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
quoted text > > Acked-by: Ingo Molnar <mingo@elte.hu>
> > Signed-off-by: Jens Axboe <jens.axboe@oracle.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@oracle.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
> >
