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SubjectRe: [PATCH 1/10] Add generic helpers for arch IPI function calls
On Sat, May 03, 2008 at 11:11:48AM -0700, Paul E. McKenney wrote:
> On Sat, May 03, 2008 at 07:49:30AM +0200, Nick Piggin wrote:
> > On Fri, May 02, 2008 at 02:59:29PM +0200, Peter Zijlstra wrote:
> > > On Fri, 2008-05-02 at 05:42 -0700, Paul E. McKenney wrote:
> > >
> > > > And here is one scenario that makes me doubt that my imagination is
> > > > faulty:
> > > >
> > > > 1. CPU 0 disables irqs.
> > > >
> > > > 2. CPU 1 disables irqs.
> > > >
> > > > 3. CPU 0 invokes smp_call_function(). But CPU 1 will never respond
> > > > because its irqs are disabled.
> > > >
> > > > 4. CPU 1 invokes smp_call_function(). But CPU 0 will never respond
> > > > because its irqs are disabled.
> > > >
> > > > Looks like inherent deadlock to me, requiring that smp_call_function()
> > > > be invoked with irqs enabled.
> > > >
> > > > So, what am I missing here?
> > >
> > > The wish to do it anyway ;-)
> > >
> > > I can imagine some situations where I'd like to try anyway and fall back
> > > to a slower path when failing.
> > >
> > > With the initial design we would simply allocate data, stick it on the
> > > queue and call the ipi (when needed).
> > >
> > > This is perfectly deadlock free when wait=0 and it just returns -ENOMEM
> > > on allocation failure.
> >
> > Yeah, I'm just talking about the wait=0 case. (btw. I'd rather the core
> > API takes some data rather than allocates some itself, eg because you
> > might want to have it on the stack).
>
> But taking data on the stack is safe only in the wait=1 case, right?
>
> > For the wait=1 case, something very clever such as processing pending
> > requests in a polling loop might be cool... however I'd rather not add
> > such complexity until someone needs it (you could stick a comment in
> > there outlining your algorithm). But I'd just rather not have peole rely
> > on it yet.
>
> In that case we may need to go back to the global lock with only one
> request being processed at a time. Otherwise, if two wait=1 requests
> happen at the same time, they deadlock waiting for each other to process
> their request. (See Keith Owens: http://lkml.org/lkml/2008/5/2/183).
>
> In other words, if you want to allow parallel calls to
> smp_call_function(), the simplest way to do it seems to be to do the
> polling loop. The other ways I have come up with thus far are uglier
> and less effective (see http://lkml.org/lkml/2008/4/30/164).
>
> Now, what I -could- do would be to prohibit the wait=1 case from
> irq-disable state from polling -- that would make sense, as the caller
> probably had a reason to mask irqs, and might not take kindly to having
> them faked behind the caller's back. ;-)
>
> > > It it doesn't return -ENOMEM I know its been queued and will be
> > > processed at some point, if it does fail, I can deal with it in another
> > > way.
> >
> > At least with IPIs I think we can guarantee they will be processed on
> > the target after we queue them.
>
> OK, so let me make sure I understand what is needed. One example might be
> some code called from scheduler_tick(), which runs with irqs disabled.
> Without the ability to call smp_call_function() directly, you have
> to fire off a work queue or something. Now, if smp_call_function()
> can hand you an -ENOMEM or (maybe) an -EBUSY, then you still have to
> fire off the work queue, but you probably only have to do it rarely,
> minimizing the performance impact.
>
> Another possibility is when it is -nice- to call smp_call_function(),
> but can just try again on the next scheduler_tick() -- ignoring dynticks
> idle for the moment. In this case, you might still test the error return
> to set a flag that you will check on the next scheduler_tick() call.
>
> Is this where you guys are coming from?
>
> And you are all OK with smp_call_function() called with irqs enabled
> never being able to fail, right? (Speaking of spaghetti code, why
> foist unnecessary failure checks on the caller...)
>
> > > I know I'd like to do that and I suspect Nick has a few use cases up his
> > > sleeve as well.
> >
> > It would be handy. The "quickly kick something off on another CPU" is
> > pretty nice in mm/ when you have per-cpu queues or caches that might
> > want to be flushed.
>
> OK, I think I might be seeing what you guys are getting at. Here is
> what I believe you guys need:
>
> 1. No deadlocks, ever, not even theoretical "low probability"
> deadlocks.
>
> 2. No failure returns when called with irqs enabled. On the other
> hand, when irqs are disabled, failure is possible. Though hopefully
> unlikely.
>
> 3. Parallel execution of multiple smp_call_function() requests
> is required, even when called with irqs disabled.
>
> 4. The wait=1 case with irqs disabled is prohibited.
>
> 5. If you call smp_call_function() with irqs disabled, then you
> are guaranteed that no other CPU's smp_call_function() handler
> will be invoked while smp_call_function() is executing.
>
> Anything I am missing?

