Messages in this thread | | | Date | Tue, 26 May 2015 04:46:07 -0700 | Subject | Re: [PATCH v2 01/11] perf,x86: Fix event/group validation | From | Stephane Eranian <> |
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On Tue, May 26, 2015 at 3:12 AM, Peter Zijlstra <peterz@infradead.org> wrote: > On Tue, May 26, 2015 at 02:24:38AM -0700, Stephane Eranian wrote: >> On Fri, May 22, 2015 at 6:40 AM, Peter Zijlstra <peterz@infradead.org> wrote: >> > On Fri, May 22, 2015 at 03:29:06PM +0200, Peter Zijlstra wrote: >> >> @@ -788,9 +788,9 @@ int x86_schedule_events(struct cpu_hw_ev >> >> x86_pmu.start_scheduling(cpuc); >> >> >> >> for (i = 0, wmin = X86_PMC_IDX_MAX, wmax = 0; i < n; i++) { >> >> + cpuc->event_constraint[i] = NULL; >> > >> > ^^^ that is new, which is esp. important in light of the >> > intel_get_event_constraints() hunk below, which would happily continue >> > life with a garbage constraint. >> > >> You've moved the constraint list from event to cpuc. Yet, it is still >> an array of pointers >> to constraints. So here you are saying, that in the case validate_group() is >> preempted and there is a context switch, there is still a risk of >> overwriting the >> constraint? I don't see how because validate_group() is using a fake_cpuc. >> So yes, the cpuc->event_constraint[] array is modified but it is not the same >> as the actual cpuc used by non-validate code. Or am I still missing something? >> >> When using dynamic constraints, we already have constraint storage in cpuc >> (to avoid calling kmalloc() in ctxsw context). Thus, I am wondering if it would >> not be easier to always use cpuc for constraint storage (no more pointers). > > No; the problem here is repeated use of the cpuc (the real one). Say one > scheduling run installs a constraint pointer for event i. Then event i > gets removed and another installed in the same spot. > > Then the next scheduling run will pick up the old pointer in > intel_get_event_constraints() as a base for the new one. > But where is the code that says: skip reinstalling the constraint in intel_get_event_constraints() because there is already a (stale) one? I don't see where that is.
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