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authorRafael J. Wysocki <rjw@sisk.pl>2011-01-31 11:06:39 +0100
committerRafael J. Wysocki <rjw@sisk.pl>2011-03-15 00:43:13 +0100
commit023d3779145ec6b7a0f38f19672a347b92feb74e (patch)
tree1ec78dc731dad2b8155faf6625617284f1b3b76d /drivers
parentdc1b83ab08f1954335692cdcd499f78c94f4c42a (diff)
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>
Diffstat (limited to 'drivers')
-rw-r--r--drivers/base/power/wakeup.c61
1 files changed, 39 insertions, 22 deletions
diff --git a/drivers/base/power/wakeup.c b/drivers/base/power/wakeup.c
index 8ec406d8f54..e5e73b5efc8 100644
--- a/drivers/base/power/wakeup.c
+++ b/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);
@@ -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;