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AJvYcCXnZ8ZyywrxuVj99DfWS07Cyh6RvviHoPeS3Dh2rWVDtU/Phty0Ep0FHDcbSJQTuidDlvMQouoBuKCva3hPmJcbMu8EAcgl X-Gm-Message-State: AOJu0YxmRonIx9mkx/753JOiptap9JSnSKm7KmUilg/S+EQzozEhzt3V 42jFXoWJoE4T3rayBNkj0GXbvqBoMTPWYjkewYfIznGBVe3vLuC7otJrg9tF1TWsWMOjvFgYP1o 8dA== X-Google-Smtp-Source: AGHT+IE1tZi1nsiH0SIgqwEaG7Vb1R0ZlJXHcVhuB6HgEl7rj1NR5a6/czPeOlKS3H+iSO3nwk8SFUENvrM= X-Received: from zagreus.c.googlers.com ([fda3:e722:ac3:cc00:7f:e700:c0a8:5c37]) (user=seanjc job=sendgmr) by 2002:a05:6a00:6f01:b0:70d:fba:c51c with SMTP id d2e1a72fcca58-7106d071bd2mr16631b3a.3.1722631891698; Fri, 02 Aug 2024 13:51:31 -0700 (PDT) Date: Fri, 2 Aug 2024 13:51:30 -0700 In-Reply-To: Precedence: bulk X-Mailing-List: kvmarm@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: 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 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.