Re: [PATCH 055/270] ARM: OMAP: counter: add locking toread_persistent_clock

[Ben Hutchings]

On Mon, 2012-11-26 at 14:55 -0200, Herton Ronaldo Krzesinski wrote:
> 3.5.7u1 -stable review patch.  If anyone has any objections, please let me know.
> 
> ------------------
> 
> From: Colin Cross <ccross@xxxxxxxxxxx>
> 
> commit 9d7d6e363b06934221b81a859d509844c97380df upstream.
> 
> read_persistent_clock uses a global variable, use a spinlock to
> ensure non-atomic updates to the variable don't overlap and cause
> time to move backwards.
> 
> Signed-off-by: Colin Cross <ccross@xxxxxxxxxxx>
> Signed-off-by: R Sricharan <r.sricharan@xxxxxx>
> Signed-off-by: Tony Lindgren <tony@xxxxxxxxxxx>
> Signed-off-by: Herton Ronaldo Krzesinski <herton.krzesinski@xxxxxxxxxxxxx>
[...]

This is also missing from 3.4.  I'm attaching the adjusted version for
3.2, which looks like it will work for 3.4.

Ben.

-- 
Ben Hutchings
Never attribute to conspiracy what can adequately be explained by stupidity.
From: Colin Cross <ccross@xxxxxxxxxxx>
Date: Mon, 8 Oct 2012 14:01:12 -0700
Subject: ARM: OMAP: counter: add locking to read_persistent_clock

commit 9d7d6e363b06934221b81a859d509844c97380df upstream.

read_persistent_clock uses a global variable, use a spinlock to
ensure non-atomic updates to the variable don't overlap and cause
time to move backwards.

Signed-off-by: Colin Cross <ccross@xxxxxxxxxxx>
Signed-off-by: R Sricharan <r.sricharan@xxxxxx>
Signed-off-by: Tony Lindgren <tony@xxxxxxxxxxx>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
 arch/arm/plat-omap/counter_32k.c |   21 ++++++++++++++-------
 1 file changed, 14 insertions(+), 7 deletions(-)

--- a/arch/arm/plat-omap/counter_32k.c
+++ b/arch/arm/plat-omap/counter_32k.c
@@ -82,22 +82,29 @@ static void notrace omap_update_sched_cl
  * nsecs and adds to a monotonically increasing timespec.
  */
 static struct timespec persistent_ts;
-static cycles_t cycles, last_cycles;
+static cycles_t cycles;
 static unsigned int persistent_mult, persistent_shift;
+static DEFINE_SPINLOCK(read_persistent_clock_lock);
+
 void read_persistent_clock(struct timespec *ts)
 {
 	unsigned long long nsecs;
-	cycles_t delta;
-	struct timespec *tsp = &persistent_ts;
+	cycles_t last_cycles;
+	unsigned long flags;
+
+	spin_lock_irqsave(&read_persistent_clock_lock, flags);
 
 	last_cycles = cycles;
 	cycles = timer_32k_base ? __raw_readl(timer_32k_base) : 0;
-	delta = cycles - last_cycles;
 
-	nsecs = clocksource_cyc2ns(delta, persistent_mult, persistent_shift);
+	nsecs = clocksource_cyc2ns(cycles - last_cycles,
+					persistent_mult, persistent_shift);
+
+	timespec_add_ns(&persistent_ts, nsecs);
+
+	*ts = persistent_ts;
 
-	timespec_add_ns(tsp, nsecs);
-	*ts = *tsp;
+	spin_unlock_irqrestore(&read_persistent_clock_lock, flags);
 }
 
 int __init omap_init_clocksource_32k(void)

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