Commit-ID: 4b07372a32c0c1505a7634ad7e607d83340ef645 Gitweb: http://git.kernel.org/tip/4b07372a32c0c1505a7634ad7e607d83340ef645 Author: Andy Lutomirski <luto@xxxxxxxxxx> AuthorDate: Thu, 16 Mar 2017 12:59:40 -0700 Committer: Ingo Molnar <mingo@xxxxxxxxxx> CommitDate: Fri, 17 Mar 2017 08:28:26 +0100 x86/perf: Clarify why x86_pmu_event_mapped() isn't racy Naively, it looks racy, but ->mmap_sem saves it. Add a comment and a lockdep assertion. Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx> Cc: Alexander Shishkin <alexander.shishkin@xxxxxxxxxxxxxxx> Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx> Cc: Borislav Petkov <bpetkov@xxxxxxx> Cc: H. Peter Anvin <hpa@xxxxxxxxx> Cc: Jiri Olsa <jolsa@xxxxxxxxxx> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx> Cc: Stephane Eranian <eranian@xxxxxxxxxx> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx> Cc: Vince Weaver <vincent.weaver@xxxxxxxxx> Link: http://lkml.kernel.org/r/03a1e629063899168dfc4707f3bb6e581e21f5c6.1489694270.git.luto@xxxxxxxxxx Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx> --- arch/x86/events/core.c | 12 ++++++++++++ 1 file changed, 12 insertions(+) diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c index e07b36c..183a972 100644 --- a/arch/x86/events/core.c +++ b/arch/x86/events/core.c @@ -2109,6 +2109,18 @@ static void x86_pmu_event_mapped(struct perf_event *event) if (!(event->hw.flags & PERF_X86_EVENT_RDPMC_ALLOWED)) return; + /* + * This function relies on not being called concurrently in two + * tasks in the same mm. Otherwise one task could observe + * perf_rdpmc_allowed > 1 and return all the way back to + * userspace with CR4.PCE clear while another task is still + * doing on_each_cpu_mask() to propagate CR4.PCE. + * + * For now, this can't happen because all callers hold mmap_sem + * for write. If this changes, we'll need a different solution. + */ + lockdep_assert_held_exclusive(¤t->mm->mmap_sem); + if (atomic_inc_return(¤t->mm->context.perf_rdpmc_allowed) == 1) on_each_cpu_mask(mm_cpumask(current->mm), refresh_pce, NULL, 1); } -- To unsubscribe from this list: send the line "unsubscribe linux-tip-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
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