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SubjectRe: [PATCH 1/3] PM / Wakeup: Add missing memory barriers
On Thu, 27 Jan 2011, Rafael J. Wysocki wrote:

> Updated patch is appended.
>
> Thanks,
> Rafael
>
> ---
> drivers/base/power/wakeup.c | 61 ++++++++++++++++++++++++++++----------------
> 1 file changed, 39 insertions(+), 22 deletions(-)
>
> Index: linux-2.6/drivers/base/power/wakeup.c
> ===================================================================
> --- linux-2.6.orig/drivers/base/power/wakeup.c
> +++ linux-2.6/drivers/base/power/wakeup.c
> @@ -24,12 +24,26 @@
> */
> bool events_check_enabled;
>
> -/* The counter of registered wakeup events. */
> -static atomic_t event_count = ATOMIC_INIT(0);
> -/* A preserved old value of event_count. */
> +/*
> + * Combined counters of registered wakeup events and wakeup events in progress.
> + * They need to be modified together atomically, so it's better to use one
> + * atomic variable to hold them both.
> + */
> +static atomic_t combined_event_count = ATOMIC_INIT(0);
> +
> +#define IN_PROGRESS_BITS (sizeof(int) * 4)
> +#define MAX_IN_PROGRESS ((1 << IN_PROGRESS_BITS) - 1)
> +
> +static void split_counters(unsigned int *cnt, unsigned int *inpr)
> +{
> + unsigned int comb = atomic_read(&combined_event_count);
> +
> + *cnt = (comb >> IN_PROGRESS_BITS);
> + *inpr = comb & MAX_IN_PROGRESS;
> +}
> +
> +/* A preserved old value of the events counter. */
> static unsigned int saved_count;
> -/* The counter of wakeup events being processed. */
> -static atomic_t events_in_progress = ATOMIC_INIT(0);
>
> static DEFINE_SPINLOCK(events_lock);
>
> @@ -333,7 +347,8 @@ static void wakeup_source_activate(struc
> ws->timer_expires = jiffies;
> ws->last_time = ktime_get();
>
> - atomic_inc(&events_in_progress);
> + /* Increment the counter of events in progress. */
> + atomic_inc(&combined_event_count);
> }
>
> /**
> @@ -420,14 +435,10 @@ static void wakeup_source_deactivate(str
> del_timer(&ws->timer);
>
> /*
> - * event_count has to be incremented before events_in_progress is
> - * modified, so that the callers of pm_check_wakeup_events() and
> - * pm_save_wakeup_count() don't see the old value of event_count and
> - * events_in_progress equal to zero at the same time.
> + * Increment the counter of registered wakeup events and decrement the
> + * couter of wakeup events in progress simultaneously.
> */
> - atomic_inc(&event_count);
> - smp_mb__before_atomic_dec();
> - atomic_dec(&events_in_progress);
> + atomic_add(MAX_IN_PROGRESS, &combined_event_count);
> }
>
> /**
> @@ -582,8 +593,10 @@ bool pm_wakeup_pending(void)
>
> spin_lock_irqsave(&events_lock, flags);
> if (events_check_enabled) {
> - ret = ((unsigned int)atomic_read(&event_count) != saved_count)
> - || atomic_read(&events_in_progress);
> + unsigned int cnt, inpr;
> +
> + split_counters(&cnt, &inpr);
> + ret = (cnt != saved_count || inpr > 0);
> events_check_enabled = !ret;
> }
> spin_unlock_irqrestore(&events_lock, flags);
> @@ -605,19 +618,22 @@ bool pm_wakeup_pending(void)
> */
> bool pm_get_wakeup_count(unsigned int *count)
> {
> - bool ret;
> + unsigned int cnt, inpr;
>
> if (capable(CAP_SYS_ADMIN))
> events_check_enabled = false;
>
> - while (atomic_read(&events_in_progress) && !signal_pending(current)) {
> + for (;;) {
> + split_counters(&cnt, &inpr);
> + if (inpr == 0 || signal_pending(current))
> + break;
> pm_wakeup_update_hit_counts();
> schedule_timeout_interruptible(msecs_to_jiffies(TIMEOUT));
> }
>
> - ret = !atomic_read(&events_in_progress);
> - *count = atomic_read(&event_count);
> - return ret;
> + split_counters(&cnt, &inpr);
> + *count = cnt;
> + return !inpr;
> }
>
> /**
> @@ -631,11 +647,12 @@ bool pm_get_wakeup_count(unsigned int *c
> */
> bool pm_save_wakeup_count(unsigned int count)
> {
> + unsigned int cnt, inpr;
> bool ret = false;
>
> spin_lock_irq(&events_lock);
> - if (count == (unsigned int)atomic_read(&event_count)
> - && !atomic_read(&events_in_progress)) {
> + split_counters(&cnt, &inpr);
> + if (cnt == count && inpr == 0) {
> saved_count = count;
> events_check_enabled = true;
> ret = true;

This looks okay. Sure, it's a little clunky, but it's better than the
alternatives.

Alan Stern



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