Messages in this thread | | | Date | Mon, 2 Dec 2019 10:20:32 -0800 | From | "Luck, Tony" <> | Subject | Re: [PATCH v10 6/6] x86/split_lock: Enable split lock detection by kernel parameter |
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On Mon, Nov 25, 2019 at 08:13:48AM -0800, Sean Christopherson wrote: > On Fri, Nov 22, 2019 at 04:30:56PM -0800, Luck, Tony wrote: > > Don't you have some horrible races between the two logical > > processors on the same core as they both try to set/clear the > > MSR that is shared at the core level? > > Yes and no. Yes, there will be races, but they won't be fatal in any way. > > - Only the split-lock bit is supported by the kernel, so there isn't a > risk of corrupting other bits as both threads will rewrite the current > hardware value. > > - Toggling of split-lock is only done in "warn" mode. Worst case > scenario of a race is that a misbehaving task will generate multiple > #AC exceptions on the same instruction. And this race will only occur > if both siblings are running tasks that generate split-lock #ACs, e.g. > a race where sibling threads are writing different values will only > occur if CPUx is disabling split-lock after an #AC and CPUy is > re-enabling split-lock after *its* previous task generated an #AC. > > - Transitioning between modes at runtime isn't supported and disabling > is tracked per task, so hardware will always reach a steady state that > matches the configured mode. I.e. split-lock is guaranteed to be > enabled in hardware once all _TIF_SLD threads have been scheduled out.
We should probably include this analysis in the commit comment. Maybe a comment or two in the code too to note that the races are mostly harmless and guaranteed to end quickly.
-Tony
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