Date:	Wed, 30 Apr 2008 04:29:34 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Jens Axboe <jens.axboe@...cle.com>
Cc:	linux-kernel@...r.kernel.org, peterz@...radead.org,
	npiggin@...e.de, linux-arch@...r.kernel.org, jeremy@...p.org,
	mingo@...e.hu
Subject: Re: [PATCH 1/10] Add generic helpers for arch IPI function calls

On Tue, Apr 29, 2008 at 06:59:36AM -0700, Paul E. McKenney wrote:
> On Tue, Apr 29, 2008 at 09:26:21AM +0200, Jens Axboe wrote:
> > This adds kernel/smp.c which contains helpers for IPI function calls. In
> > addition to supporting the existing smp_call_function() in a more efficient
> > manner, it also adds a more scalable variant called smp_call_function_single()
> > for calling a given function on a single CPU only.
> > 
> > The core of this is based on the x86-64 patch from Nick Piggin, lots of
> > changes since then. "Alan D. Brunelle" <Alan.Brunelle@...com> has
> > contributed lots of fixes and suggestions as well.
> 
> Looks much better, but there still appears to be a potential deadlock
> with a CPU spinning waiting (indirectly) for a grace period to complete.
> Such spinning can prevent the grace period from ever completing.
> 
> See "!!!".

One additional question...  Why not handle memory allocation failure
by pretending that the caller to smp_call_function() had specified
"wait"?  The callee is in irq context, so cannot block, right?

							Thanx, Paul

> > Acked-by: Ingo Molnar <mingo@...e.hu>
> > Signed-off-by: Jens Axboe <jens.axboe@...cle.com>
> > ---
> >  arch/Kconfig        |    3 +
> >  include/linux/smp.h |   31 ++++-
> >  init/main.c         |    1 +
> >  kernel/Makefile     |    1 +
> >  kernel/smp.c        |  415 +++++++++++++++++++++++++++++++++++++++++++++++++++
> >  5 files changed, 449 insertions(+), 2 deletions(-)
> >  create mode 100644 kernel/smp.c
> > 
> > diff --git a/arch/Kconfig b/arch/Kconfig
> > index 694c9af..a5a0184 100644
> > --- a/arch/Kconfig
> > +++ b/arch/Kconfig
> > @@ -36,3 +36,6 @@ config HAVE_KPROBES
> > 
> >  config HAVE_KRETPROBES
> >  	def_bool n
> > +
> > +config USE_GENERIC_SMP_HELPERS
> > +	def_bool n
> > diff --git a/include/linux/smp.h b/include/linux/smp.h
> > index 55232cc..b22b4fc 100644
> > --- a/include/linux/smp.h
> > +++ b/include/linux/smp.h
> > @@ -7,9 +7,19 @@
> >   */
> > 
> >  #include <linux/errno.h>
> > +#include <linux/list.h>
> > +#include <linux/spinlock.h>
> > +#include <linux/cpumask.h>
> > 
> >  extern void cpu_idle(void);
> > 
> > +struct call_single_data {
> > +	struct list_head list;
> > +	void (*func) (void *info);
> > +	void *info;
> > +	unsigned int flags;
> > +};
> > +
> >  #ifdef CONFIG_SMP
> > 
> >  #include <linux/preempt.h>
> > @@ -53,9 +63,27 @@ extern void smp_cpus_done(unsigned int max_cpus);
> >   * Call a function on all other processors
> >   */
> >  int smp_call_function(void(*func)(void *info), void *info, int retry, int wait);
> > -
> > +int smp_call_function_mask(cpumask_t mask, void(*func)(void *info), void *info,
> > +				int wait);
> >  int smp_call_function_single(int cpuid, void (*func) (void *info), void *info,
> >  				int retry, int wait);
> > +void __smp_call_function_single(int cpuid, struct call_single_data *data);
> > +
> > +/*
> > + * Generic and arch helpers
> > + */
> > +#ifdef CONFIG_USE_GENERIC_SMP_HELPERS
> > +void generic_smp_call_function_single_interrupt(void);
> > +void generic_smp_call_function_interrupt(void);
> > +void init_call_single_data(void);
> > +void arch_send_call_function_single_ipi(int cpu);
> > +void arch_send_call_function_ipi(cpumask_t mask);
