Re: [PATCH 1/10] Add generic helpers for arch IPI function calls

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Fri, May 02, 2008 at 04:12:34AM +0200, Nick Piggin wrote:
> On Thu, May 01, 2008 at 07:02:41PM -0700, Paul E. McKenney wrote:
> > On Wed, Apr 30, 2008 at 02:37:17PM +0200, Jens Axboe wrote:
> > > > Here are some (probably totally broken) ideas:
> > > > 
> > > > 1.	Global lock so that only one smp_call_function() in the
> > > > 	system proceeds.  Additional calls would be spinning with
> > > > 	irqs -enabled- on the lock, avoiding deadlock.  Kind of
> > > > 	defeats the purpose of your list, though...
> > > 
> > > That is what we used to do, that will obviously work. But defeats most
> > > of the purpose, unfortunately :-)
> > > 
> > > > 2.	Maintain a global mask of current targets of smp_call_function()
> > > > 	CPUs.  A given CPU may proceed if it is not a current target
> > > > 	and if none of its target CPUs are already in the mask.
> > > > 	This mask would be manipulated under a global lock.
> > > > 
> > > > 3.	As in #2 above, but use per-CPU counters.  This allows the
> > > > 	current CPU to proceed if it is not a target, but also allows
> > > > 	concurrent smp_call_function()s to proceed even if their
> > > > 	lists of target CPUs overlap.
> > > > 
> > > > 4.	#2 or #3, but where CPUs can proceed freely if their allocation
> > > > 	succeeded.
> > > > 
> > > > 5.	If a given CPU is waiting for other CPUs to respond, it polls
> > > > 	its own list (with irqs disabled), thus breaking the deadlock.
> > > > 	This means that you cannot call smp_call_function() while holding
> > > > 	a lock that might be acquired by the called function, but that
> > > > 	is not a new prohibition -- the only safe way to hold such a
> > > > 	lock is with irqs disabled, and you are not allowed to call
> > > > 	the smp_call_function() with irqs disabled in the first place
> > > > 	(right?).
> > > > 
> > > > #5 might actually work...
> > > 
> > > Yeah, #5 sounds quite promising. I'll see if I can work up a patch for
> > > that, or if you feel so inclined, I'll definitely take patches :-)
> > > 
> > > The branch is 'generic-ipi' on git://git.kernel.dk/linux-2.6-block.git
> > > The link is pretty slow, so it's best pull'ed off of Linus base. Or just
> > > grab the patches from the gitweb interface:
> > > 
> > > http://git.kernel.dk/?p=linux-2.6-block.git;a=shortlog;h=refs/heads/generic-ipi
> > 
> > And here is an untested patch for getting rid of the fallback element,
> > and eliminating the "wait" deadlocks.
> 
> Hey this is coming along really nicely, thanks guys.
> 
> The only problem I have with this is that if you turn IRQs off, you
> probably don't expect call function functions to be processed under
> you (sure that doesn't happen now, but it could if anybody actually
> starts to call IPIs under irq off).

OK -- for some reason, I was thinking that it was illegal to
invoke smp_call_function() with irqs disabled...

Ah, I see it -- smp_call_function_mask() says:

 * You must not call this function with disabled interrupts or from a
 * hardware interrupt handler or from a bottom half handler.

So we have no problem with smp_call_function, then.

OK, so smp_call_function() -can- be invoked with irqs disabled?
Hmmm...  I will give this some thought.

> What I _really_ wanted to do is just keep the core API as a non-deadlocky
> one that has its data passed into it; and then implemented the fallbacky,
> deadlocky one on top of that. In places where it makes sense, callers
> could then use the new API if they want to.

I don't believe that you can make the fallback non-deadlocky...  Perhaps
a failure of imagination on my part, of course, but I am beginning to
doubt that...

> We could make another rule that smp_call_function might also run functions,
> but IMO that is starting to turn into spaghetti ;) Clever spaghetti though,
> I give you that!

Well, given that you cannot call smp_call_function_mask() with irqs
disabled, my approach -does- work in that case, as an irq might come
in just after you called the function but before irqs were disabled.

So, how many places is smp_call_function() invoked with irqs disabled?

