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PM / Wakeup: Combine atomic counters to avoid reordering issues 

The memory barrier in wakeup_source_deactivate() is supposed to
prevent the callers of pm_wakeup_pending() and pm_get_wakeup_count()
from seeing the new value of events_in_progress (0, in particular)
and the old value of event_count at the same time.  However, if
wakeup_source_deactivate() is executed by CPU0 and, for instance,
pm_wakeup_pending() is executed by CPU1, where both processors can
reorder operations, the memory barrier in wakeup_source_deactivate()
doesn't affect CPU1 which can reorder reads.  In that case CPU1 may
very well decide to fetch event_count before it's modified and
events_in_progress after it's been updated, so pm_wakeup_pending()
may fail to detect a wakeup event.  This issue can be addressed by
using a single atomic variable to store both events_in_progress
and event_count, so that they can be updated together in a single
atomic operation.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
This commit is contained in:
Rafael J. Wysocki 2011-01-31 11:06:39 +01:00
parent dc1b83ab08
commit 023d377914
1 changed files with 39 additions and 22 deletions
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61
drivers/base/power/wakeup.c
View File

@ -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);
@ -307,7 +321,8 @@ static void wakeup_source_activate(struct wakeup_source *ws)
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);
}
/**
@ -394,14 +409,10 @@ static void wakeup_source_deactivate(struct wakeup_source *ws)
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);
}
/**
@ -556,8 +567,10 @@ 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);
@ -579,19 +592,22 @@ 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;
}
/**
@ -605,11 +621,12 @@ bool pm_get_wakeup_count(unsigned int *count)
*/
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;