> > +extern spinlock_t call_function_lock;
> > +#else
> > +static inline void init_call_single_data(void)
> > +{
> > +}
> > +#endif
> > 
> >  /*
> >   * Call a function on all processors
> > @@ -112,7 +140,6 @@ static inline void smp_send_reschedule(int cpu) { }
> >  })
> >  #define smp_call_function_mask(mask, func, info, wait) \
> >  			(up_smp_call_function(func, info))
> > -
> >  #endif /* !SMP */
> > 
> >  /*
> > diff --git a/init/main.c b/init/main.c
> > index 1687b01..81b652e 100644
> > --- a/init/main.c
> > +++ b/init/main.c
> > @@ -778,6 +778,7 @@ static void __init do_pre_smp_initcalls(void)
> >  {
> >  	extern int spawn_ksoftirqd(void);
> > 
> > +	init_call_single_data();
> >  	migration_init();
> >  	spawn_ksoftirqd();
> >  	if (!nosoftlockup)
> > diff --git a/kernel/Makefile b/kernel/Makefile
> > index 6c5f081..7e275d4 100644
> > --- a/kernel/Makefile
> > +++ b/kernel/Makefile
> > @@ -28,6 +28,7 @@ obj-$(CONFIG_DEBUG_RT_MUTEXES) += rtmutex-debug.o
> >  obj-$(CONFIG_RT_MUTEX_TESTER) += rtmutex-tester.o
> >  obj-$(CONFIG_GENERIC_ISA_DMA) += dma.o
> >  obj-$(CONFIG_SMP) += cpu.o spinlock.o
> > +obj-$(CONFIG_USE_GENERIC_SMP_HELPERS) += smp.o
> >  obj-$(CONFIG_DEBUG_SPINLOCK) += spinlock.o
> >  obj-$(CONFIG_PROVE_LOCKING) += spinlock.o
> >  obj-$(CONFIG_UID16) += uid16.o
> > diff --git a/kernel/smp.c b/kernel/smp.c
> > new file mode 100644
> > index 0000000..36d3eca
> > --- /dev/null
> > +++ b/kernel/smp.c
> > @@ -0,0 +1,415 @@
> > +/*
> > + * Generic helpers for smp ipi calls
> > + *
> > + * (C) Jens Axboe <jens.axboe@...cle.com> 2008
> > + *
> > + */
> > +#include <linux/init.h>
> > +#include <linux/module.h>
> > +#include <linux/percpu.h>
> > +#include <linux/rcupdate.h>
> > +#include <linux/smp.h>
> > +
> > +static DEFINE_PER_CPU(struct call_single_queue, call_single_queue);
> > +static LIST_HEAD(call_function_queue);
> > +__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 {
> > +	struct call_single_data csd;
> > +	spinlock_t lock;
> > +	unsigned int refs;
> > +	cpumask_t cpumask;
> > +	struct rcu_head rcu_head;
> > +};
> > +
> > +struct call_single_queue {
> > +	struct list_head list;
> > +	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;
> > +
> > +	for_each_possible_cpu(i) {
> > +		struct call_single_queue *q = &per_cpu(call_single_queue, i);
> > +
> > +		spin_lock_init(&q->lock);
> > +		INIT_LIST_HEAD(&q->list);
> > +	}
> > +}
> > +
> > +static void csd_flag_wait(struct call_single_data *data)
> > +{
> > +	/* Wait for response */
> > +	do {
> > +		/*
> > +		 * We need to see the flags store in the IPI handler
> > +		 */
> > +		smp_mb();
> > +		if (!(data->flags & CSD_FLAG_WAIT))
> > +			break;
> > +		cpu_relax();
> > +	} while (1);
> > +}
> > +
> > +/*
> > + * 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)
> > +{
> > +	struct call_single_queue *dst = &per_cpu(call_single_queue, cpu);
> > +	int wait = data->flags & CSD_FLAG_WAIT, ipi;
> > +	unsigned long flags;
> > +
> > +	spin_lock_irqsave(&dst->lock, flags);
> > +	ipi = list_empty(&dst->list);
> > +	list_add_tail(&data->list, &dst->list);
> > +	spin_unlock_irqrestore(&dst->lock, flags);
> > +
> > +	if (ipi)
> > +		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();
> 
> !!! What if this CPU recently did a smp_call_function_mask() in no-wait mode,
> and this CPU's fallback element is still waiting for a grace period
> to elapse?  Wouldn't we end up with a deadlock, with this CPU spinning
> preventing the grace period from completing, thus preventing the element
> from ever being freed?
