diff options
| -rw-r--r-- | include/linux/stop_machine.h | 64 | ||||
| -rw-r--r-- | kernel/smp.c | 26 |
2 files changed, 58 insertions, 32 deletions
diff --git a/include/linux/stop_machine.h b/include/linux/stop_machine.h index 3132262a404d..72820503514c 100644 --- a/include/linux/stop_machine.h +++ b/include/linux/stop_machine.h @@ -88,55 +88,73 @@ static inline void print_stop_info(const char *log_lvl, struct task_struct *task #endif /* CONFIG_SMP */ /* - * stop_machine "Bogolock": stop the entire machine, disable - * interrupts. This is a very heavy lock, which is equivalent to - * grabbing every spinlock (and more). So the "read" side to such a - * lock is anything which disables preemption. + * stop_machine "Bogolock": stop the entire machine, disable interrupts. + * This is a very heavy lock, which is equivalent to grabbing every raw + * spinlock (and more). So the "read" side to such a lock is anything + * which disables preemption. */ #if defined(CONFIG_SMP) || defined(CONFIG_HOTPLUG_CPU) /** * stop_machine: freeze the machine on all CPUs and run this function * @fn: the function to run - * @data: the data ptr for the @fn() - * @cpus: the cpus to run the @fn() on (NULL = any online cpu) + * @data: the data ptr to pass to @fn() + * @cpus: the cpus to run @fn() on (NULL = run on each online CPU) * - * Description: This causes a thread to be scheduled on every cpu, - * each of which disables interrupts. The result is that no one is - * holding a spinlock or inside any other preempt-disabled region when - * @fn() runs. + * Description: This causes a thread to be scheduled on every CPU, which + * will run with interrupts disabled. Each CPU specified by @cpus will + * run @fn. While @fn is executing, there will no other CPUs holding + * a raw spinlock or running within any other type of preempt-disabled + * region of code. * - * This can be thought of as a very heavy write lock, equivalent to - * grabbing every spinlock in the kernel. + * When @cpus specifies only a single CPU, this can be thought of as + * a reader-writer lock where readers disable preemption (for example, + * by holding a raw spinlock) and where the insanely heavy writers run + * @fn while also preventing any other CPU from doing any useful work. + * These writers can also be thought of as having implicitly grabbed every + * raw spinlock in the kernel. * - * Protects against CPU hotplug. + * When @fn is a no-op, this can be thought of as an RCU implementation + * where readers again disable preemption and writers use stop_machine() + * in place of synchronize_rcu(), albeit with orders of magnitude more + * disruption than even that of synchronize_rcu_expedited(). + * + * Although only one stop_machine() operation can proceed at a time, + * the possibility of blocking in cpus_read_lock() means that the caller + * cannot usefully rely on this serialization. + * + * Return: 0 if all invocations of @fn return zero. Otherwise, the + * value returned by an arbitrarily chosen member of the set of calls to + * @fn that returned non-zero. */ int stop_machine(cpu_stop_fn_t fn, void *data, const struct cpumask *cpus); /** * stop_machine_cpuslocked: freeze the machine on all CPUs and run this function * @fn: the function to run - * @data: the data ptr for the @fn() - * @cpus: the cpus to run the @fn() on (NULL = any online cpu) + * @data: the data ptr to pass to @fn() + * @cpus: the cpus to run @fn() on (NULL = run on each online CPU) + * + * Same as above. Avoids nested calls to cpus_read_lock(). * - * Same as above. Must be called from with in a cpus_read_lock() protected - * region. Avoids nested calls to cpus_read_lock(). + * Context: Must be called from within a cpus_read_lock() protected region. */ int stop_machine_cpuslocked(cpu_stop_fn_t fn, void *data, const struct cpumask *cpus); /** * stop_core_cpuslocked: - stop all threads on just one core * @cpu: any cpu in the targeted core - * @fn: the function to run - * @data: the data ptr for @fn() + * @fn: the function to run on each CPU in the core containing @cpu + * @data: the data ptr to pass to @fn() * - * Same as above, but instead of every CPU, only the logical CPUs of a - * single core are affected. + * Same as above, but instead of every CPU, only the logical CPUs of the + * single core containing @cpu are affected. * * Context: Must be called from within a cpus_read_lock() protected region. * - * Return: 0 if all executions of @fn returned 0, any non zero return - * value if any returned non zero. + * Return: 0 if all invocations of @fn return zero. Otherwise, the + * value returned by an arbitrarily chosen member of the set of calls to + * @fn that returned non-zero. */ int stop_core_cpuslocked(unsigned int cpu, cpu_stop_fn_t fn, void *data); diff --git a/kernel/smp.c b/kernel/smp.c index c5e1da7a88da..4649fa4872ff 100644 --- a/kernel/smp.c +++ b/kernel/smp.c @@ -86,13 +86,15 @@ int smpcfd_dead_cpu(unsigned int cpu) int smpcfd_dying_cpu(unsigned int cpu) { /* - * The IPIs for the smp-call-function callbacks queued by other - * CPUs might arrive late, either due to hardware latencies or - * because this CPU disabled interrupts (inside stop-machine) - * before the IPIs were sent. So flush out any pending callbacks - * explicitly (without waiting for the IPIs to arrive), to - * ensure that the outgoing CPU doesn't go offline with work - * still pending. + * The IPIs for the smp-call-function callbacks queued by other CPUs + * might arrive late, either due to hardware latencies or because this + * CPU disabled interrupts (inside stop-machine) before the IPIs were + * sent. So flush out any pending callbacks explicitly (without waiting + * for the IPIs to arrive), to ensure that the outgoing CPU doesn't go + * offline with work still pending. + * + * This runs with interrupts disabled inside the stopper task invoked by + * stop_machine(), ensuring mutually exclusive CPU offlining and IPI flush. */ __flush_smp_call_function_queue(false); irq_work_run(); @@ -418,6 +420,10 @@ void __smp_call_single_queue(int cpu, struct llist_node *node) */ static int generic_exec_single(int cpu, call_single_data_t *csd) { + /* + * Preemption already disabled here so stopper cannot run on this CPU, + * ensuring mutually exclusive CPU offlining and last IPI flush. + */ if (cpu == smp_processor_id()) { smp_call_func_t func = csd->func; void *info = csd->info; @@ -638,8 +644,10 @@ int smp_call_function_single(int cpu, smp_call_func_t func, void *info, int err; /* - * prevent preemption and reschedule on another processor, - * as well as CPU removal + * Prevent preemption and reschedule on another CPU, as well as CPU + * removal. This prevents stopper from running on this CPU, thus + * providing mutual exclusion of the below cpu_online() check and + * IPI sending ensuring IPI are not missed by CPU going offline. */ this_cpu = get_cpu(); |