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[35.185.214.157]) by smtp.gmail.com with ESMTPSA id c9sm4061004pgq.58.2021.11.04.07.18.30 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 04 Nov 2021 07:18:30 -0700 (PDT) Date: Thu, 4 Nov 2021 14:18:26 +0000 From: Sean Christopherson To: Like Xu Subject: Re: [PATCH v3 01/16] perf: Ensure perf_guest_cbs aren't reloaded between !NULL check and deref Message-ID: References: <20210922000533.713300-1-seanjc@google.com> <20210922000533.713300-2-seanjc@google.com> <77e3a76a-016b-8945-a1d5-aae4075e2147@gmail.com> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <77e3a76a-016b-8945-a1d5-aae4075e2147@gmail.com> Cc: Wanpeng Li , kvm@vger.kernel.org, Peter Zijlstra , Guo Ren , linux-riscv@lists.infradead.org, Vincent Chen , Will Deacon , kvmarm@lists.cs.columbia.edu, Stefano Stabellini , Jiri Olsa , Joerg Roedel , linux-csky@vger.kernel.org, Alexander Shishkin , Ingo Molnar , xen-devel@lists.xenproject.org, Albert Ou , Zhu Lingshan , Arnaldo Carvalho de Melo , Greentime Hu , Paul Walmsley , Namhyung Kim , Boris Ostrovsky , Artem Kashkanov , linux-arm-kernel@lists.infradead.org, Jim Mattson , Juergen Gross , Nick Hu , linux-kernel@vger.kernel.org, linux-perf-users@vger.kernel.org, Palmer Dabbelt , Marc Zyngier , Paolo Bonzini , Vitaly Kuznetsov X-BeenThere: kvmarm@lists.cs.columbia.edu X-Mailman-Version: 2.1.14 Precedence: list List-Id: Where KVM/ARM decisions are made List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Errors-To: kvmarm-bounces@lists.cs.columbia.edu Sender: kvmarm-bounces@lists.cs.columbia.edu On Thu, Nov 04, 2021, Like Xu wrote: > On 22/9/2021 8:05 am, Sean Christopherson wrote: > > diff --git a/kernel/events/core.c b/kernel/events/core.c > > index 464917096e73..80ff050a7b55 100644 > > --- a/kernel/events/core.c > > +++ b/kernel/events/core.c > > @@ -6491,14 +6491,21 @@ 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); > > So per Paolo's comment [1], does it help to use > smp_store_release(perf_guest_cbs, cbs) > or > rcu_assign_pointer(perf_guest_cbs, cbs) > here? Heh, if by "help" you mean "required to prevent bad things on weakly ordered architectures", then yes, it helps :-) If I'm interpeting Paolo's suggestion correctly, he's pointing out that oustanding stores to the function pointers in @cbs need to complete before assigning a non-NULL pointer to perf_guest_cbs, otherwise a perf event handler may see a valid pointer with half-baked callbacks. I think smp_store_release() with a comment would be appropriate, assuming my above interpretation is correct. > [1] https://lore.kernel.org/kvm/37afc465-c12f-01b9-f3b6-c2573e112d76@redhat.com/ > > > 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 -EINVAL; > > + > > + WRITE_ONCE(perf_guest_cbs, NULL); > > + synchronize_rcu(); > > return 0; > > } > > EXPORT_SYMBOL_GPL(perf_unregister_guest_info_callbacks); > > _______________________________________________ kvmarm mailing list kvmarm@lists.cs.columbia.edu https://lists.cs.columbia.edu/mailman/listinfo/kvmarm