> 
> > +}
> > +
> > +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);
> > +}
> 
> Good, now there is no need to worry about readers seeing a recycling
> event on the fallback element.  I just need to go check to see what
> readers do if there is no memory available and the fallback element
> is in use...
> 
> > +
> > +/*
> > + * Invoked by arch to handle an IPI for call function. Must be called with
> > + * interrupts disabled.
> > + */
> > +void generic_smp_call_function_interrupt(void)
> > +{
> > +	struct call_function_data *data;
> > +	int cpu = get_cpu();
> > +
> > +	/*
> > +	 * It's ok to use list_for_each_rcu() here even though we may delete
> > +	 * 'pos', since list_del_rcu() doesn't clear ->next
> > +	 */
> > +	rcu_read_lock();
> > +	list_for_each_entry_rcu(data, &call_function_queue, csd.list) {
> 
> Much improved over the earlier list_for_each_entry()!!!
> 
> > +		int refs;
> > +
> > +		if (!cpu_isset(cpu, data->cpumask))
> > +			continue;
> > +
> > +		data->csd.func(data->csd.info);
> > +
> > +		spin_lock(&data->lock);
> > +		cpu_clear(cpu, data->cpumask);
> > +		WARN_ON(data->refs == 0);
> > +		data->refs--;
> > +		refs = data->refs;
> > +		spin_unlock(&data->lock);
> > +
> > +		if (refs)
> > +			continue;
> > +
> > +		spin_lock(&call_function_lock);
> > +		list_del_rcu(&data->csd.list);
> > +		spin_unlock(&call_function_lock);
> > +
> > +		if (data->csd.flags & CSD_FLAG_WAIT) {
> > +			/*
> > +			 * serialize stores to data with the flag clear
> > +			 * and wakeup
> > +			 */
> > +			smp_wmb();
> > +			data->csd.flags &= ~CSD_FLAG_WAIT;
> > +		} else
> > +			call_rcu(&data->rcu_head, rcu_free_call_data);
> > +	}
> > +	rcu_read_unlock();
> > +
> > +	put_cpu();
> > +}
> > +
> > +/*
> > + * Invoked by arch to handle an IPI for call function single. Must be called
> > + * from the arch with interrupts disabled.
> > + */
> > +void generic_smp_call_function_single_interrupt(void)
> > +{
> > +	struct call_single_queue *q = &__get_cpu_var(call_single_queue);
> > +	LIST_HEAD(list);
> > +
> > +	/*
> > +	 * Need to see other stores to list head for checking whether
> > +	 * list is empty without holding q->lock
> > +	 */
> > +	smp_mb();
> > +	while (!list_empty(&q->list)) {
> > +		unsigned int data_flags;
> > +
> > +		spin_lock(&q->lock);
> > +		list_replace_init(&q->list, &list);
> > +		spin_unlock(&q->lock);
> > +
> > +		while (!list_empty(&list)) {
> > +			struct call_single_data *data;
> > +
> > +			data = list_entry(list.next, struct call_single_data,
> > +						list);
> > +			list_del(&data->list);
> > +
> > +			/*
> > +			 * 'data' can be invalid after this call if
> > +			 * flags == 0 (when called through
> > +			 * generic_exec_single(), so save them away before
> > +			 * making the call.
