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charset="utf-8" Recently while revising RCU's cpu online checks, there was some discussion around how IPIs synchronize with hotplug. Add comments explaining how preemption disable creates mutual exclusion with CPU hotplug's stop_machine mechanism. The key insight is that stop_machine() atomically updates CPU masks and flushes IPIs with interrupts disabled, and cannot proceed while any CPU (including the IPI sender) has preemption disabled. Cc: Andrea Righi Cc: Paul E. McKenney Cc: Frederic Weisbecker Cc: rcu@vger.kernel.org Co-developed-by: Frederic Weisbecker Signed-off-by: Joel Fernandes Acked-by: Paul E. McKenney --- v1->v2: Reworded a bit more (minor nit). kernel/cpu.c | 4 ++++ kernel/smp.c | 12 ++++++++++++ 2 files changed, 16 insertions(+) diff --git a/kernel/cpu.c b/kernel/cpu.c index a59e009e0be4..a8ce1395dd2c 100644 --- a/kernel/cpu.c +++ b/kernel/cpu.c @@ -1310,6 +1310,10 @@ static int takedown_cpu(unsigned int cpu) =20 /* * So now all preempt/rcu users must observe !cpu_active(). + * + * stop_machine() waits for all CPUs to enable preemption. This lets + * take_cpu_down() atomically update CPU masks and flush last IPI + * before new IPIs can be attempted to be sent. */ err =3D stop_machine_cpuslocked(take_cpu_down, NULL, cpumask_of(cpu)); if (err) { diff --git a/kernel/smp.c b/kernel/smp.c index 974f3a3962e8..842691467f9e 100644 --- a/kernel/smp.c +++ b/kernel/smp.c @@ -93,6 +93,9 @@ int smpcfd_dying_cpu(unsigned int cpu) * explicitly (without waiting for the IPIs to arrive), to * ensure that the outgoing CPU doesn't go offline with work * still pending. + * + * This runs in stop_machine's atomic context with interrupts disabled, + * thus CPU offlining and IPI flushing happen together atomically. */ __flush_smp_call_function_queue(false); irq_work_run(); @@ -418,6 +421,10 @@ void __smp_call_single_queue(int cpu, struct llist_nod= e *node) */ static int generic_exec_single(int cpu, call_single_data_t *csd) { + /* + * Preemption must be disabled by caller to mutually exclude with + * stop_machine() atomically updating CPU masks and flushing IPIs. + */ if (cpu =3D=3D smp_processor_id()) { smp_call_func_t func =3D csd->func; void *info =3D csd->info; @@ -640,6 +647,11 @@ int smp_call_function_single(int cpu, smp_call_func_t = func, void *info, /* * prevent preemption and reschedule on another processor, * as well as CPU removal + * + * get_cpu() disables preemption, ensuring mutual exclusion with + * stop_machine() where CPU offlining and last IPI flushing happen + * atomically versus this code. This guarantees here that the cpu_online() + * check and IPI sending are safe without losing IPIs due to offlining. */ this_cpu =3D get_cpu(); =20 --=20 2.43.0