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Sender: Ingo Molnar <mingo.kernel.org@gmail.com>
Date:   Tue, 24 Oct 2023 18:51:47 +0200
From:   Ingo Molnar <mingo@kernel.org>
To:     Mario Limonciello <mario.limonciello@amd.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Borislav Petkov <bp@alien8.de>,
        Thomas Gleixner <tglx@linutronix.de>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Sandipan Das <sandipan.das@amd.com>,
        "H . Peter Anvin" <hpa@zytor.com>, linux-kernel@vger.kernel.org,
        x86@kernel.org, linux-pm@vger.kernel.org, rafael@kernel.org,
        pavel@ucw.cz, linux-perf-users@vger.kernel.org,
        Ingo Molnar <mingo@redhat.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Jiri Olsa <jolsa@kernel.org>,
        Namhyung Kim <namhyung@kernel.org>,
        Ian Rogers <irogers@google.com>,
        Adrian Hunter <adrian.hunter@intel.com>
Subject: Re: [PATCH 2/2] perf/x86/amd: Don't allow pre-emption in
 amd_pmu_lbr_reset()
Message-ID: <ZTf2IxAVPUFq91F4@gmail.com>
References: <20231023160018.164054-1-mario.limonciello@amd.com>
 <20231023160018.164054-3-mario.limonciello@amd.com>
 <ZTd6BYr17ycdHR2a@gmail.com>
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* Ingo Molnar <mingo@kernel.org> wrote:

> 
> * Mario Limonciello <mario.limonciello@amd.com> wrote:
> 
> > Fixes a BUG reported during suspend to ram testing.
> > 
> > ```
> > [  478.274752] BUG: using smp_processor_id() in preemptible [00000000] code: rtcwake/2948
> > [  478.274754] caller is amd_pmu_lbr_reset+0x19/0xc0
> > ```
> > 
> > Cc: stable@vger.kernel.org # 6.1+
> > Fixes: ca5b7c0d9621 ("perf/x86/amd/lbr: Add LbrExtV2 branch record support")
> > Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
> > ---
> >  arch/x86/events/amd/lbr.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/x86/events/amd/lbr.c b/arch/x86/events/amd/lbr.c
> > index eb31f850841a..5b98e8c7d8b7 100644
> > --- a/arch/x86/events/amd/lbr.c
> > +++ b/arch/x86/events/amd/lbr.c
> > @@ -321,7 +321,7 @@ int amd_pmu_lbr_hw_config(struct perf_event *event)
> >  
> >  void amd_pmu_lbr_reset(void)
> >  {
> > -	struct cpu_hw_events *cpuc = this_cpu_ptr(&cpu_hw_events);
> > +	struct cpu_hw_events *cpuc = get_cpu_ptr(&cpu_hw_events);
> >  	int i;
> >  
> >  	if (!x86_pmu.lbr_nr)
> > @@ -335,6 +335,7 @@ void amd_pmu_lbr_reset(void)
> >  
> >  	cpuc->last_task_ctx = NULL;
> >  	cpuc->last_log_id = 0;
> > +	put_cpu_ptr(&cpu_hw_events);
> >  	wrmsrl(MSR_AMD64_LBR_SELECT, 0);
> >  }
> 
> Weird, amd_pmu_lbr_reset() is called from these places:
> 
>   - amd_pmu_lbr_sched_task(): during task sched-in during 
>     context-switching, this should already have preemption disabled.
> 
>   - amd_pmu_lbr_add(): this gets indirectly called by amd_pmu::add 
>     (amd_pmu_add_event()), called by event_sched_in(), which too should have 
>     preemption disabled.
> 
> I clearly must have missed some additional place it gets called in.

Just for completeness, the additional place I missed is 
amd_pmu_cpu_reset():

                static_call(amd_pmu_branch_reset)();

... and the amd_pmu_branch_reset static call is set up with 
amd_pmu_lbr_reset, which is why git grep missed it.

Anyway, amd_pmu_cpu_reset() is very much something that should run 
non-preemptable to begin with, so your patch only papers over the real 
problem AFAICS.

Thanks,

	Ingo