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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 Cc: Peter Zijlstra , Will Deacon , Paolo Bonzini , Alexander Shishkin , Jiri Olsa , Namhyung Kim , James Morse , Alexandru Elisei , Suzuki K Poulose , Vitaly Kuznetsov , Wanpeng Li , Jim Mattson , Joerg Roedel , Stefano Stabellini , linux-arm-kernel@lists.infradead.org, linux-perf-users@vger.kernel.org, linux-kernel@vger.kernel.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 , Zhu Lingshan , Juergen Gross , Ingo Molnar , Albert Ou , Palmer Dabbelt , Vincent Chen , Paul Walmsley , Greentime Hu , Boris Ostrovsky , Marc Zyngier , Nick Hu , Guo Ren , Mark Rutland , Arnaldo Carvalho de Melo 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-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <77e3a76a-016b-8945-a1d5-aae4075e2147@gmail.com> Precedence: bulk List-ID: X-Mailing-List: linux-perf-users@vger.kernel.org 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); > >