On the off-chance that the answer to the above question is "no", here
is a crude patch on top of Jens's earlier patch.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---

arch/Kconfig | 2 -
kernel/smp.c | 107 ++++++++++++++++++-----------------------------------------
2 files changed, 34 insertions(+), 75 deletions(-)

diff --git a/arch/Kconfig b/arch/Kconfig
index a5a0184..5ae9360 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -38,4 +38,4 @@ config HAVE_KRETPROBES
def_bool n

config USE_GENERIC_SMP_HELPERS
- def_bool n
+ def_bool y
diff --git a/kernel/smp.c b/kernel/smp.c
index 36d3eca..d7e8dd1 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;
@@ -48,7 +44,7 @@ void __cpuinit init_call_single_data(void)
}
}

-static void csd_flag_wait(struct call_single_data *data)
+static void csd_flag_wait(struct call_single_data *data, int poll)
{
/* Wait for response */
do {
@@ -59,6 +55,8 @@ static void csd_flag_wait(struct call_single_data *data)
if (!(data->flags & CSD_FLAG_WAIT))
break;
cpu_relax();
+ if (poll)
+ generic_smp_call_function_interrupt();
} while (1);
}

@@ -66,7 +64,7 @@ static void csd_flag_wait(struct call_single_data *data)
* Insert a previously allocated call_single_data element for execution
* on the given CPU. data must already have ->func, ->info, and ->flags set.
*/
-static void generic_exec_single(int cpu, struct call_single_data *data)
+static void generic_exec_single(int cpu, struct call_single_data *data, int poll)
{
struct call_single_queue *dst = &per_cpu(call_single_queue, cpu);
int wait = data->flags & CSD_FLAG_WAIT, ipi;
@@ -81,39 +79,7 @@ static void generic_exec_single(int cpu, struct call_single_data *data)
arch_send_call_function_single_ipi(cpu);

if (wait)
- 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;
- }
+ csd_flag_wait(data, poll);
}

static void rcu_free_call_data(struct rcu_head *head)
@@ -122,10 +88,7 @@ static void rcu_free_call_data(struct rcu_head *head)

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 +185,6 @@ 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
@@ -244,40 +205,39 @@ 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;
unsigned long flags;
/* prevent preemption and reschedule on another processor */
int me = get_cpu();
+ int irqsdisabled = irqs_disabled();

/* Can deadlock when called with interrupts disabled */
- WARN_ON(wait && irqs_disabled());
+ WARN_ON(wait && irqsdisabled);

if (cpu == me) {
local_irq_save(flags);
func(info);
local_irq_restore(flags);
} else {
- struct call_single_data *data;
-
- if (wait) {
- struct call_single_data d;
+ struct call_single_data *data = NULL;

- 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) {
+ if (irqsdisabled) {
+ put_cpu();
+ return -ENOMEM;
}
+ data = &d;
+ data->flags = CSD_FLAG_WAIT;
}

data->func = func;
data->info = info;
- generic_exec_single(cpu, data);
+ generic_exec_single(cpu, data, !irqsdisabled);
}

put_cpu();
@@ -300,7 +260,7 @@ void __smp_call_function_single(int cpu, struct call_single_data *data)
/* Can deadlock when called with interrupts disabled */
WARN_ON((data->flags & CSD_FLAG_WAIT) && irqs_disabled());

- generic_exec_single(cpu, data);
+ generic_exec_single(cpu, data, !irqs_disabled());
}

/**
@@ -320,13 +280,15 @@ 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 *data;
+ struct call_function_data d;
+ struct call_function_data *data = NULL;
cpumask_t allbutself;
unsigned long flags;
int cpu, num_cpus;
+ int irqsdisabled = irqs_disabled();

/* Can deadlock when called with interrupts disabled */
- WARN_ON(wait && irqs_disabled());
+ WARN_ON(wait && irqsdisabled);

cpu = smp_processor_id();
allbutself = cpu_online_map;
@@ -345,21 +307,18 @@ 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) {
+ if (unlikely(irqsdisabled)) {
+ put_cpu();
+ return -ENOMEM;
}
+ data = &d;
+ data->csd.flags = CSD_FLAG_WAIT;
}

spin_lock_init(&data->lock);
@@ -382,7 +341,7 @@ int smp_call_function_mask(cpumask_t mask, void (*func)(void *), void *info,

/* optionally wait for the CPUs to complete */
if (wait)
- csd_flag_wait(&data->csd);
+ csd_flag_wait(&data->csd, !irqsdisabled);

return 0;
}

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