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[35.185.214.157]) by smtp.gmail.com with ESMTPSA id z11sm4163558pff.144.2021.09.16.14.41.04 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 16 Sep 2021 14:41:04 -0700 (PDT) Date: Thu, 16 Sep 2021 21:41:01 +0000 From: Sean Christopherson To: Peter Zijlstra Cc: Ingo Molnar , Arnaldo Carvalho de Melo , Will Deacon , Mark Rutland , Catalin Marinas , Marc Zyngier , Guo Ren , Nick Hu , Greentime Hu , Vincent Chen , Paul Walmsley , Palmer Dabbelt , Albert Ou , Thomas Gleixner , Borislav Petkov , x86@kernel.org, Paolo Bonzini , Boris Ostrovsky , Juergen Gross , Alexander Shishkin , Jiri Olsa , Namhyung Kim , James Morse , Alexandru Elisei , Suzuki K Poulose , "H. Peter Anvin" , Vitaly Kuznetsov , Wanpeng Li , Jim Mattson , Joerg Roedel , Stefano Stabellini , linux-perf-users@vger.kernel.org, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, kvmarm@lists.cs.columbia.edu, linux-csky@vger.kernel.org, linux-riscv@lists.infradead.org, kvm@vger.kernel.org, xen-devel@lists.xenproject.org, Artem Kashkanov , Like Xu , Zhu Lingshan Subject: Re: [PATCH v2 01/13] perf: Ensure perf_guest_cbs aren't reloaded between !NULL check and deref Message-ID: References: <20210828003558.713983-1-seanjc@google.com> <20210828003558.713983-2-seanjc@google.com> <20210828194421.GB4353@worktop.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210828194421.GB4353@worktop.programming.kicks-ass.net> Precedence: bulk List-ID: X-Mailing-List: linux-csky@vger.kernel.org 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 :-)