							Thanx, Paul

> > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> > ---
> > 
> >  smp.c |   80 +++++++++++-------------------------------------------------------
> >  1 file changed, 14 insertions(+), 66 deletions(-)
> > 
> > diff --git a/kernel/smp.c b/kernel/smp.c
> > index 36d3eca..9df96fa 100644
> > --- a/kernel/smp.c
> > +++ b/kernel/smp.c
> > @@ -17,7 +17,6 @@ __cacheline_aligned_in_smp DEFINE_SPINLOCK(call_function_lock);
> >  enum {
> >  	CSD_FLAG_WAIT		= 0x01,
> >  	CSD_FLAG_ALLOC		= 0x02,
> > -	CSD_FLAG_FALLBACK	= 0x04,
> >  };
> >  
> >  struct call_function_data {
> > @@ -33,9 +32,6 @@ struct call_single_queue {
> >  	spinlock_t lock;
> >  };
> >  
> > -static DEFINE_PER_CPU(struct call_function_data, cfd_fallback);
> > -static DEFINE_PER_CPU(unsigned long, cfd_fallback_used);
> > -
> >  void __cpuinit init_call_single_data(void)
> >  {
> >  	int i;
> > @@ -59,6 +55,7 @@ static void csd_flag_wait(struct call_single_data *data)
> >  		if (!(data->flags & CSD_FLAG_WAIT))
> >  			break;
> >  		cpu_relax();
> > +		generic_smp_call_function_interrupt();
> >  	} while (1);
> >  }
> >  
> > @@ -84,48 +81,13 @@ static void generic_exec_single(int cpu, struct call_single_data *data)
> >  		csd_flag_wait(data);
> >  }
> >  
> > -/*
> > - * We need to have a global per-cpu fallback of call_function_data, so
> > - * we can safely proceed with smp_call_function() if dynamic allocation
> > - * fails and we cannot fall back to on-stack allocation (if wait == 0).
> > - */
> > -static noinline void acquire_cpu_fallback(int cpu)
> > -{
> > -	while (test_and_set_bit_lock(0, &per_cpu(cfd_fallback_used, cpu)))
> > -		cpu_relax();
> > -}
> > -
> > -static noinline void free_cpu_fallback(struct call_single_data *csd)
> > -{
> > -	struct call_function_data *data;
> > -	int cpu;
> > -
> > -	data = container_of(csd, struct call_function_data, csd);
> > -
> > -	/*
> > -	 * We could drop this loop by embedding a cpu variable in
> > -	 * csd, but this should happen so extremely rarely (if ever)
> > -	 * that this seems like a better idea
> > -	 */
> > -	for_each_possible_cpu(cpu) {
> > -		if (&per_cpu(cfd_fallback, cpu) != data)
> > -			continue;
> > -
> > -		clear_bit_unlock(0, &per_cpu(cfd_fallback_used, cpu));
> > -		break;
> > -	}
> > -}
> > -
> >  static void rcu_free_call_data(struct rcu_head *head)
> >  {
> >  	struct call_function_data *data;
> >  
> >  	data = container_of(head, struct call_function_data, rcu_head);
> >  
> > -	if (data->csd.flags & CSD_FLAG_ALLOC)
> > -		kfree(data);
> > -	else
> > -		free_cpu_fallback(&data->csd);
> > +	kfree(data);
> >  }
> >  
> >  /*
> > @@ -222,8 +184,6 @@ void generic_smp_call_function_single_interrupt(void)
> >  				data->flags &= ~CSD_FLAG_WAIT;
> >  			} else if (data_flags & CSD_FLAG_ALLOC)
> >  				kfree(data);
> > -			else if (data_flags & CSD_FLAG_FALLBACK)
> > -				free_cpu_fallback(data);
> >  		}
> >  		/*
> >  		 * See comment on outer loop
> > @@ -244,6 +204,7 @@ void generic_smp_call_function_single_interrupt(void)
> >  int smp_call_function_single(int cpu, void (*func) (void *info), void *info,
> >  			     int retry, int wait)
> >  {
> > +	struct call_single_data d = NULL;
> >  	unsigned long flags;
> >  	/* prevent preemption and reschedule on another processor */
> >  	int me = get_cpu();
> > @@ -258,21 +219,14 @@ int smp_call_function_single(int cpu, void (*func) (void *info), void *info,
> >  	} else {
> >  		struct call_single_data *data;
> >  
> > -		if (wait) {
> > -			struct call_single_data d;
> > -
> > -			data = &d;
> > -			data->flags = CSD_FLAG_WAIT;
> > -		} else {
> > +		if (!wait) {
> >  			data = kmalloc(sizeof(*data), GFP_ATOMIC);
> >  			if (data)
> >  				data->flags = CSD_FLAG_ALLOC;
> > -			else {
> > -				acquire_cpu_fallback(me);
> > -
> > -				data = &per_cpu(cfd_fallback, me).csd;
> > -				data->flags = CSD_FLAG_FALLBACK;
> > -			}
> > +		}
> > +		if (!data) {
> > +			data = &d;
> > +			data->flags = CSD_FLAG_WAIT;
> >  		}
> >  
> >  		data->func = func;
> > @@ -320,6 +274,7 @@ void __smp_call_function_single(int cpu, struct call_single_data *data)
> >  int smp_call_function_mask(cpumask_t mask, void (*func)(void *), void *info,
> >  			   int wait)
> >  {
> > +	struct call_function_data d;
> >  	struct call_function_data *data;
> >  	cpumask_t allbutself;
> >  	unsigned long flags;
> > @@ -345,21 +300,14 @@ int smp_call_function_mask(cpumask_t mask, void (*func)(void *), void *info,
> >  		return smp_call_function_single(cpu, func, info, 0, wait);
> >  	}
> >  
> > -	if (wait) {
> > -		struct call_function_data d;
> > -
> > -		data = &d;
> > -		data->csd.flags = CSD_FLAG_WAIT;
> > -	} else {
> > +	if (!wait) {
> >  		data = kmalloc(sizeof(*data), GFP_ATOMIC);
> >  		if (data)
> >  			data->csd.flags = CSD_FLAG_ALLOC;
> > -		else {
> > -			acquire_cpu_fallback(cpu);
> > -
> > -			data = &per_cpu(cfd_fallback, cpu);
> > -			data->csd.flags = CSD_FLAG_FALLBACK;
> > -		}
> > +	}
> > +	if (!data) {
> > +		data = &d;
> > +		data->csd.flags = CSD_FLAG_WAIT;
> >  	}
> >  
> >  	spin_lock_init(&data->lock);