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[87.16.204.231]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-48fe537c788sm298640545e9.12.2026.05.19.02.36.11 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 19 May 2026 02:36:12 -0700 (PDT) Date: Tue, 19 May 2026 11:36:04 +0200 From: Stefano Garzarella To: ziyu zhang Cc: "David S . Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Simon Horman , Andy King , George Zhang , Dmitry Torokhov , virtualization@lists.linux.dev, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, baijiaju1990@gmail.com, r33s3n6@gmail.com, gality369@gmail.com, zhenghaoran154@gmail.com, hanguidong02@gmail.com, zzzccc427@gmail.com Subject: Re: [PATCH net] vsock: keep poll shutdown state consistent Message-ID: References: <20260516034745.260442-1-ziyuzhang201@gmail.com> Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii; format=flowed Content-Disposition: inline In-Reply-To: On Mon, May 18, 2026 at 08:39:37PM +0800, ziyu zhang wrote: >On Mon, 18 May 2026 at 18:16, Stefano Garzarella wrote: >> >> On Sat, May 16, 2026 at 11:47:45AM +0800, Ziyu Zhang wrote: >> >vsock_poll() reads vsk->peer_shutdown before taking the socket >> >lock to set EPOLLHUP and EPOLLRDHUP, then reads it again under the >> >lock to report EOF readability. A shutdown packet can update >> >peer_shutdown while poll is waiting for the lock, so one poll invocation >> >can report EPOLLIN without the corresponding HUP/RDHUP bits. >> > >> >Keep non-connectible sockets on a single lockless READ_ONCE() >> >> Should this be paired with WRITE_ONCE() on writers? > >Yes, since the poll path uses lockless READ_ONCE() snapshots of >peer_shutdown, the writer side should be annotated with WRITE_ONCE() as >well. I will add that in v2. > >> >> >snapshot. For connectible sockets, defer shutdown-derived poll bits >> >until after lock_sock() and use one READ_ONCE() snapshot for both EOF >> >readability and HUP/RDHUP. This preserves shutdowns that arrive before >> >the lock is acquired and keeps all peer-shutdown-derived bits consistent >> >for a poll pass. >> > >> >Fixes: d021c344051a ("VSOCK: Introduce VM Sockets") >> >Signed-off-by: Ziyu Zhang >> >--- >> > net/vmw_vsock/af_vsock.c | 40 ++++++++++++++++++++++++++-------------- >> > 1 file changed, 26 insertions(+), 14 deletions(-) >> > >> >diff --git a/net/vmw_vsock/af_vsock.c b/net/vmw_vsock/af_vsock.c >> >index adcba1b7b..bed42347b 100644 >> >--- a/net/vmw_vsock/af_vsock.c >> >+++ b/net/vmw_vsock/af_vsock.c >> >@@ -1122,6 +1122,25 @@ static int vsock_shutdown(struct socket *sock, int mode) >> > return err; >> > } >> > >> >+static __poll_t vsock_poll_shutdown(struct sock *sk, u32 peer_shutdown) >> >+{ >> >+ __poll_t mask = 0; >> >+ >> >+ /* INET sockets treat local write shutdown and peer write shutdown as a >> >+ * case of EPOLLHUP set. >> >+ */ >> >+ if (sk->sk_shutdown == SHUTDOWN_MASK || >> >+ ((sk->sk_shutdown & SEND_SHUTDOWN) && >> >+ (peer_shutdown & SEND_SHUTDOWN))) >> >+ mask |= EPOLLHUP; >> >+ >> >+ if (sk->sk_shutdown & RCV_SHUTDOWN || >> >+ peer_shutdown & SEND_SHUTDOWN) >> >+ mask |= EPOLLRDHUP; >> >+ >> >+ return mask; >> >+} >> >+ >> > static __poll_t vsock_poll(struct file *file, struct socket *sock, >> > poll_table *wait) >> > { >> >@@ -1139,19 +1158,9 @@ static __poll_t vsock_poll(struct file *file, struct socket *sock, >> > /* Signify that there has been an error on this socket. */ >> > mask |= EPOLLERR; >> > >> >- /* INET sockets treat local write shutdown and peer write shutdown as a >> >- * case of EPOLLHUP set. >> >- */ >> >- if ((sk->sk_shutdown == SHUTDOWN_MASK) || >> >- ((sk->sk_shutdown & SEND_SHUTDOWN) && >> >- (vsk->peer_shutdown & SEND_SHUTDOWN))) { >> >- mask |= EPOLLHUP; >> >- } >> >- >> >- if (sk->sk_shutdown & RCV_SHUTDOWN || >> >- vsk->peer_shutdown & SEND_SHUTDOWN) { >> >- mask |= EPOLLRDHUP; >> >- } >> >+ if (!sock_type_connectible(sk->sk_type)) >> >+ mask |= vsock_poll_shutdown(sk, >> >+ READ_ONCE(vsk->peer_shutdown)); >> >> Can we move this in the `if (sock->type == SOCK_DGRAM)` branch ? >> >> Not a strong opinion about that, but in any case IMO we should add a >> comment here to explain why we are doing only for not connectible >> sockets. >> >> That said, if we use WRITE_ONCE in the writers, do we really need to >> move this after the lock_sock for the connectable ones? > >Yes, I will move the non-connectible handling into the SOCK_DGRAM branch >and add a comment there. > >For connectible sockets, my current understanding is that >READ_ONCE()/WRITE_ONCE() make the individual lockless accesses explicit, but >they do not make two separate reads in one vsock_poll() invocation observe the >same peer_shutdown value. So I still think using one peer_shutdown snapshot >after lock_sock() is useful for keeping the returned mask internally >consistent. Please let me know if you think WRITE_ONCE() is enough for this >case. What will be the issue of "do not make two separate reads in one vsock_poll() invocation observe the same peer_shutdown value." ? In any case, I'm not against it; I just want to understand the issue better. Thanks, Stefano