linux-kernel - Re: [PATCH 00/15] perf: KVM: Fix, optimize, and clean up callbacks

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Date:   Fri, 27 Aug 2021 12:47:25 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Like Xu <like.xu.linux@...il.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        James Morse <james.morse@....com>,
        Alexandru Elisei <alexandru.elisei@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Stefano Stabellini <sstabellini@...nel.org>,
        linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        linux-csky@...r.kernel.org, linux-riscv@...ts.infradead.org,
        kvm@...r.kernel.org, xen-devel@...ts.xenproject.org,
        Artem Kashkanov <artem.kashkanov@...el.com>,
        Zhu Lingshan <lingshan.zhu@...el.com>,
        Will Deacon <will@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Marc Zyngier <maz@...nel.org>, Guo Ren <guoren@...nel.org>,
        Nick Hu <nickhu@...estech.com>,
        Greentime Hu <green.hu@...il.com>,
        Vincent Chen <deanbo422@...il.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>, x86@...nel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Jason Baron <jbaron@...mai.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ard Biesheuvel <ardb@...nel.org>
Subject: Re: [PATCH 00/15] perf: KVM: Fix, optimize, and clean up callbacks

On Fri, Aug 27, 2021 at 04:01:45PM +0800, Like Xu wrote:
> On 27/8/2021 3:44 pm, Peter Zijlstra wrote:

> > You just have to make sure all static_call() invocations that started
> > before unreg are finished before continuing with the unload.
> > synchronize_rcu() can help with that.
> 
> Do you mean something like that:
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 64e310ff4f3a..e7d310af7509 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -8465,6 +8465,7 @@ void kvm_arch_exit(void)
>  #endif
>  	kvm_lapic_exit();
>  	perf_unregister_guest_info_callbacks(&kvm_guest_cbs);
> +	synchronize_rcu();
> 
>  	if (!boot_cpu_has(X86_FEATURE_CONSTANT_TSC))
>  		cpufreq_unregister_notifier(&kvmclock_cpufreq_notifier_block,
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index e466fc8176e1..63ae56c5d133 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -6508,6 +6508,7 @@ EXPORT_SYMBOL_GPL(perf_register_guest_info_callbacks);
>  int perf_unregister_guest_info_callbacks(struct perf_guest_info_callbacks *cbs)
>  {
>  	perf_guest_cbs = NULL;
> +	arch_perf_update_guest_cbs();

I'm thinking the synchronize_rcu() should go here, and access to
perf_guest_cbs should be wrapped to yell when called with preemption
enabled.

But yes..

>  	return 0;
>  }
>  EXPORT_SYMBOL_GPL(perf_unregister_guest_info_callbacks);
> 
> > 
> > This is module unload 101. Nothing specific to static_call().
> >