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smp: Document preemption and stop_machine() mutual exclusion

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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 <arighi@nvidia.com>
Cc: Paul E. McKenney <paulmck@kernel.org>
Cc: Frederic Weisbecker <frederic@kernel.org>
Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: rcu@vger.kernel.org
Acked-by: Paul E. McKenney <paulmck@kernel.org>
Co-developed-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
This commit is contained in:
Joel Fernandes 2025-07-15 16:01:51 -04:00 • committed by Paul E. McKenney
parent fc6f89dc70
commit cf4fc66746
1 changed files with 17 additions and 9 deletions
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26
kernel/smp.c
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@ -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();