>
> --
> Mathieu Desnoyers
> OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68
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