Messages in this thread | | | From | Namhyung Kim <> | Date | Mon, 13 Dec 2021 13:10:58 -0800 | Subject | Re: [PATCH v2] perf/core: Fix cgroup event list management |
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Hi Peter,
On Mon, Dec 13, 2021 at 12:30 PM Peter Zijlstra <peterz@infradead.org> wrote: > > On Sun, Dec 12, 2021 at 10:59:36PM -0800, Namhyung Kim wrote: > > The active cgroup events are managed in the per-cpu cgrp_cpuctx_list. > > This list is accessed from current cpu and not protected by any locks. > > But from the commit ef54c1a476ae ("perf: Rework > > perf_event_exit_event()"), this assumption does not hold true anymore. > > > > In the perf_remove_from_context(), it can remove an event from the > > context without an IPI when the context is not active. I think it > > "I tihnk" just doesn't cut it. That means I have to completely reverse > engineer your patch and it's assumptions. Which is more work for me :-(
If you are talking about my wording, ok I will use more assertive words with the facts in the future. I should say that it doesn't work with cgroup events which can change the active states during context switches.
> > > assumes task event context, but it's possible for cpu event context > > only with cgroup events can be inactive at the moment - and it might > > become active soon. > > > > If the event is enabled when it's about to be closed, it might call > > perf_cgroup_event_disable() and list_del() with the cgrp_cpuctx_list > > on a different cpu. > > > > This resulted in a crash due to an invalid list pointer access during > > the cgroup list traversal on the cpu which the event belongs to. > > > > The following program can crash my box easily.. > > Unless that's already public, you've just given the script kiddos ammo, > surely we don't need that.
Understood, will take more care in the future.
But this requires root access to create cgroups or the sysctl perf_event_paranoid of 0 to open cpu/cgroup events which is restricted in the most distro. Anyway, I should be careful, sorry about that.
> > > Let's use IPI to prevent such crashes. > > Let's just not do random things and hope stuff 'works'. Either it is > correct or it is not.
Right, but in this case, it's back to use IPI by removing the optimization for cgroup events. I'll update the description.
> > > Similarly, I think perf_install_in_context() should use IPI for the > > cgroup events too. > > Let's be sure, ok?
I see.
> > > Reported-by: kernel test robot <lkp@intel.com> # for build error > > That's complete garbage, please don't do that.
Got it.
> > > Cc: Marco Elver <elver@google.com> > > Signed-off-by: Namhyung Kim <namhyung@kernel.org> > > --- > > v2) simply use IPI for cgroup events > > > > kernel/events/core.c | 7 +++++-- > > 1 file changed, 5 insertions(+), 2 deletions(-) > > > > diff --git a/kernel/events/core.c b/kernel/events/core.c > > index 30d94f68c5bd..9460c083acd9 100644 > > --- a/kernel/events/core.c > > +++ b/kernel/events/core.c > > @@ -2388,7 +2388,7 @@ static void perf_remove_from_context(struct perf_event *event, unsigned long fla > > * event_function_call() user. > > */ > > raw_spin_lock_irq(&ctx->lock); > > - if (!ctx->is_active) { > > + if (!ctx->is_active && !is_cgroup_event(event)) { > > __perf_remove_from_context(event, __get_cpu_context(ctx), > > ctx, (void *)flags); > > raw_spin_unlock_irq(&ctx->lock); > > @@ -2857,11 +2857,14 @@ perf_install_in_context(struct perf_event_context *ctx, > > * perf_event_attr::disabled events will not run and can be initialized > > * without IPI. Except when this is the first event for the context, in > > * that case we need the magic of the IPI to set ctx->is_active. > > + * Similarly, cgroup events for the context also needs the IPI to > > + * manipulate the cgrp_cpuctx_list. > > * > > * The IOC_ENABLE that is sure to follow the creation of a disabled > > * event will issue the IPI and reprogram the hardware. > > */ > > - if (__perf_effective_state(event) == PERF_EVENT_STATE_OFF && ctx->nr_events) { > > + if (__perf_effective_state(event) == PERF_EVENT_STATE_OFF && > > + ctx->nr_events && !is_cgroup_event(event)) { > > raw_spin_lock_irq(&ctx->lock); > > if (ctx->task == TASK_TOMBSTONE) { > > raw_spin_unlock_irq(&ctx->lock); > > > > base-commit: 73743c3b092277febbf69b250ce8ebbca0525aa2 > > What's junk like that doing ?
It was recommended by the kernel test robot. I think it uses the info to find where this patch applies to.
Thanks, Namhyung
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