On Sat, May 28, 2016 at 08:27:43PM +0800, Minfei Huang wrote:
> +++ b/arch/x86/kernel/pvclock.c
> @@ -61,11 +61,14 @@ void pvclock_resume(void)
> u8 pvclock_read_flags(struct pvclock_vcpu_time_info *src)
> {
> unsigned version;
> - cycle_t ret;
> u8 flags;
>
> do {
> - version = __pvclock_read_cycles(src, &ret, &flags);
> + version = src->version;
> + /* Make the latest version visible */
> + smp_rmb();
> +
> + flags = src->flags;
Using a seqcount to load a single byte is insane ;-)
> /* Make sure that the version double-check is last. */
> smp_rmb();
> } while ((src->version & 1) || version != src->version);
What's wrong with:
u8 flags = READ_ONCE(src->flags);
?
(and have the flags store be done using WRITE_ONCE() of course).
Sure, if your total state is larger than one word you need the
seqcount for integrity, but reading _one_ byte, shees.
