Re: [PATCH 1/3 v2] call_function_many: fix list delete vs add race

[Milton Miller]

On Tue, 1 Feb 2011 about 14:00:26 -0800, "Paul E. McKenney" wrote:
> On Tue, Feb 01, 2011 at 01:12:18AM -0600, Milton Miller wrote:
> > Peter pointed out there was nothing preventing the list_del_rcu in
> > smp_call_function_interrupt from running before the list_add_rcu in
> > smp_call_function_many.   Fix this by not setting refs until we
> > have gotten the lock for the list.  Take advantage of the wmb in
> > list_add_rcu to save an explicit additional one.
> 
> Finally getting a chance to give this full attention...
> 
> I don't know how to do this patch-by-patch, so I applied these
> three patches to mainline and looked at the result.  This probably
> gets some comments that are irrelevant to these patches, but so it
> goes.
> 
> Starting with smp_call_function_many():
> 
> o	The check for refs is redundant:
> 
> 		/* some callers might race with other cpus changing the mask */
> 		if (unlikely(!refs)) {
> 			csd_unlock(&data->csd);
> 			return;
> 		}
> 
> 	The memory barriers and atomic functions in
> 	generic_smp_call_function_interrupt() prevent the callback from
> 	being reused before the cpumask bits have all been cleared, right?

The issue is not the cpumask in the csd, but the mask passed in from the
caller.  If other cpus clear the mask between the cpumask_first and and
cpumask_next above (where we established there were at least two cpus not
ourself) and the cpumask_copy, then this can happen.  Both Mike Galbraith
and Jan Beulich saw this in practice (Mikes case was mm_cpumask(mm)).

This was explained in the changelog for the second patch.

> 	Furthermore, csd_lock() contains a full memory barrier that pairs
> 	with the full memory barrier in csd_unlock(), so if csd_lock()
> 	returns, we are guaranteed to see the effects of all accesses
> 	to the prior incarnation of this structure.
> 
> 	Yes, there might be another CPU that got a pointer to this
> 	callback just as another CPU removed it.  That CPU will see
> 	the callback as having their bit of the CPU mask zero until
> 	we set it, and they will further see ->refs as being zero
> 	until we set it.  And we don't set ->refs until after we
> 	re-insert the callback.
> 
> 	So I we can drop this "if" statement entirely.

Disagree, see above.

> 
> o	The smp_mb() below look extraneous.  The comment in
> 	generic_exec_single() agrees -- it says that the IPI
> 	code is required to impose ordering.  Besides,
> 	there is a lock in arch_send_call_function_ipi_mask(),
> 	and in conjunction with the unlock below, this makes
> 	a full memory barrier.  So I believe that we can drop
> 	the smp_mb() shown below.  (But not the comment!!!)
> 
> 		raw_spin_unlock_irqrestore(&call_function.lock, flags);
> 
> 		/*
> 		 * Make the list addition visible before sending the ipi.
> 		 * (IPIs must obey or appear to obey normal Linux cache
> 		 * coherency rules -- see comment in generic_exec_single).
> 		 */
> 		smp_mb();


Well, this says the generic code guarantees mb before calling IPI ...
this has been here since 2.6.30 and would need at least an arch audit
so I vote to leave this here for now.

After we are confident in the locking, we can remove the internal bugs
and warn on the cpumask and refs, leaving the one for mask clear and
the ones for cpu online and calling context.

> 
> 		/* Send a message to all CPUs in the map */
> 		arch_send_call_function_ipi_mask(data->cpumask);
> 
> Next up: generic_smp_call_function_interrupt()
> 
> o	Shouldn't the smp_mb() at the top of the function be supplied
> 	in arch-specific code for those architectures that require it?
> 

Again, this would require an arch audit, I vote to leave for now.
This says the generic code will guarantee that an IPI is received after
the list is visiable with a mb on each side (as long as the arch makes
IPI appear as ordered by smp_mb()).

