linux-kernel - Re: [PATCH] perf/core: fix perf_proc_update

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Message-ID: <20190117220924.GN6118@tassilo.jf.intel.com>
Date:   Thu, 17 Jan 2019 14:09:24 -0800
From:   Andi Kleen <ak@...ux.intel.com>
To:     Stephane Eranian <eranian@...gle.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>, mingo@...e.hu,
        "Liang, Kan" <kan.liang@...el.com>, Jiri Olsa <jolsa@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...hat.com>
Subject: Re: [PATCH] perf/core: fix perf_proc_update_handler() bug

On Thu, Jan 17, 2019 at 01:03:20PM -0800, Stephane Eranian wrote:
> On Thu, Jan 10, 2019 at 5:17 PM Stephane Eranian <eranian@...gle.com> wrote:
> >
> > The perf_proc_update_handler() handles /proc/sys/kernel/perf_event_max_sample_rate
> > syctl variable.  When the PMU IRQ handler timing monitoring is disabled, i.e,
> > when /proc/sys/kernel/perf_cpu_time_max_percent is equal to 0 or 100,
> > then no modification to sysctl_perf_event_sample_rate is allowed to prevent
> > possible hang from wrong values.
> >
> > The problem is that the test to prevent modification is made after the
> > sysctl variable is modified in perf_proc_update_handler().
> >
> > You get an error:
> >
> > $ echo 10001 >/proc/sys/kernel/perf_event_max_sample_rate
> > echo: write error: invalid argument
> >
> > But the value is still modified causing all sorts of inconsistencies:
> >
> > $ cat /proc/sys/kernel/perf_event_max_sample_rate
> > 10001
> >
> > This patch fixes the problem by moving the parsing of the value after
> > the test.
> >
> > Signed-off-by: Stephane Eranian <eranian@...gle.com>
> 
> Ping.

Reviewed-by: Andi Kleen <ak@...ux.intel.com>

-Andi