On Wed, 13 Feb 2019, Will Deacon wrote:

On Fri, Feb 08, 2019 at 08:30:25PM +0100, Thomas Gleixner wrote:

quoted

How is the in kernel ktime_get() correctness guaranteed then?

Luck.

I think we'll have to introduce a dummy dependent stack read into our
counter accessor so that it's ordered by the smp_rmb(). Example diff
below, which I'll roll into a proper patch series later on.

Yikes.

 static inline u64 arch_counter_get_cntpct(void)
 {
+	u64 cnt, tmp;
+
 	isb();
-	return arch_timer_reg_read_stable(cntpct_el0);
+	cnt = arch_timer_reg_read_stable(cntpct_el0);
+
+	/*
+	 * This insanity brought to you by speculative, out-of-order system
+	 * register reads, sequence locks and Thomas Gleixner.

ROTFL


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