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kernel: events: fix walking the waitq race condition

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Fixes race condition for k_event_post_internal() in an
SMP environment while walking the waitq. Uses z_sched_waitq_walk()
to safely walk the waitq by using a sched_spinlock.
It should be noted that since walking the wait queue is an
operation of indeterminant length, there exists the possibility
that the sched_spinlock (which is a highly used and contended-for
lock) may be locked for an indeterminant amount of time. However,
it is expected that few threads will be waiting on any given kernel
event object, which should ameliorate this risk.

Fixes #54317

Signed-off-by: Aastha Grover <aastha.grover@intel.com>
This commit is contained in:
Aastha Grover 2023-03-01 14:27:33 -05:00 committed by Carles Cufí
parent cf20ff0765
commit a2dccf1283
1 changed files with 32 additions and 19 deletions
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51
kernel/events.c
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@ -39,6 +39,11 @@
#define K_EVENT_WAIT_RESET 0x02 /* Reset events prior to waiting */
struct event_walk_data {
struct k_thread *head;
uint32_t events;
};
void z_impl_k_event_init(struct k_event *event)
{
event->events = 0;
@ -84,14 +89,35 @@ static bool are_wait_conditions_met(uint32_t desired, uint32_t current,
return match != 0;
}
static int event_walk_op(struct k_thread *thread, void *data)
{
unsigned int wait_condition;
struct event_walk_data *event_data = data;
wait_condition = thread->event_options & K_EVENT_WAIT_MASK;
if (are_wait_conditions_met(thread->events, event_data->events,
wait_condition)) {
/*
* The wait conditions have been satisfied. Add this
* thread to the list of threads to unpend.
*/
thread->next_event_link = event_data->head;
event_data->head = thread;
}
return 0;
}
static void k_event_post_internal(struct k_event *event, uint32_t events,
uint32_t events_mask)
{
k_spinlock_key_t key;
struct k_thread *thread;
unsigned int wait_condition;
struct k_thread *head = NULL;
struct event_walk_data data;
data.head = NULL;
key = k_spin_lock(&event->lock);
SYS_PORT_TRACING_OBJ_FUNC_ENTER(k_event, post, event, events,
@ -100,7 +126,7 @@ static void k_event_post_internal(struct k_event *event, uint32_t events,
events = (event->events & ~events_mask) |
(events & events_mask);
event->events = events;
data.events = events;
/*
* Posting an event has the potential to wake multiple pended threads.
* It is desirable to unpend all affected threads simultaneously. To
@ -112,23 +138,10 @@ static void k_event_post_internal(struct k_event *event, uint32_t events,
* 3. Ready each of the threads in the linked list
*/
_WAIT_Q_FOR_EACH(&event->wait_q, thread) {
wait_condition = thread->event_options & K_EVENT_WAIT_MASK;
z_sched_waitq_walk(&event->wait_q, event_walk_op, &data);
if (are_wait_conditions_met(thread->events, events,
wait_condition)) {
/*
* The wait conditions have been satisfied. Add this
* thread to the list of threads to unpend.
*/
thread->next_event_link = head;
head = thread;
}
}
if (head != NULL) {
thread = head;
if (data.head != NULL) {
thread = data.head;
do {
z_unpend_thread(thread);
arch_thread_return_value_set(thread, 0);