linux-kernel - Re: [PATCH v2 01/13] perf: Ensure perf_guest_cbs aren't reloaded between !NULL check and deref

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Message-ID: <YUO57TlEGlUk2Q03@google.com>
Date:   Thu, 16 Sep 2021 21:41:01 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Will Deacon <will@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        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>,
        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>,
        Like Xu <like.xu.linux@...il.com>,
        Zhu Lingshan <lingshan.zhu@...el.com>
Subject: Re: [PATCH v2 01/13] perf: Ensure perf_guest_cbs aren't reloaded
 between !NULL check and deref

On Sat, Aug 28, 2021, Peter Zijlstra wrote:
> On Fri, Aug 27, 2021 at 05:35:46PM -0700, Sean Christopherson wrote:
> > diff --git a/kernel/events/core.c b/kernel/events/core.c
> > index 464917096e73..2126f6327321 100644
> > --- a/kernel/events/core.c
> > +++ b/kernel/events/core.c
> > @@ -6491,14 +6491,19 @@ struct perf_guest_info_callbacks *perf_guest_cbs;
> >  
> >  int perf_register_guest_info_callbacks(struct perf_guest_info_callbacks *cbs)
> >  {
> > -	perf_guest_cbs = cbs;
> > +	if (WARN_ON_ONCE(perf_guest_cbs))
> > +		return -EBUSY;
> > +
> > +	WRITE_ONCE(perf_guest_cbs, cbs);
> > +	synchronize_rcu();
> 
> You're waiting for all NULL users to go away? :-) IOW, we can do without
> this synchronize_rcu() call.

Doh, right.  I was thinking KVM needed to wait for in-progress NMI to exit to
ensure guest PT interrupts are handled correctly, but obviously the NMI handler
needs to exit for that CPU to get into a guest...

> >  	return 0;
> >  }
> >  EXPORT_SYMBOL_GPL(perf_register_guest_info_callbacks);
> >  
> >  int perf_unregister_guest_info_callbacks(struct perf_guest_info_callbacks *cbs)
> >  {
> > -	perf_guest_cbs = NULL;
> 
> 	if (WARN_ON_ONCE(perf_guest_cbs != cbs))
> 		return -EBUSY;
> 
> ?

Works for me.  I guess I'm more optimistic about people not being morons :-)