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Fri, 02 Aug 2024 13:51:31 -0700 (PDT) Date: Fri, 2 Aug 2024 13:51:30 -0700 In-Reply-To: Mime-Version: 1.0 References: <20240802200136.329973-1-seanjc@google.com> <20240802200136.329973-3-seanjc@google.com> Message-ID: Subject: Re: [PATCH 2/2] KVM: Protect vCPU's "last run PID" with rwlock, not RCU From: Sean Christopherson To: Steve Rutherford Cc: Marc Zyngier , Oliver Upton , Paolo Bonzini , linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev, kvm@vger.kernel.org, linux-kernel@vger.kernel.org Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20240802_135133_305124_40DD7862 X-CRM114-Status: GOOD ( 12.45 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Fri, Aug 02, 2024, Steve Rutherford wrote: > On Fri, Aug 2, 2024 at 1:01=E2=80=AFPM Sean Christopherson wrote: > > @@ -4178,9 +4181,9 @@ static int vcpu_get_pid(void *data, u64 *val) > > { > > struct kvm_vcpu *vcpu =3D data; > > > > - rcu_read_lock(); > > - *val =3D pid_nr(rcu_dereference(vcpu->pid)); > > - rcu_read_unlock(); > > + read_lock(&vcpu->pid_lock); > > + *val =3D pid_nr(vcpu->pid); > > + read_unlock(&vcpu->pid_lock); > > return 0; > > } > > > > @@ -4466,7 +4469,7 @@ static long kvm_vcpu_ioctl(struct file *filp, > > r =3D -EINVAL; > > if (arg) > > goto out; > > - oldpid =3D rcu_access_pointer(vcpu->pid); > > + oldpid =3D vcpu->pid; >=20 > Overall this patch looks correct, but this spot took me a moment, and > I want to confirm. This skips the reader lock since writing only > happens just below, under the vcpu lock, and we've already taken that > lock? Yep, exactly.