On Fri, Oct 04, 2013 at 09:56:23PM +0200, Oleg Nesterov wrote:
> Hmm. perhaps you meant that this should be done before rcu_sync_enter()
> increments ->gp_count. Perhaps this can work, but the code will be more
> complex and this way rcu_sync_exit() will always schedule the callback?
Yah, however see below.
> And again, we do want to increment ->gp_count asap to disable this cb
> if it is already pending.
Ah indeed, I forgot about that. We'd have to wait until we'd get
scheduled to increment gp_count again. However I think we can fix this
and the above problem by introduction of rcu_sync_busy() which checks
for eiter gp_count or pending waiters.
But yes, slightly more complex code :/
That would yield something like so I suppose:
void rcu_sync_enter(struct rcu_sync_struct *rss)
{
bool need_wait, need_sync;
spin_lock_irq(&rss->rss_lock);
if (rss->exclusive && rss->gp_count) {
__wait_event_locked(rss->gp_wait, rss->gp_count);
rss->gp_count++;
need_wait = need_sync = false;
} else {
need_wait = rss->gp_count++;
need_sync = rss->gp_state == GP_IDLE;
if (need_sync)
rss->gp_state = GP_PENDING;
}
spin_unlock_irq(&rss->lock);
if (need_sync) {
rss->sync();
rss->gp_state = GP_PASSED;
wake_up_all(&rss->gp_wait);
} else if (need_wait) {
wait_event(rss->gp_wait, rss->gp_state == GP_PASSED);
} else {
BUG_ON(rss->gp_state != GP_PASSED);
}
}
static bool rcu_sync_busy(struct rcu_sync_struct *rss)
{
return rss->gp_count ||
(rss->exclusive && waitqueue_active(&rss->gp_wait));
}
static void rcu_sync_func(struct rcu_head *rcu)
{
struct rcu_sync_struct *rss =
container_of(rcu, struct rcu_sync_struct, cb_head);
unsigned long flags;
BUG_ON(rss->gp_state != GP_PASSED);
BUG_ON(rss->cb_state == CB_IDLE);
spin_lock_irqsave(&rss->rss_lock, flags);
if (rcu_sync_busy(rss)) {
/*
* A new rcu_sync_begin() has happened; drop the callback.
*/
rss->cb_state = CB_IDLE;
} else if (rss->cb_state == CB_REPLAY) {
/*
* A new rcu_sync_exit() has happened; requeue the callback
* to catch a later GP.
*/
rss->cb_state = CB_PENDING;
rss->call(&rss->cb_head, rcu_sync_func);
} else {
/*
* We're at least a GP after rcu_sync_exit(); eveybody will now
* have observed the write side critical section. Let 'em rip!.
*/
rss->cb_state = CB_IDLE;
rss->gp_state = GP_IDLE;
}
spin_unlock_irqrestore(&rss->rss_lock, flags);
}
void rcu_sync_exit(struct rcu_sync_struct *rss)
{
spin_lock_irq(&rss->rss_lock);
if (!--rss->gp_count) {
if (!rcu_sync_busy(rss)) {
if (rss->cb_state == CB_IDLE) {
rss->cb_state = CB_PENDING;
rss->call(&rss->cb_head, rcu_sync_func);
} else if (rss->cb_state == CB_PENDING) {
rss->cb_state = CB_REPLAY;
}
} else {
__wake_up_locked(&rss->gp_wait, TASK_NORMAL, 1);
}
}
spin_unlock_irq(&rss->rss_lock);
}
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