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    SubjectRe: [PATCH 3/3 V2] pvclock: Get rid of __pvclock_read_cycles in function pvclock_read_flags
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    On 09/06/2016 14:16, Peter Zijlstra wrote:
    > 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 ;-)

    Only if you know that the writer will not write that byte twice within a
    critical section...

    Which I guess we do know in this case because the write side is just a
    memcpy, but it's still a bit safer when it's not specified by the
    pvclock API. It's not a fast path anyway, it runs literally twice at
    startup.

    Paolo

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