> > +			 */
> > +			data_flags = data->flags;
> > +
> > +			data->func(data->info);
> > +
> > +			if (data_flags & CSD_FLAG_WAIT) {
> > +				smp_wmb();
> > +				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);
> 
> Here is is OK not to wait for a grace period -- this CPU owns the element,
> so no one ease should be accessing it.
> 
> > +		}
> > +		/*
> > +		 * See comment on outer loop
> > +		 */
> > +		smp_mb();
> > +	}
> > +}
> > +
> > +/*
> > + * smp_call_function_single - Run a function on a specific CPU
> > + * @func: The function to run. This must be fast and non-blocking.
> > + * @info: An arbitrary pointer to pass to the function.
> > + * @retry: Unused
> > + * @wait: If true, wait until function has completed on other CPUs.
> > + *
> > + * Returns 0 on success, else a negative status code.
> > + */
> > +int smp_call_function_single(int cpu, void (*func) (void *info), void *info,
> > +			     int retry, int wait)
> > +{
> > +	unsigned long flags;
> > +	/* prevent preemption and reschedule on another processor */
> > +	int me = get_cpu();
> > +
> > +	/* Can deadlock when called with interrupts disabled */
> > +	WARN_ON(wait && irqs_disabled());
> > +
> > +	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;
> > +
> > +			data = &d;
> > +			data->flags = CSD_FLAG_WAIT;
> > +		} else {
> > +			data = kmalloc(sizeof(*data), GFP_ATOMIC);
> > +			if (data)
> > +				data->flags = CSD_FLAG_ALLOC;
> > +			else {
> > +				acquire_cpu_fallback(me);
> 
> Ah -- per-CPU fallback element, and we are presumably not permitted
> to block during the course of the call.  But then someone might be
> using it after we exit this function!!!   More on this elsewhere...
> 
> Hmmm...  What about the no-wait case?
> 
> Ah, not a problem, because we always use the "d" element that is allocated
> on our stack.  But aren't we then passing a reference to an on-stack
> variable out of scope?  Don't we need to allocate "d" at the top level
> of the function?  Isn't gcc within its rights to get rid of this local
> (perhaps re-using it for a compiler temporary or some such) as soon as
> its scope ends?
> 
> > +
> > +				data = &per_cpu(cfd_fallback, me).csd;
> > +				data->flags = CSD_FLAG_FALLBACK;
> > +			}
> > +		}
> > +
> > +		data->func = func;
> > +		data->info = info;
> > +		generic_exec_single(cpu, data);
> 
> If we are using the on-stack variable, generic_exec_single() is
> guaranteed to wait until all other CPUs are done with it.
> 
> > +	}
> > +
> > +	put_cpu();
> > +	return 0;
> > +}
> > +EXPORT_SYMBOL(smp_call_function_single);
> > +
> > +/**
> > + * __smp_call_function_single(): Run a function on another CPU
> > + * @cpu: The CPU to run on.
> > + * @data: Pre-allocated and setup data structure
> > + *
> > + * Like smp_call_function_single(), but allow caller to pass in a pre-allocated
> > + * data structure. Useful for embedding @data inside other structures, for
> > + * instance.
> > + *
> > + */
> > +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);
> > +}
> > +
> > +/**
> > + * smp_call_function_mask(): Run a function on a set of other CPUs.
> > + * @mask: The set of cpus to run on.
> > + * @func: The function to run. This must be fast and non-blocking.
