smp: Document preemption and stop_machine() mutual exclusion

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.

[ Apply peterz feedback. ]

Cc: Andrea Righi <[email protected]>
Cc: Paul E. McKenney <[email protected]>
Cc: Frederic Weisbecker <[email protected]>
Cc: Peter Zijlstra (Intel) <[email protected]>
Cc: [email protected]
Acked-by: Paul E. McKenney <[email protected]>
Co-developed-by: Frederic Weisbecker <[email protected]>
Signed-off-by: Joel Fernandes <[email protected]>
Signed-off-by: Paul E. McKenney <[email protected]>

Author: Joel Fernandes

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();