Messages in this thread | | | Date | Thu, 9 Jun 2016 14:35:57 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH 3/3 V2] pvclock: Get rid of __pvclock_read_cycles in function pvclock_read_flags |
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On Thu, Jun 09, 2016 at 02:26:59PM +0200, Paolo Bonzini wrote: > > > 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.
Fair enough; just thought it was really silly code.
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