Messages in this thread | | | Subject | Re: [PATCH v5 1/4] ftrace: Implement fs notification for tracing_max_latency | From | Viktor Rosendahl <> | Date | Wed, 4 Sep 2019 21:00:32 +0200 |
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On 9/4/19 1:39 PM, Steven Rostedt wrote: > On Tue, 3 Sep 2019 15:25:59 +0200 > Viktor Rosendahl <viktor.rosendahl@gmail.com> wrote: > >> +void latency_fsnotify_stop(void) >> +{ >> + /* Make sure all CPUs see caller's previous actions to stop tracer */ >> + smp_wmb(); > > These memory barriers just look wrong. What exactly are you trying to protect here? > > Where's the matching rmbs? >
Thanks for reviewing.
However, since these functions will disappear when I take the irq_work facility into use, we should perhaps not spend too much time discussing what would have been.
There are no matching rmbs, I was thinking that the smp_wmb() would merely enforce the order of the memory writes, as seen by other CPUs, so that the tracer would be stopped, before the latency fsnotify is disabled.
E.g. in case of the preemptirqsoff tracer the idea was that it doesn't matter exactly when a CPU sees the "tracer_enabled = 0;" in stop_irqsoff_tracer() but that it needs to be seen before the writes in latency_fsnotify_stop() are seen.
best regards,
Viktor
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