> o	So, the check for the current CPU's bit in the mask...
> 
> 	o	If the bit is clear, then we are looking at an callback
> 		that this CPU already processed or that was not intended
> 		for this CPU in the first place.
> 
> 		Of course, this callback might be reused immediately
> 		after we do the check.  In that case, there should
> 		be an IPI waiting for us shortly.  If the IPI beat
> 		the callback to us, that seems to me to be a bug
> 		in the architecture rather than in this code.

agreed

> 
> 	o	If the bit is set, then we need to process this callback.
> 		IRQs are disabled, so we cannot race with ourselves
> 		-- our bit will remain set until we clear it.
> 		The list_add_rcu() in smp_call_function_many()
> 		in conjunction with the list_for_each_entry_rcu()
> 		in generic_smp_call_function_interrupt() guarantees
> 		that all of the field except for ->refs will be seen as
> 		initialized in the common case where we are looking at
> 		an callback that has just been enqueued.
> 
> 		In the uncommon case where we picked up the pointer
> 		in list_for_each_entry_rcu() just before the last
> 		CPU removed the callback and when someone else
> 		immediately recycled it, all bets are off.  We must
> 		ensure that we see all initialization via some other
> 		means.
> 
> 		OK, so where is the memory barrier that pairs with the
> 		smp_rmb() between the ->cpumask and ->refs checks?
> 		It must be before the assignment to ->cpumask.  One
> 		candidate is the smp_mb() in csd_lock(), but that does
> 		not make much sense.  What we need to do is to ensure
> 		that if we see our bit in ->cpumask, that we also see
> 		the atomic decrement that previously zeroed ->refs.

We have a full mb in csd_unlock on the cpu that zeroed refs and a full
mb in csd_lock on the cpu that sets mask and later refs.

We rely on the atomic returns to order the two atomics, and the 
atomic_dec_return to establish a single cpu as the last.  After
that atomic is performed we do a full mb in unlock.   At this
point all cpus must have visibility to all this prior processing.
On the owning cpu we then do a full mb in lock.

How can any of the second party writes after the paired mb in lock be
visible and not all of the prior third party writes?

How can any cpu see the old refs after the new mask is set?  And if they
could, how can we guarantee the ordering between the cpumask clearing
and the unlock?

We rely on the statment that the cpumask memory will not change to an
intermediate value has bits set that will be later cleared, that is we
rely on the and of the incoming mask and online mask to occur before the
store.  We are broken if we copy online mask then and in the source mask..

The multiple non-atomic writes to data, func, and mask must be occur
after the mb in lock and before the wmb in list_add_rcu.  This wmb
must force all processing except for the set of refs, and we require
atomic_set to be atomic wrt atomic_dec_return.



> 		But some third CPU did that atomic decrement, which
> 		brings transitivity into play, which leads me to believe
> 		that this smp_rmb() might need to be upgraded to a
> 		full smp_mb().

I'm not sure I understand the need for this upgrade.  We are only
performing reads on this cpu, how is smp_mb stronger than smp_rmb?

All the prior writes were ordered by atomic returns or a full mb.


> 
> o	After we verify that the ->refs field is non-zero, we pick
> 	up the ->csd.func and ->csd.info fields, but with no intervening
> 	memory barrier.  There needs to be at least an smp_rmb()
> 	following the test of the ->refs field.  Since there is
> 	no transitivity required, smp_rmb() should do the trick.
>
> 
> 	If this seems unnecessary, please keep in mind that there
> 	is nothing stopping the compiler and the CPU from reordering
> 	the statements in smp_call_function_many() that initialize
> 	->csd.func, ->csd.info, and ->cpumask.

Ok I agree, we need either wmb before setting cpumask or rmb before
reading data & func after wmb is set.

> 
> 	In contrast, given the suggested smp_rmb(), we are guaranteed
> 	of the identity of the callback from the time we test ->refs
> 	until the time someone calls csd_unlock(), which cannot precede
> 	the time that we atomically decrement ->refs.
> 
> o	It seems silly to pick up the ->csd.func field twice.  Why
> 	not use the local variable?