> > + * @info: An arbitrary pointer to pass to the function.
> > + * @wait: If true, wait (atomically) until function has completed on other CPUs.
> > + *
> > + * Returns 0 on success, else a negative status code.
> > + *
> > + * If @wait is true, then returns once @func has returned.
> > + *
> > + * You must not call this function with disabled interrupts or from a
> > + * hardware interrupt handler or from a bottom half handler.
> > + */
> > +int smp_call_function_mask(cpumask_t mask, void (*func)(void *), void *info,
> > +			   int wait)
> > +{
> > +	struct call_function_data *data;
> > +	cpumask_t allbutself;
> > +	unsigned long flags;
> > +	int cpu, num_cpus;
> > +
> > +	/* Can deadlock when called with interrupts disabled */
> > +	WARN_ON(wait && irqs_disabled());
> > +
> > +	cpu = smp_processor_id();
> > +	allbutself = cpu_online_map;
> > +	cpu_clear(cpu, allbutself);
> > +	cpus_and(mask, mask, allbutself);
> > +	num_cpus = cpus_weight(mask);
> > +
> > +	/*
> > +	 * If zero CPUs, return. If just a single CPU, turn this request
> > +	 * into a targetted single call instead since it's faster.
> > +	 */
> > +	if (!num_cpus)
> > +		return 0;
> > +	else if (num_cpus == 1) {
> > +		cpu = first_cpu(mask);
> > +		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;
> 
> !!! We are passing "d" out of its scope -- need to move the declaration
> to the top level.
> 
> > +	} else {
> > +		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;
> > +		}
> > +	}
> > +
> > +	spin_lock_init(&data->lock);
> > +	data->csd.func = func;
> > +	data->csd.info = info;
> > +	data->refs = num_cpus;
> > +
> > +	/*
> > +	 * need to see above stores before the cpumask is valid for the CPU
> > +	 */
> > +	smp_wmb();
> > +	data->cpumask = mask;
> > +
> > +	spin_lock_irqsave(&call_function_lock, flags);
> > +	list_add_tail_rcu(&data->csd.list, &call_function_queue);
> > +	spin_unlock_irqrestore(&call_function_lock, flags);
> > +
> > +	/* Send a message to all CPUs in the map */
> > +	arch_send_call_function_ipi(mask);
> > +
> > +	/* optionally wait for the CPUs to complete */
> > +	if (wait)
> > +		csd_flag_wait(&data->csd);
> 
> And there are no references to "d" past here.  Preemption appears to be
> disabled by all callers.  So good.
> 
> Still might have the fallback element waiting for a grace period
> past this point, which would interfere with a subsequent call to this
> function!!!
> 
> > +
> > +	return 0;
> > +}
> > +EXPORT_SYMBOL(smp_call_function_mask);
> > +
> > +/**
> > + * smp_call_function(): Run a function on all other CPUs.
> > + * @func: The function to run. This must be fast and non-blocking.
> > + * @info: An arbitrary pointer to pass to the function.
> > + * @natomic: Unused
> > + * @wait: If true, wait (atomically) until function has completed on other CPUs.
> > + *
> > + * Returns 0 on success, else a negative status code.
> > + *
> > + * If @wait is true, then returns once @func has returned; otherwise
> > + * it returns just before the target cpu calls @func.
> > + *
> > + * You must not call this function with disabled interrupts or from a
> > + * hardware interrupt handler or from a bottom half handler.
> > + */
> > +int smp_call_function(void (*func)(void *), void *info, int natomic, int wait)
> > +{
> > +	int ret;
> > +
> > +	preempt_disable();
> > +	ret = smp_call_function_mask(cpu_online_map, func, info, wait);
> > +	preempt_enable();
> > +	return ret;
> > +}
> > +EXPORT_SYMBOL(smp_call_function);
> > -- 
> > 1.5.5.1.57.g5909c
> > 
--
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