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Date: Thu, 27 Jan 2011 22:19:54 +0100
From: "Rafael J. Wysocki" <rjw@...ell.com>
To: Alan Stern <stern@...land.harvard.edu>
Cc: "Linux-pm mailing list" <linux-pm@...ts.linux-foundation.org>,
LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/3] PM / Wakeup: Add missing memory barriers
On Thursday, January 27, 2011, Alan Stern wrote:
> On Wed, 26 Jan 2011, Rafael J. Wysocki wrote:
>
> > > Ideally you could do away with the need for synchronization entirely.
> > > For example, events_in_progress and event_count could be stored as two
> > > 16-bit values stuffed into a single atomic variable. Then they could
> > > both be read or updated simultaneously.
> >
> > OK, the patch below appears to work for me. Can you have a look at it, please?
> >
> > Rafael
> >
> >
> > ---
> > drivers/base/power/wakeup.c | 82 +++++++++++++++++++++++++++++++-------------
> > 1 file changed, 58 insertions(+), 24 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 +24,48 @@
> > */
> > 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. */
> > +#define EVENT_COUNT_BITS (sizeof(atomic_t) * 4)
>
> This should be sizeof(int), since atomic_t variables store int values.
> In principle, the atomic_t might include other things along with the
> stored value (it used to, on some architectures).
OK
> > +#define MAX_EVENT_COUNT ((1 << EVENT_COUNT_BITS) - 1)
> > +
> > +/* Combined counters of registered wakeup events and events in progress. */
> > +static atomic_t combined_event_count = ATOMIC_INIT(0);
> > +
>
> Comment here, explaining that this is needed so that the in_progress
> and count parts can be operated on simultaneously.
OK
> > +static unsigned int split_counters(unsigned int *inpr, unsigned int *cnt)
> > +{
> > + unsigned int comb = atomic_read(&combined_event_count);
> > +
> > + *inpr = (comb >> EVENT_COUNT_BITS);
> > + *cnt = comb & MAX_EVENT_COUNT;
>
> The inpr part is bounded, whereas the cnt part increments without
> limit. Therefore inpr should occupy the lower bits and cnt should
> occupy the upper bits, where overflow isn't an issue.
Well, it wouldn't matter if merge_counters() weren't buggy (cnt should have
been masked before applying the | in there). Plus the simplifications below
require inpr to be the lower bits, right?
> > + return comb;
> > +}
> > +
> > +static unsigned int merge_counters(unsigned int inpr, unsigned int cnt)
> > +{
> > + return (inpr << EVENT_COUNT_BITS) | cnt;
> > +}
> > +
> > +static void update_events_in_progress(void)
> > +{
> > + unsigned int cnt, inpr, old, new;
> > +
> > + do {
> > + old = split_counters(&inpr, &cnt);
> > + new = merge_counters(inpr + 1, cnt);
> > + } while (atomic_cmpxchg(&combined_event_count, old, new) != old);
> > +}
>
> Just atomic_inc(&combined_event_count) -- after inpr has been moved to
> the lower bits.
OK
> > +
> > +static void update_counters(void)
> > +{
> > + unsigned int cnt, inpr, old, new;
> > +
> > + do {
> > + old = split_counters(&inpr, &cnt);
> > + new = merge_counters(inpr - 1, cnt + 1);
> > + } while (atomic_cmpxchg(&combined_event_count, old, new) != old);
> > +}
>
> Just atomic_add(MAX_EVENT_COUNT, &combined_event_count).
Right.
> The rest looks fine.
OK
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;
--
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