I added the local variable for the warn and just left the compiler to
recognise the duplicate load.   But yes, we could use the local to make
the call too.  At the time I choose fewer loc changed.

> 
> o	The cpumask_test_and_clear_cpu() and the atomic_dec_return()
> 	are both atomic operations that return values, so they act
> 	as full memory barriers.
> 
> 	The cpumask_test_and_clear_cpu() needs to be atomic despite
> 	the identity guarantee because CPUs might be concurrently
> 	trying to clear bits in the same word.

Ok and in addition, since these are necessarly ordered then we
can assert that the test_and_clear_cpu must execute before the
decrement of refs, which is requried for total ordering to say
that all cpus are done clearing their mask bit and decrementing
refs before we do the csd_unlock, which allows the reuse.

> 
> Here is the corresponding (untested) patch.
> 
> Thoughts?
> 
> 							Thanx, Paul
> 
> ------------------------------------------------------------------------
> 
> smp_call_function: additional memory-order tightening.
> 
> The csd_lock() and csd_unlock() interaction guarantees that the
> smp_call_function_many() function sees the results of interactions
> with prior incarnations of the callback, so the check is not needed.
> Instead, tighter memory ordering is required in the companion
> generic_smp_call_function_interrupt() function to ensure proper
> interaction with partially initialized callbacks.
> 
> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> 
> diff --git a/kernel/smp.c b/kernel/smp.c
> index 064bb6e..4c8b005 100644
> --- a/kernel/smp.c
> +++ b/kernel/smp.c
> @@ -209,13 +209,19 @@ void generic_smp_call_function_interrupt(void)
>  		if (!cpumask_test_cpu(cpu, data->cpumask))
>  			continue;
>  
> -		smp_rmb();
> +		smp_mb(); /* If we see our bit set above, we need to see */
> +			  /* all the processing associated with the prior */
> +			  /* incarnation of this callback. */

Again, I want more justification.  Why is mb required over rmb?  
We are ordering two reads.  If we could see an old value before
stored by another cpu then what does the mb in lock and unlock mean?

>  
>  		if (atomic_read(&data->refs) == 0)
>  			continue;
>  
> +		smp_rmb(); /* If we see non-zero ->refs, we need to see all */
> +			   /* other initialization for this incarnation of */
> +			   /* this callback. */
> +

/* Need to read func and info after refs to see the new values, pairs
with the wmb before setting refs. */


>  		func = data->csd.func;			/* for later warn */
> -		data->csd.func(data->csd.info);
> +		func(data->csd.info);

Ok, and can merge with the add of rmb.

>  
>  		/*
>  		 * If the cpu mask is not still set then func enabled
> @@ -492,12 +498,6 @@ void smp_call_function_many(const struct cpumask *mask,
>  	cpumask_clear_cpu(this_cpu, data->cpumask);
>  	refs = cpumask_weight(data->cpumask);
>  
> -	/* some callers might race with other cpus changing the mask */
> -	if (unlikely(!refs)) {
> -		csd_unlock(&data->csd);
> -		return;
> -	}
> -

Again, this is needed, you were considering the csd mask but this
is protecting against the cpumask argument is the source of the
cpumask_copy changing.

>  	/*
>  	 * We reuse the call function data without waiting for any grace
>  	 * period after some other cpu removes it from the global queue.
> @@ -527,8 +527,9 @@ void smp_call_function_many(const struct cpumask *mask,
>  	 * Make the list addition visible before sending the ipi.
>  	 * (IPIs must obey or appear to obey normal Linux cache
>  	 * coherency rules -- see comment in generic_exec_single).
> +	 * The unlock above combined with the lock in the IPI
> +	 * code covers this requirement.
>  	 */
> -	smp_mb();

again, this requires the arch audit.

>  
>  	/* Send a message to all CPUs in the map */
>  	arch_send_call_function_ipi_mask(data->cpumask);

milton
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