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SubjectRe: [PATCH 1/10] Add generic helpers for arch IPI function calls
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);


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